THE CHEMICAL ELEMENTS 
 
tf 
 
 o 
 
 B 
 
THE CHEMICAL 
 ELEMENTS 
 
 BY 
 
 M. M. PATTISON MUIR, M.A. 
 
 FELLOW AND PRELECTOR IN CHEMISTRY 
 
 OF GONVILLE AND CAIUS COLLEGE 
 
 CAMBRIDGE 
 
 HODDER AND STOUGHTON 
 PUBLISHERS, LONDON* 
 
ITbe ^Useful fmowlefcge 
 
 PLANT LIFE. By GRANT ALLEN. 
 ARCHITECTURE. By P. L. WATERHOUSE. 
 THE STARS. By G. F. CHAMBERS, F.R.A S. 
 
 THE SOLAR SYSTEM. By GEORGE F. 
 CHAMBERS, F.R.A.S. 
 
 FOREST AND STREAM. By JAMES ROD- 
 WAY. 
 
 THE MIND. By Prof. J. M. BALDWIN' 
 
 THE RELIGIONS OF THE WORLD. By 
 the Rev. E. D. PRICE, F G.S. 
 
 EXTINCT CIVILIZATIONS OF THE EAST. 
 By ROBERT E. ANDERSON, M.A., F.A.S. 
 
 THE CHEMICAL ELEMENTS. By M. M. 
 PATTISON MUIR, M.A. 
 
 A PIECE OF COAL. By E. A. MARTIN. 
 
 THE EARTH IN PAST AGES. By H. G. 
 SEELEY, F.R.S. 
 
 BIRD-LIFE. By W. P. PYCRAFT. 
 LONDON : HODDER AND STOUGHTON 
 
PREFACE. 
 
 I HAVE made an. attempt to put forth in some 
 kind of orderly sequence a few of the chief guid- 
 ing conceptions of chemistry ; endeavouring, on 
 the one hand, to avoid technical details, and, on 
 the other hand, to illustrate these conceptions by 
 describing many common facts. I have tried to 
 keep the attention of the reader fixed on the 
 great generalisations of chemistry; and I have 
 done my best to shew that these generalisations 
 find constant application in the occurrences of 
 everyday life. 
 
 M. M. PATTISON MUIR. 
 
 CAMBRIDGE, October 1896. 
 
CONTENTS. 
 
 CHAP. PAGE 
 
 I. INTRODUCTION 9 
 
 II. THE CHARACTER OF CHEMICAL CHANGE . 19 
 
 in. THE CHEMIST'S CLASSIFICATION OF MATERIAL 
 
 THINGS 33 
 
 IV. THE COMPOSITION OF HOMOGENEOUS BODIES 54 
 
 V. THE STUDY OF PROPERTIES ... 76 
 VI. CONNEXIONS BETWEEN COMPOSITION AND 
 
 PROPERTIES 90 
 
 VII. THE COMPOSITIONS AND PROPERTIES OF SOME 
 
 COMPOUNDS OF CARBON . . . 114 
 
 VIII. THE ATOM AND THE MOLECULE . . . 141 
 
 IX. MOLECULAR ARCHITECTURE . . . 167 
 
 X. SUMMARY AND CONCLUSION 186 
 
THE STOKY OF 
 
 THE CHEMICAL ELEMENTS, 
 
 CHAPTER I. 
 
 INTRODUCTION. 
 
 ANYONE who observes with some care the 
 material occurrences which are constantly tak- 
 ing place around him must notice, that although 
 these events differ much from one another, very 
 many of them have this in common that in them 
 there is change of substance, or change of form, 
 or change of properties. As a fire burns there 
 is a ohange of coal, or wood, to smoke, flame, and 
 ashes; although we see no new thing produced 
 when coal gas is burning, yet our sense of smell 
 helps our sense of sight, for we know that if the 
 gas were not changed into some other thing when 
 it is burnt we should smell the unchanged gas ; 
 when an oil lamp is burning badly some of the 
 substances that are formed from the oil make 
 known their presence by unbearable odours. 
 From the dry and winter-stricken trees the 
 return of spring calls forth green buds, and 
 summer changes these to leaves, and autumn 
 dyes those leaves in crimson, purple, and gold ; 
 the blossoms on the fruit-trees that herald the 
 
10 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 near approach of a milder season change after 
 a time into fruits, and on the surface of these 
 fruits the colour deepens as under the influence 
 of the sun new substances are produced within 
 them; the fields are ploughed and the seed is 
 sown, and from these seeds corn and other kinds 
 of grain spring up in due time, and then the pale 
 green ears acquire a warmer hue till the waving 
 of the golden grain tells that the time has come 
 for this cycle of changes to be completed. Water 
 changes to ice, and rain to hail and snow ; the 
 mountain side that to-day is dry and silent to- 
 morrow is vocal with the sound of dashing 
 brooks ; and the mountains themselves crumble 
 slowly and their rocks are changed to the dust of 
 the plains. 
 
 And if we turn to man's dealings with material 
 nature outside himself we shall find that it is 
 hardly an exaggeration to say that the whole of 
 these dealings consist in taking advantage of 
 changes of substance, properties, or form that occur 
 in material things without his intervention, or in 
 accomplishing novel changes by creating condi- 
 tions different from those that prevail where he 
 does not interfere. 
 
 The timber which is used in making houses, 
 furniture, ships, and many other things that 
 enter into our daily lives, is one product of those 
 complicated changes that are brought about by 
 the growing tree in the food it obtains from the 
 air by its leaves and from the soil by its roots. 
 There are plants which change the substances 
 they absorb by their roots and leaves into sugar, 
 and when these plants are crushed and the liquid 
 
INTRODUCTION. 11 
 
 part of the sweet juice which exudes is boiled away 
 sugar remains. In the living laboratories of other 
 plants are produced, from materials the same as, 
 or very like, those that the sugar-cane changes 
 into sugar, dye stuffs such as madder and indigo, 
 or sweet-smelling resins such as frankincense 
 and myrrh, or essential oils as cedarwood oil, 
 oil of bergamot, and attar of roses, or gums as 
 gum tragacanth and gum arabic, or such things 
 as guttapercha and indiarubber, or substances 
 such as quinine, morphine, and salicylic acid that 
 afford some relief to those who suffer pain. 
 
 Most of the substances that are used as human 
 food are the products of changes that occur in 
 those living organisms, plants and animals. The 
 plants absorb comparatively simple substances 
 from the air and the soil, and from these they 
 build up more complex things such as starch and 
 sugar ; plants are eaten by animals which carry 
 further the building up processes that the plants 
 began ; and both vegetable and animal substances 
 are consumed by man and are changed into the 
 highly complex materials that form the organism 
 which is for ever synthesising itself from simpler 
 kinds of matter, and is constantly being resolved 
 into less complex forms of substance. 
 
 The manufacturing industries are so many 
 examples of the intelligent direction to definite 
 ends of changes in material things. Iron ore is 
 brought in the blast furnace under such condi- 
 tions that the red earthy stone is converted into 
 lumps of cast iron ; and when this iron is melted 
 and air is blown into the fluid mass it undergoes 
 another transmutation into hard, lustrous, malle- 
 
12 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 able steel. Sulphur, or it may be pyrites, is burnt 
 in a stream of air, and the suffocating gas so pro- 
 duced is conducted into large leaden chambers 
 where it mixes with steam and with another red- 
 dish coloured gas from which it removes oxygen ; 
 the product of these changes is oil of vitriol, an 
 acrid, corrosive, liquid that is used in almost 
 every manufacturing process. 
 
 Common salt is heated with oil of vitriol, and 
 a mixture of the product with coal and chalk is 
 melted in a furnace ; the black solid formed by 
 this operation is lixiviated with water, and when 
 the liquid is evaporated carbonate of soda is 
 obtained. Carbonate of soda is transformed into 
 caustic soda by boiling with milk of lime ; and 
 when caustic soda is heated with fat, olive oil, or 
 cottonseed oil, the products are glycerin and 
 soap. Scrap iron is heated with pearl ash and 
 hoofs, horns, or fragments of hides or leather ; 
 the product is treated with water, and large, 
 yellow, transparent crystals of yellow prussiate 
 of potash are obtained by filtering this liquid and 
 evaporating it. The yellow prussiate is dissolved 
 in water and a solution of iron in an acid is 
 added to this liquid ; the result is the deep blue 
 pigment called Prussian blue. Sand, or finely 
 ground quartz, is mixed with carbonate of soda, 
 or pearlash, and the mixture is melted in pots 
 placed in a very hot furnace ; when the melted 
 mass is poured on to a flat metal table a sheet of 
 transparent, colourless, glass is formed. 
 
 Coal tar is distilled ; a portion of the colour- 
 less liquid which distils over is mixed with aqua 
 fortis, and the heavy reddish oil, smelling like oil 
 
INTRODUCTION. 13 
 
 of almonds, which is produced by this treatment 
 is warmed for some time with iron filings and 
 concentrated vinegar, and is then distilled. The 
 colourless liquid that distils over is called aniline ; 
 and from that liquid is produced, by suitable 
 treatment, a vast number of coloured substances 
 wherewith cotton, woollen, and other goods may 
 be dyed every shade and variety of colour, both 
 beautiful and crude. 
 
 The juice of grapes acquires new properties 
 after being exposed to the air for a time ; the 
 liquid loses some of its sugar, and in place thereof 
 a volatile liquid, endowed with strange powers 
 and potencies, is produced ; the juice of the fruit 
 has changed into "wine that maketh glad the 
 heart of man." When barley that has begun to 
 germinate is heated to a proper temperature, and 
 is then mashed with water, and the liquid so pro- 
 duced is mixed with yeast and kept slightly warm, 
 a change occurs similar to that which happens in 
 grape juice exposed to the air ; sugar disappears, 
 and alcohol and carbonic acid gas are formed : 
 the decoction of malted barley has been trans- 
 muted into beer. 
 
 And what is the art of cooking but the intelli- 
 gent direction of certain possible transmutations 
 in the substance and properties of food-stuffs so 
 as to produce new materials that nourish the 
 body, please the palate, and enliven the wits of 
 man 1 
 
 The story of chemistry is the story of the 
 unravelling of such changes and transmutations 
 of material things as those that have been referred 
 to in the preceding paragraphs. 
 
14 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 It was supposed in former days that anything 
 might be changed into anything else, provided 
 the proper conditions could be arranged. On this 
 supposition was based the tedious art of alchemy ; 
 for it was not on a vague guess, but on what they 
 took to be a universal law, to wit, the infinite 
 mutability of material things, that the long- 
 suffering, much-enduring, alchemists rested their 
 strange processes. Noticing the profound and un- 
 expected transmutations that occur so frequently 
 in things, the alchemists gave an affirmative 
 answer to the question which these transmuta- 
 tions kept dinning in their ears ; Can any mate- 
 rial thing be changed into any other thing ? And 
 then driven to seek for some stable, unchanging, 
 reality, by the innate longing of humanity for 
 rest, the alchemists hit upon the enticing hypo- 
 thesis of The One Thing, which they called also 
 the Essence, the Stone of Wisdom, the Blessed 
 Water, the Philosopher's Stone, and many other 
 names. 
 
 Those early readers of the story of the changes 
 of material nature found that a language had to 
 be constructed wherein they might express their 
 reading of the legend ; and the fundamental con- 
 ception of their rendering found expression in 
 such sentences as these : " There abides in 
 nature a certain form of matter which, being dis- 
 covered and brought by art to perfection, con- 
 verts to itself, proportionally, all imperfect bodies 
 that it touches." 
 
 ' ; ... For 'twere absurd 
 
 To think that nature in the earth bred gold 
 
INTRODUCTION. 15 
 
 Perfect in the instant ; something went before. 
 There must be remote matter. 
 
 Nor can this .remote matter suddenly 
 Progress so from extreme unto extreme 
 As to grow gold, and leap o'er all the means. 
 Nature doth first beget the imperfect, then 
 Proceeds she to the perfect." 
 
 And, basing their work on this conception of 
 a perfect matter which was able to convert to 
 itself all imperfect bodies, the alchemists recog- 
 nised that much labour was required to gain 
 that perfect matter ; they recognised, as one of 
 themselves stated, that " Nature moves not by 
 the theorie of men, but by their practice, and 
 surely wit and reason can perform no miracles 
 unlesse the hands supplie them." In setting their 
 hands to supply the material conditions favour- 
 able to the movement of nature, they were con- 
 vinced that nature moved in the direction they 
 thought she ought to move in and by ways which 
 they were able to mark out for her progress. 
 
 Many centuries passed before the investigators 
 of nature shewed some willingness to accept their 
 true position as parts of the nature they sought 
 to comprehend. Most of the earlier seekers 
 after a knowledge of nature began by building 
 walls each of which marked off one portion of 
 natural occurrences from the rest. I do not 
 suppose they built these walls consciously, rather 
 they thought they saw walls where no barriers 
 were. The alchemists were very busy within 
 their intellectual sheep-pens, as the chemists of 
 to-day are most active in the larger walled 
 
16 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 gardens of their intellectual domain; but the 
 alchemists made fewer breaches in their encircling 
 barriers than we make now in ours. And I think 
 we recognise, what our predecessors could not 
 acknowledge, that the territorial divisions which 
 must be established are of our own making, and 
 are made only for the purpose of helping us to 
 understand the infinitely complex whole. 
 
 The history of science may be described as the 
 history of the breaking down, and the crumbling 
 away, of artificially constructed barriers. All 
 the great men of science have been famous wall- 
 breakers. The instruments of demolition which 
 these men wielded have been derided by the 
 ignorant wise men who declared the walls would 
 never fall ; the noise of the crashing barriers has 
 travelled slowly to the ears of those who did not 
 wish to hear, and in the vacant places solid con- 
 structions have continued to be evident to the 
 eyes which did not desire to see. 
 
 It is worthy of remark that the central con- 
 ception of the alchemists, which, nevertheless, 
 led to their pigeon-holed representation of 
 nature, was the unity of natural phenomena. If 
 their arguments were translated into the language 
 of to-day they would be somewhat as follows 
 Plants grow from seeds, and the plant is a very 
 different thing from the seed ; animals become 
 larger, stronger, and more complete as they grow 
 older ; the plant may well be called more perfect 
 than the seed, and the full grown animal more 
 perfect than the immature animal ; both plants 
 and animals grow, come to their prime, and 
 decay; and there are degrees of perfection in 
 
INTRODUCTION. 17 
 
 the animal and vegetable worlds. Now we 
 may suppose the argument of the alchemist to 
 continue nature works in simple ways ; man is 
 the crown and glory of creation, and it is foolish 
 to suppose that man cannot understand nature ; 
 to preserve the unity of nature it is necessary 
 that minerals and metals and all inanimate things 
 should grow, and change, from less perfect to 
 more perfect forms ; as there are degrees of per- 
 fectness and dignity among living things, so 
 must there be degrees of perfectness and dignity 
 among all things ; some metals disappear in acrid 
 liquids, and these metals are generally easily 
 worn away, they are readily melted and burnt to 
 dross ; but some other metals are not swallowed 
 up by corrosive liquids, nor are they worn away 
 with ease, nor readily changed in the fire ; there 
 are evidently noble and base metals, perfect and 
 imperfect metals; and as the less perfect seed 
 under proper conditions produces the more per- 
 fect plant, and the plant is rendered yet more 
 perfect by cultivation, so the imperfect metals 
 change slowly into those which are more perfect, 
 and this slow process of change can be hastened 
 by man's art and device. 
 
 The argument from analogy was pushed by the 
 alchemists further than this. What they said 
 may be rendered somewhat in this way : It is 
 evident that living things are more perfect than 
 inanimate things ; if therefore there are changes 
 from less perfect to more perfect forms among 
 living things, much more must changes from 
 immature to mature forms be constantly pro- 
 ceeding among dead things like minerals and 
 
18 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 metals ; and as these minerals and metals are 
 less perfect than plants and animals, it is prob- 
 able that the plasticity of the minerals and the 
 metals will be greater than the plasticity of living 
 animals and living plants ; hence there will not 
 be those marked lines of demarcation in the 
 mineral world which we find in the animal and 
 vegetable worlds ; it will be a comparatively easy 
 thing to grow a noble metal like gold from 
 ignoble metals like lead or copper, although it is 
 an impossible thing to change one kind of animal 
 into another or one sort of plant into another 
 sort of plant. 
 
 A vague conception of the unity of nature of 
 this kind led to little accurate knowledge ; the 
 conception could not be applied strictly and in 
 detail to natural occurrences ; all that could be 
 done was to perform a vast number of inaccurate 
 and incomplete experiments, and to state the 
 results in the loose and slipshod language of the 
 vague but sonorous hypothesis which prompted 
 the experiments. And so although the hypothesis 
 postulated the unity of nature there was no unity 
 in the experimental results which were collected 
 to support the hypothesis. The resemblances 
 and the dissimilarities between natural occur- 
 rences are not to be discovered by reasoning 
 about an order of nature which exists only in 
 the brain of the reasoner. A man who sets out 
 to discover what is must endeavour to put aside 
 all his notions of what ought to be ; it is only 
 when he has gained a solid foundation of verified 
 and accurately stated facts that he may venture 
 to make a definite guess concerning the cause of 
 
THE CHARACTER OF CHEMICAL CHANGE. 19 
 
 the facts he has collected, but unless he makes 
 clearly stated guesses that is, scientific hypo- 
 theses he will remain a mere collector of half 
 facts and will never become a scientific investi- 
 gator. 
 
 We shall endeavour in the following chapters 
 to trace the methods which chemists have used 
 to elucidate a certain class of changes that 
 material things undergo, and to state and illus- 
 trate some of the more important general results 
 they have attained by the application of these 
 methods. 
 
 CHAPTER II. 
 
 THE CHARACTER OF CHEMICAL CHANGE. 
 
 As the changes which material things undergo 
 were examined with some care, a distinction 
 came to be made between those changes wherein 
 the properties and also the composition of the 
 matter affected are modified, and those wherein 
 the composition of the matter is not altered, 
 although its properties are changed. 
 
 A few simple experiments will illustrate the 
 more prominent differences between these two 
 kinds of changes. 
 
 If a piece of thin, clean, iron wire is held in 
 the hot, non-luminous flame of a Bunsen burner,* 
 
 * The Bunsen burner is an ordinary gas jet over which 
 tits a cylindrical metal tube pierced near the lower end with 
 two large holes ; the mixture of air and gas is lighted at 
 
20 THE STOKY OF THE CHEMICAL ELEMENTS. 
 
 the iron glows and emits light. If the wire is 
 removed from the flame after a few seconds, an 
 examination of the wire will show that the iron 
 has not been changed into anything else so far as 
 can be judged by the senses of sight and touch. 
 
 When a few chips of wood are placed in a dry 
 test tube which is then gradually heated by a 
 Bunsen burner, the wood soon darkens in colour, 
 and after some minutes moisture is seen forming 
 on the upper parts of the tube, and something is 
 given off with a very decided odour. If the 
 heating is continued for a few minutes, and a 
 lighted taper is then brought to the mouth of 
 the tube, the vapours that are coming off take 
 fire. The senses of sight and smell assure us 
 that the wood has been changed by heating it, 
 into things which are quite different from wood ; 
 and this conclusion is verified by the test of the 
 lighted taper. 
 
 If a few small pieces of sugar are heated in a dry 
 test tube, in the same way as the chips of wood 
 were heated in the last experiment, the sugar 
 chars arid gives off badly-smelling vapours, which 
 ignite when a lighted taper is brought to the 
 mouth of the tube after the heating has been 
 continued for some minutes. The sugar is evi- 
 dently destroyed, and things which are not sugar 
 are formed in its stead. 
 
 When a few grains of dry salammoniac are 
 
 the top of this tube. A considerable volume of air enters 
 through the holes in the metal tube, and when the mixture 
 of gas and air which issues from the top of the tube is 
 ignited, the whole of the gas is completely burnt and a 
 flame is obtained which is very hot, and is not luminous 
 as it is free from particles of unburnt carbon. 
 
THE CHARACTER OF CHEMICAL CHANGE 21 
 
 placed in a dry test tube, and the lower end of 
 the tube is heated very gently in the flame of a 
 Bunsen burner, the salammoniac gradually dis- 
 appears from the heated part of the tube, and a 
 white solid slowly forms on the upper, cool part 
 of the tube. So far as appearance goes, this 
 white solid is the same as the salammoniac that 
 was heated ; if minute portions of the salam- 
 moniac and the white deposit are tasted they 
 seem to be identical ; and if a little of each is 
 shaken with water both dissolve easily in the 
 water ; moreover, if both are boiled with a little 
 soda lye, the smell of ammonia (spirits of harts- 
 horn) is very perceptible in each case. The 
 senses of sight and taste tell us that the salam- 
 moniac was not changed into any new substance 
 by heating it, although it was transformed from 
 a solid to a vapour, which again became a 
 solid when it was cooled ; and this conclusion is 
 strengthened by the results of the tests with 
 water and soda lye. 
 
 So far as they go, these rough experiments 
 indicate that the iron remained iron when it was 
 taken out of the flame, and that the salammoniac 
 was not changed into any new thing by heating 
 until it became a vapour, and then cooling this 
 vapour : at the same time the iron certainly 
 acquired a new property as long as it was kept 
 in the hot gas-flame, for under these condi- 
 tions it emitted light; and the salammoniac 
 passed from the solid to the gaseous state when 
 it was heated. A change of properties was 
 effected in each case, but this change was not 
 permanent, nor did it seem to be accompanied 
 
22 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 by any change of composition. On the other 
 hand, the senses of sight and smell assured us 
 that both wood and sugar were changed into 
 new things with new properties by the action of 
 heat : in these cases change of properties was 
 undoubtedly accompanied by change of composi- 
 tion ; and both changes were permanent, for the 
 things produced by heating wood did not change 
 back into wood, nor were the things formed by 
 heating sugar re-transformed into sugar, when 
 the vessels in which the heatings were conducted 
 were removed from the source of heat. 
 
 The iron seemed to remain iron, and the sal- 
 ammoniac seemed to be still salammoniac. But 
 u things are not what they seem ; " we must have 
 more conclusive proof. 
 
 The alchemists used to say that during the 
 burning of chalk to lime, an igneous principle, or a 
 fiery essence, passed into the chalk from the fire, 
 and that the caustic properties of burnt lime 
 were due to the presence in it of this igneous 
 principle. Why should not we also assert that 
 the things formed by heating wood or sugar are 
 produced by the addition to the wood or sugar 
 of a calorific principle from the heat, which prin- 
 ciple remains permanently in the burnt wood or 
 sugar ? Why should not we maintain that a 
 fiery essence passes into iron that is placed in a 
 hot flame, but escapes when the iron is removed 
 from the flame. 
 
 The balance has forced the chemist to throw 
 aside the alchemical explanation, and it has 
 enabled him to find an explanation more con- 
 sistent with the facts by revealing facts which 
 
THE CHARACTER OF CHEMICAL CHANGE. 23 
 
 were unknown to the alchemist. If any kind of 
 matter is added to chalk during the conversion 
 of chalk into lime in a kiln, the lime must weigh 
 more than the chalk. But when the lime formed 
 by burning a weighed quantity of chalk is weighed 
 it is found that instead of weighing more, the lime 
 weighs less, than the chalk from which it has been 
 obtained. If one pound of chalk, for instance, is 
 burnt to lime, rather more than half a pound of 
 lime is produced. Some material substance it 
 may be more than one substance has been taken 
 away from the chalk in the kiln; and lime is 
 chalk deprived of one, or more than one, of its 
 constituents. Or, perhaps, the burning effected 
 the removal of something from the chalk and at 
 the same time caused the addition of something 
 else, the net result of these two occurrences being 
 a loss of weight. 
 
 The result of a few experiments on the change 
 that takes place in the lime-kiln, arranged so 
 that determinations are made of the relative 
 quantities of matter concerned in this change, is 
 to present us with a definite subject for further 
 inquiry. Putting aside speculations about igne- 
 ous principles and the like reserving all specu- 
 lations until we have a fixed basis of accurately 
 ascertained quantitative facts whereon to rest our 
 theories we must seek to solve the questions 
 which are directly suggested by the result of our 
 first application of the instrument of research 
 which the balance-maker has put into our hands. 
 
 What did the chalk lose when it was heated ? 
 We did not see anything leaving the chalk. 
 Well then we must aid our sense of sight ; we 
 
24 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 must try to arrange an experiment in such a way 
 that if anything escapes that something may be 
 caught, weighed, and examined. And we must 
 also make the conditions of our experiment such 
 that if anything combines with the chalk, during 
 the heating of it, we shall be able to detect the 
 removal of this something from the materials that 
 are in contact with the chalk during the experi- 
 ment. But to conduct such experiments as these 
 means that we acquaint ourselves with the com- 
 position of chalk, and the composition of the air 
 in contact with which the chalk is heated, and 
 that we know also the composition of the vessel 
 wherein the chalk is exposed to the action of 
 heat. A large inquiry is at once opened ; and 
 that inquiry must be prosecuted by accurate, 
 that is by quantitative, experiments. We shall 
 have to determine whether chalk has a definite 
 composition, or whether its composition is not 
 always the same , we must resolve chalk and 
 lime into their components, and re-build these 
 substances from those components, and we can- 
 not do this without giving a very definite mean- 
 ing to the words composition and components. 
 
 And as with the chalk so with the wood, the 
 sugar, the salammoniac, and the iron. If the 
 iron was not changed into a new thing when it 
 was heated until it glowed, the weight of the iron 
 before the experiment was made must be the 
 same as the weight of the material when the 
 experiment was finished ; and that we may posi- 
 tively assert that no change of composition accom- 
 panied the temporary change of properties from 
 dull iron to a glowing, light-emitting substance, 
 
THE CHARACTER OF CHEMICAL CHANGE. 25 
 
 we must know definitely the composition of the 
 iron before it was heated and after it was heated, 
 and we must determine whether the composition 
 of the air that surrounded the glowing iron did 
 or did not undergo any change. Similar experi- 
 ments must be made with salammoniac ; the 
 composition of the substance must be determined, 
 and the composition of the white sublimate * which 
 seemed to be unchanged salammoniac must be 
 compared with that of the salammoniac before 
 heating ; moreover the loss of weight suffered by 
 the salammoniac must be determined and the 
 weight of the white sublimate must be found, for 
 if no change of composition accompanied the 
 change from the solid to the gas and back to the 
 solid these two weights must be the same. 
 Methods must also be devised and experiments 
 conducted whereby we shall be able to determine 
 the exact compositions of wood and sugar, and the 
 compositions of the things that are produced when 
 wood and sugar are heated ; and the experiments 
 must take into account the possibility of the 
 removal of material things from, or the gain of 
 such things by, the surrounding air and the vessels 
 wherein the experiments are conducted. 
 
 It was only as chemists, and naturalists in 
 general, recognised that quantities of matter 
 could be measured by their weights, that they 
 began to find a trustworthy method of examining 
 and disentangling the transformations that matter 
 undergoes. However strange, unexpected, and 
 profound a change in the properties of some mate- 
 
 * A solid obtained by cooling the vapour formed by 
 heating a solid is called a siiblimate. 
 
26 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 rial substance may be, the balance enables us to 
 find out whether the change has or has not been 
 accompanied by the addition to, or the removal 
 from, the changing substance of some other kind 
 of matter. 
 
 Imagine a box with many different things in it : 
 let there be balls of wood of different colours ; 
 lumps of iron, lead, copper, silver, and other 
 metals ; pieces of suet ; parcels of tea, spices, 
 sugar, coffee and such things; let some of the 
 things in the box be large, and let other things 
 be extremely small. Let it be arranged that 
 nothing is to be taken from the box, nor is any- 
 thing to be brought into the box, from outside. 
 Let there be two people. One of these examines 
 an object in the box, say a parcel of tea ; he then 
 turns his back, while the other person either 
 takes out some of the contents of the parcel of 
 tea, or mixes other things with the tea in the 
 parcel, or does both of these, or does not alter 
 the contents of the parcel but dashes a coat of 
 paint over the outside of it ; the first person has 
 then to determine which of the four processes 
 has taken place. It is evident that a couple of 
 weighings would help towards the solution of the 
 problem but would not suffice to solve it. If the 
 parcel weighs the same after the process as before 
 the process, either nothing has been taken out, or 
 if anything has been removed an equal weight of 
 something else (or of some other things) has been 
 put into the parcel. If the parcel weighs less 
 at the end, than it did at the beginning, of the 
 experiment, something has been taken out of it ; 
 but something may also have been put in, pro- 
 
THE CHARACTER OF CHEMICAL CHANGE. 27 
 
 vided the weight of what has been put in is less 
 than the weight of what has been taken out. A 
 complete solution of the problem would be pos- 
 sible only when the properties of each distinct 
 substance in the box were known, and the quan- 
 tity of each was known : in other words, to solve 
 the problem it would be necessary to examine 
 the various substances carefully, arid to determine 
 the quantity of each in the box, by weighing it, 
 both at the beginning and at the close of the pro- 
 cess ; if the same weight of each substance was 
 found in the box, outside the parcel, at the end 
 as was found at the beginning of the proceedings, 
 evidently nothing had been taken out of the 
 parcel of tea nor had anything been put into it. 
 
 One of the people in our imaginary case is deal- 
 ing with a question which resembles those the 
 chemist has to explain. 
 
 " Had I to play at that foolish game," some 
 one may say, " I would open the parcel and look 
 in; and with a little rummaging I would soon 
 find out whether the other player had changed 
 the contents of the parcel or not." But however 
 keen your eyesight you might easily miss a few 
 grains of pepper among the tea ; and although 
 your sense of smell is very delicate a fraction of 
 a grain of cinnamon might escape your nose. 
 And when you play the real game, the chemist's 
 game, the parcels you are set to rummage are 
 either very hard to open, or they have a way of 
 falling to pieces while you are looking at them ; 
 and if you succeed in opening the parcels and 
 taking out the contents, the things you arrange 
 in heaps according to their kinds are strangely 
 
28 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 different from the appearance of the things when 
 they were mixed together in the parcels. 
 
 The illustration is crude ; but it may help us a 
 little now, and ib will perhaps help us more at a 
 later stage of our inquiries. 
 
 The box which contains the things the chemist 
 has to deal with is the Earth. There are other 
 boxes outside the earth, but the chemist has as yet 
 only one instrument for opening the lids of these 
 a little that he may peep in at the contents ; that 
 instrument, the spectroscope, enabled the chemist 
 to see a certain kind of matter in the sun which 
 was not known to exist in the earth, and which 
 for that reason was called helium, but within the 
 last year or two the chemist has been using the 
 same instrument to search odd nooks and cran- 
 nies of his earthly domain, and he has discovered 
 that the solar matter he called helium exists shut 
 up in some out of the way minerals that are found 
 on the surface of the earth. 
 
 Let us turn back to the experiments wherein 
 wood and sugar were changed into substances 
 quite different from themselves by the action of 
 heat. The results of these experiments proved, 
 it was said that the compositions of the wood 
 and the sugar were changed. What new sub- 
 stances then were formed when these things, 
 wood and sugar, were heated"? And what are 
 the differences between the compositions of the 
 new substances and those of the things from 
 which these new substances have been produced ? 
 
 When sugar is resolved into its simplest con- 
 stituents these constituents are found to be 
 three very different substances : one of these is 
 
THE CHARACTER OF CHEMICAL CHANGE. 29 
 
 carbon, a substance well known to everyone by 
 the name of charcoal ; another of the three sub- 
 stances is a colourless, odourless, gas to which 
 the name oxygen is given ; and the third is also 
 a colourless, odourless, gas, but a gas that differs 
 much from oxygen ; to this gas is given the name 
 hydrogen. From a determinate weight of sugar 
 certain determinate weights of these three kinds 
 of matter can be obtained ; the weight of carbon, 
 or the weight of oxygen, or the weight of 
 hydrogen is less than the weight of the sugar 
 from which the carbon, oxygen, or hydrogen is 
 obtained ; and the sum of the weights of carbon, 
 oxygen, and hydrogen is equal to the weight of 
 the sugar from which these substances are pro- 
 duced. By no process has it been found possible 
 to obtain from a specified weight of carbon any 
 substance different in properties from carbon 
 and weighing less than the weight of carbon 
 used ; and what is true of carbon is true also of 
 oxygen and hydrogen. These things carbon, 
 oxygen, and hydrogen, can be changed into 
 other things, but the changes are always effected 
 by adding other substances to the carbon, 
 hydrogen, or oxygen ; in other words, the new 
 substances obtained always weigh more than the 
 carbon, the oxygen, or the hydrogen weighed; 
 and when a new substance is formed by adding 
 other things to carbon, oxygen, or hydrogen, the 
 weight of carbon, oxygen, or hydrogen used, but 
 no more than this weight, can always be obtained 
 again from the new substance so formed. 
 
 These three kinds of matter, which are called 
 carbon, oxygen, and hydrogen, respectively, are 
 
30 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 said to be the simplest substances into which 
 sugar can be resolved ; it would be better to say 
 that these are the simplest substances into which 
 sugar has been resolved. The word simplest is 
 used as a convenient term to express trie fact 
 that no one has been able to separate carbon, or 
 oxgyen, or hydrogen, into parts that are unlike 
 one another; it is true that substances very 
 unlike carbon, oxygen, or hydrogen are easily 
 obtained from these kinds of matter, but no one 
 of the processes whereby such substances are 
 formed is a process of separation or sorting, for 
 the new substance always weighs more than the 
 carbon, oxygen, or hydrogen weighed, and the 
 new substance is always formed by adding some 
 other kind of matter to the carbon, the oxygen, 
 or the hydrogen. 
 
 Wood, like sugar, is composed of carbon, 
 oxygen, and hydrogen; but a specified weight 
 of wood contains weights of these three kinds of 
 matter different from the weights of them that 
 are contained in a weight of sugar the same as 
 the specified weight of wood ; in other words 
 the qualitative composition of wood is the same 
 as that of sugar, but the quantitative compositions 
 of these two substances are different. 
 
 When sugar, or wood, is heated, new substances 
 are produced : some of the carbon that was in 
 the sugar makes its appearance as black charcoal 
 in the vessel wherein the heating is conducted ; 
 gases are given off, some of which are com- 
 bustible and some smell strongly, and these gases 
 are composed of carbon, oxygen, and hydrogen, 
 or of two of these substances. 
 
THE CHARACTER OF CHEMICAL CHANGE. 31 
 
 Experiments with weighed quantities of sal- 
 ammoniac have proved that this substance can 
 be resolved into two gases, one called ammonia 
 (commonly known, when dissolved in water, as 
 spirits of hartshorn), and the other called hydro- 
 chloric add gas (commonly known, when dissolved 
 in water, as spirits of salt). The weight of 
 ammonia, or the weight of hydrochloric acid gas, 
 that can be obtained from salammoniac is always 
 less than the weight of the salammoniac from 
 which these gases are obtained ; and when sal- 
 ammoniac is formed by causing ammonia and 
 hydrochloric acid gas to combine, the sum of the 
 weights of these gases that disappear is exactly 
 equal to the weight of salammoniac that is 
 formed. But ammonia and hydrochloric acid 
 gas are not the simplest constituents of sal- 
 ammoniac ; for each of these gases can be re- 
 solved into two substances quite unlike itself. 
 Ammonia can be separated into two parts, each 
 very different from the other and both quite 
 dissimilar from ammonia ; one of those parts, or 
 constituents, of ammonia is hydrogen one of 
 the gases that can be obtained from sugar and 
 the other is a colourless, odourless, very inert 
 gas called nitrogen. Hydrochloric acid gas also 
 can be separated into two dissimilar portions or 
 kinds of matter ; hydrogen is one of these, and 
 the other, called chlorine, is a yellow, very badly 
 smelling, suffocating, gas. The weight of nitrogen, 
 and the weight of hydrogen, obtainable from a 
 determinate weight of ammonia is less than the 
 weight of ammonia that is separated into nitrogen 
 and hydrogen : similarly hydrochloric acid gas 
 
32 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 yields less than its own weight of hydrogen, and 
 less than its own weight of chlorine. Finally, 
 the sum of the weights of nitrogen and hydrogen 
 that can be obtained from ammonia is exactly 
 equal to the weight of ammonia from which they 
 are obtained ; and the sum of the weights of 
 hydrogen and chlorine that can be obtained from 
 hydrochloric acid gas is exactly equal to the 
 weight of that gas from which they are obtained. 
 
 No process is known whereby either nitrogen 
 or chlorine can be resolved, or separated, into 
 dissimilar kinds of matter : if a new kind of 
 matter is produced from nitrogen, or chlorine, 
 the new substance always weighs more than the 
 nitrogen, or the chlorine, from which it has been 
 produced ; and the new substance is always 
 formed by adding some other kind (or kinds) of 
 matter to the nitrogen or the chlorine. 
 
 Nitrogen and hydrogen, then, are the simplest 
 constituents of ammonia; and chlorine and 
 hydrogen are the simplest constituents of hydro- 
 chloric acid gas. Salammoniac can be resolved 
 into ammonia and hydrochloric acid gas ; but the 
 simplest constituents of salammoniac are the 
 three gases nitrogen, hydrogen, and chlorine, for 
 ammonia can be separated into nitrogen and 
 hydrogen, and hydrochloric acid gas can be re- 
 solved into chlorine and hydrogen. 
 
 We have then two classes of substances ; those 
 that have been separated into unlike parts or 
 constituents, and those that have not been 
 separated into dissimilar portions ; it is con- 
 venient to speak of the latter as simpler than 
 the former. 
 
CHEMICAL CLASSIFICATION OF MATTER. 33 
 
 Iron belongs to the class of substances that 
 have not been divided into dissimilar con- 
 stituents ; when iron is changed into something 
 different from itself the change always consists 
 in the adding on to the iron of some other 
 substance, or some other substances. Iron is 
 one of the relatively simple kinds of matter. 
 
 In the next chapter certain examples of 
 changes of matter will be considered in some 
 detail, with the object of becoming acquainted 
 with the methods that are used in investigating 
 those transmutations which are classed together 
 as chemical changes. 
 
 CHAPTER III 
 
 1 THE CHEMIST'S CLASSIFICATION OF MATERIAL 
 THINGS 
 
 ALL the wonderful changes that proceed in 
 material things around us, and all the changes 
 that can be produced in these things by man's 
 device, are either occurrences wherein the com- 
 position, as well as the properties, of the matter 
 affected, undergo modification, or they are 
 events in which the composition of the matter 
 dealt with remains unaltered although its pro- 
 perties are changed. Chemistry is concerned 
 with those phenomena which are marked by 
 change of composition accompanying change of 
 properties; and the object of chemistry is to 
 study those transmutations that occur simul- 
 c 
 
34 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 taneously in the composition and properties of 
 definite kinds of matter, and to express the 
 results of this study in general statements which 
 hold good in all particular cases, and which 
 connect, or at anyrate suggest connections be- 
 tween, the chemical aspects of natural pheno- 
 mena and the other parts of these phenomena. 
 The chemical study of any occurrence presents 
 two lines of inquiry ; the compositions of all the 
 substances that take part in the occurrence 
 must be studied, and the properties of all these 
 substances must be studied. It is on these lines 
 that our examination of the chemist's dealings 
 with nature must proceed. The chemical ex- 
 amination of any natural occurrence is an 
 examination of only a part, it may be a small 
 part, of the whole event ; but the chemist is 
 bound to make his part of the inquiry as com- 
 plete as. he can. Nature defies complete analysis; 
 nevertheless, the student of each part of nature's 
 working must do his best to take out the folds ; 
 remembering always that much will remain for 
 the workers in other branches of natural know- 
 ledge, and, not forgetting, that when the men of 
 science have completed their inquiries, there is 
 need of the realising grasp of the man of im- 
 agination the man who sees and can express 
 his vision to bind together the parts into a 
 unity which satisfies. 
 
 All material things can be placed in one or 
 other of two classes ; they are either things like 
 iron or things like wood : that is to say, they 
 are either things which have not been separated 
 into dissimilar parts, or they are things which 
 
CHEMICAL CLASSIFICATION OF MATTER. 35 
 
 have been separated into unlike portions. But 
 chemistry does not deal with all material things ; 
 this branch of natural knowledge concerns itself 
 with what I have called, on p. 34, " definite 
 kinds of matter." It is sometimes said that 
 chemistry deals with changes occurring in 
 " homogeneous bodies." What is meant by 
 these expressions, ** definite kinds of matter," 
 and " homogeneous bodies"? They are used 
 to convey the same meaning; what, then, is 
 their meaning? 
 
 You are handed a cup of coffee at the break- 
 fast table. You sip a little to find whether 
 there is sugar in it or not ; your sense of taste 
 detects the presence or absence of sugar, The 
 coffee is too strong ; you pour hot milk into it 
 that the neutral taste of the milk may tone 
 down the aromatic flavour of the coffee ; and if 
 you put in too much milk you remedy that by 
 adding more coffee. Each ingredient of the 
 beverage you are drinking infusion in water 
 of coffee beans, milk, and sugar carries its own 
 special properties with it, and retains these pro- 
 perties in the mixture you consume. You would 
 certainly be surprised if the addition of a lump 
 of sugar made the coffee bitter, or if pouring 
 in milk deepened the colour of the liquid in 
 your cup. A cup of coffee is not a "definite 
 kind of matter"; it is not a "homogeneous 
 body." It is made of, at least, three different 
 substances, and each of these retains its own 
 properties in the product of the mixture of 
 them. A cup of coffee is a mixture. A chemist 
 asked to examine this mixture would examine 
 
36 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 the composition and properties of the milk, the 
 sugar, and the coffee beans, separately. " The 
 liquid you drink," he would say, " has no distinct 
 and precise properties of its own ; one man 
 takes his coffee blacky another has his mostly all 
 milk, a third can't drink coffee with sugar in 
 it, and a fourth puts half-a-dozen lumps of sugar 
 into a single cup : yet these different mixtures 
 are all called cofiee." The ordinary coffee- 
 drinker builds up his beverage on a foundation 
 of coffee-beans; and he is able to distinguish, 
 by his sense of taste, the different parts of the 
 building. 
 
 Now suppose a man is a connoisseur in coffee ; 
 he cannot drink his coffee unless it is made from 
 freshly -roasted beans ; and, that the process may 
 be stopped exactly at the right point, he roasts 
 his beans each morning before breakfast. The 
 roasting process brings about a deep seated and 
 permanent change in the beans ; light-coloured 
 and nearly odourless things are changed into 
 mahogany-brown and odoriferous substances ; 
 from somewhat insipid things are produced things 
 whose taste is pleasing to the palate. As the 
 chemical changes proceed in the roasting beans, 
 the sense of smell enables the trained operator 
 to follow these changes step by step. It is surely 
 impossible that something with that all-penetrat- 
 ing odour was actually in the unroasted beans, 
 and that the roasting is merely drawing out what 
 was already there ? The senses declare that new 
 substances are being formed as the roasting pro- 
 ceeds. The fastidious coffee-drinker who roasts 
 his own beans morning by morning daiiy carries 
 
CHEMICAL CLASSIFICATION OF MATTER. 37 
 
 out a process of practical chemistry, and every 
 morning he impresses on the senses of the house- 
 hold the singular accuracy of the schoolboy's 
 designation of that branch of science. It is true 
 that the sense of smell does not enable an ordinary 
 person to determine whether the odoriferous 
 emanations from the roasting beans consist of 
 several ingredients, differing in gradations of 
 piquancy, or whether the smelling thing is one 
 and undivided. But at anyrate a marked change, 
 and a fairly definite change, is being brought 
 about: the unroasted material is not a mixture 
 of odourless beans and a smelling substance ; the 
 roasting undoubtedly calls into existence kinds 
 of matter that were not there before. 
 
 Take another example. Beer is a mixture of 
 many things ; its chief constituents are water, the 
 soluble parts of hops, fruit-sugar and other sub- 
 stances from malt, and alcohol. If a bottle of 
 beer is distilled until about three-fourths of it 
 has disappeared, and the vapour that comes off' 
 is condensed to a liquid, this colourless liquid 
 contains all the alcohol that was in the beer. If 
 the colourless liquid is distilled repeatedly, and 
 the last distillate is boiled with burnt lime and 
 then distilled again, almost pure alcohol is 
 obtained. And it is possible to separate the 
 small quantity of water that is mixed with this 
 alcohol, and so to produce what is called "absolute 
 alcohol." By taking advantage of certain pro- 
 perties of alcohol and water, respectively, the 
 alcohol that is contained in beer can be separated 
 from the water and the other ingredients of the 
 beer. The alcohol retains its own properties in 
 
38 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 the mixture of alcohol and other things we call 
 beer. The beer can be sorted out, by distillation, 
 into unlike parts of which alcohol is one ; and a 
 mixture of these parts has the properties of beer. 
 But by no mere sorting process can alcohol be 
 separated into things unlike itself : nor can 
 alcohol be produced by mixing other things. It 
 is tiue that alcohol has been separated into unlike 
 parts or constituents ; it has been separated into 
 carbon (a substance well known as charcoal), and 
 hydrogen (a very light, colourless, inflammable 
 gas), and oxygen (also a colourless gas, but not 
 inflammable). But the process whereby these 
 kinds of matter are obtained from alcohol is a 
 process of breaking up , it is not a sorting out, 
 like sifting coarser from finer sand, or an unravel- 
 ling, like disentangling skeins of blue and yellow 
 silk. By no process can carbon be removed 
 from alcohol except by breaking up and disin- 
 tegrating the alcohol. Alcohol cannot, by any 
 stretching of the term, be called a mixture of 
 carbon, hydrogen, and oxygen. Alcohol is as 
 definite and homogeneous a kind of matter as 
 any of its three constituents ; only it is possible 
 to obtain from a definite weight of alcohol certain 
 definite weights of carbon, hydrogen, and oxygen, 
 such that, while the weight of each is less than 
 the weight of alcohol used, the sum of the weights 
 of the three is equal to the weight of alcohol used ; 
 whereas no one has succeeded in subjecting car- 
 bon, hydrogen, or oxygen to a process of disin- 
 tegration, or simplification, similar to this. 
 
 Chemistry, then, is concerned with the changes 
 of composition ar^d properties that take place 
 
CHEMICAL CLASSIFICATION OF MATTER. 39 
 
 when one definite kind of matter is transmuted 
 into other definite kinds of matter, or when 
 several homogeneous bodies interact so as to 
 produce other homogeneous bodies. 
 
 It is true that most of the definite kinds of 
 matter with which the chemist has to deal are 
 given to him mixed one with another. Pure 
 water is a perfectly distinct substance ; the pro- 
 perties of any portion, however small, of a 
 quantity of pure water are identical with the 
 properties of any other portion. But pure water- 
 is never found in nature ; one may even say that 
 no man has ever seen or handled absolutely pure 
 water; it is an ideal substance, to which some 
 specimens of highly purified water have nearly 
 approached. Natural waters are complicated mix- 
 tures ; but the proportion of impurities, that is, 
 substances which are not water, in some kinds of 
 lake waters, and in the rain that falls in places far 
 from human habitations, is so small that such water 
 may be spoken of in ordinary language as pure. 
 
 Again, the substances that are commonly used as 
 fuel are mixtures, each of several distinct kinds 
 of matter or homogeneous substances. Take for 
 instance paraffin oil. If a homogeneous liquid is 
 boiled in a flask with rather a long neck, a ther- 
 mometer placed in the neck of the flask will 
 register the same temperature as long as any- 
 boiling liquid remains in the flask. But if 
 paraffin oil is boiled under these conditions, the 
 temperature of the vapour that is given off is 
 shown by the thermometer to change very con- 
 siderably. By condensing to liquid all the vapour 
 that comes off while the thermometer varies but 
 
40 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 slightly ; then changing the collecting vessel, and 
 receiving in it the liquid that distils over while 
 the thermometer marks another small variation 
 of temperature ; then again changing the vessel ; 
 and so on : and by repeating these processes 
 many times with each of the several portions of 
 liquid in the collecting vessels : by such a series 
 of tedious operations, paraffin oil can be sorted 
 out into many distinct portions, each of which 
 is a homogeneous liquid with its own distinct 
 and characteristic properties. 
 
 Once more, the rocks, clays, sands, and minerals 
 that are spoken of under definite names in 
 mineralogical or geological books are mixtures, 
 and some of them are very complicated mixtures. 
 The geologist classifies rocks under such names 
 as slate, schist, felspar, mica, quartz, sandstone, 
 limestone, and the like ; and a classification of this 
 kind is sufficient for his purposes. All the rocks 
 called schist, for instance, are more like one 
 another to the geological eye than they are like 
 any other rocks ; when the geologist has divided 
 the schists into such subclasses as mica schist, 
 hornblende schist, and chlorite schist, he has carried 
 the sorting process far enough for the purpose of 
 distinguishing broadly one variety of rock from 
 another, so that the story of the formation of 
 masses of rock may be stated in intelligible 
 terms. But if the chemist has to deal with such 
 mixtures of definite substances as different kinds 
 of limestone, his first business is to pick out, by 
 some suitable device or appropriate machine, the 
 definite kinds of matter of which each variety of 
 limestone is composed, and his next business is 
 
CHEMICAL CLASSIFICATION OF MATTER. 41 
 
 to find out how far he can carry the process of 
 simplification of each of these definite kinds of 
 matter. When he has determined which of those 
 simplest kinds of matter whereinto he has suc- 
 ceeded in disintegrating all material substances 
 enter into the composition of each distinct and 
 definite substance obtained from the various lime- 
 stones, and when he has determined the quantity 
 of each simplest kind of matter in a definite quan- 
 tity of each homogeneous substance sorted out of 
 the limestones, then he has performed his task so 
 far as the composition of those mixtures that the 
 geologist calls limestone is concerned. 
 
 Although the chemist does not begin his proper 
 work until the mixtures that are found in nature 
 have been sorted into their distinct and hoim> 
 geneous components ; although, in other words, 
 chemistry is not concerned, strictly speaking, 
 with mixtures, but only with those definite kinds 
 of matter the smallest parts of which that can be 
 obtained by any kind of sorting process which 
 does not involve the breaking up of the substance 
 are identical in properties with one another ; 
 although this is true, nevertheless the chemist is 
 so often obliged to preface his proper work by 
 acting as a sifter of mixtures, that I should like 
 to say a little more on the meaning of the terms 
 mixture and homogeneous body, and to illustrate the 
 intermingling of these two classes of substances 
 in some well-known material of everyday life. 
 
 I shall try to illustrate this point of funda- 
 mental importance by sketching the processes 
 whereby beer is manufactured. Malt is made by 
 causing germination to begin in barley, and then 
 
42 THE STORY OF THti CHEMICAL ELEMENTS. 
 
 stopping this process by heating in kilns. A 
 definite substance called diastase is produced in 
 the germinating barley, and this substance is able 
 to convert starch into sugar. The malt is crushed 
 and agitated with water in tuns, at a temperature 
 of 50 to 70 C. [120 to 170 F.]. The liquor 
 that is drawn off is known as wort ; it is rich in 
 sugar produced by the action of diastase on the 
 starch of the barley. The wort is now boiled 
 with hops, whereby some of the bitter substances 
 in the hops go into solution and impart their 
 flavour to the wort. The liquor is then cooled 
 rapidly, a little yeast is added to it, and it is kept 
 at the temperature which experience has shewn 
 to be most favourable to the change of sugar, by 
 the action on it of yeast, into alcohol and carbonic 
 acid gas. The fermented liquid is stored in closed 
 barrels or bottles ; and a change, called secondary 
 fermentation, goes on slowly, and produces a very 
 little more alcohol, and a small quantity of car- 
 bonic acid gas which collects in the liquid, and, 
 by escaping when the barrels, or the bottles, are 
 opened, causes brisk effervescence. 
 
 The principal chemical changes that take place 
 in the making of beer are the conversion of starch 
 into sugar by the action of diastase, and the 
 transmutation of sugar into alcohol and carbonic 
 acid by the action of yeast. In these changes, 
 definite kinds of matter are converted into other 
 definite kinds of matter ; and each change of one 
 homogeneous body into other homogeneous bodies 
 takes place under definite conditions. The homo- 
 geneous kinds of matter chiefly concerned in the 
 processes are starch, sugar, alcohol, and carbonic 
 
CHEMICAL CLASSIFICATION OF MATTER. 43 
 
 acid gas. Starch, sugar, and alcohol have been sepa- 
 rated each into three dissimilar constituents called 
 carbon, hydrogen, and oxygen, respectively ; and 
 carbonic acid gas has been separated into carbon 
 and oxygen. But no one of those three homo- 
 geneous bodies, which are the constituents of 
 starch, sugar, alcohol, and carbonic acid gas, has 
 itself been disintegrated into unlike portions. 
 These three definite kinds of matter are the sim- 
 plest constituents of the other four definite kinds 
 of matter. A definite quantity of any one of the 
 four things, starch, sugar, alcohol, or carbonic 
 acid gas, is always composed of fixed and unalter- 
 able quantities of its simplest constituents. For 
 instance, 100 Ibs. of pure fruit sugar are always 
 composed of 40 Ibs. of carbon, 53| Ibs. of oxygen, 
 and 6 1 Ibs. of hydrogen ; and 100 grains of any 
 specimen of pure carbonic acid gas are always 
 composed of 27 '27 grains of carbon and 72 '73 
 grains of oxygen. Moreover, if pure fruit sugar 
 were completely changed into alcohol and car- 
 bonic acid gas, and into these substances only, 
 51 '1 oz. of alcohol, and 48*9 oz. of carbonic acid 
 gas would always be obtained from 100 oz. of the 
 fruit sugar ; and if the whole of the sugar were 
 not changed into alcohol and carbonic acid gas, 
 the weights of the three substances, sugar, alcohol, 
 and carbonic acid gas, concerned in the change 
 would always be related in the proportion of the 
 numbers 100: 51'1 : 48 '9, or of whole multiples 
 of these numbers. 
 
 These definite chemical changes namely, the 
 conversion of starch into sugar, and the transfor- 
 mation of sugar into alcohol and carbonic acid 
 
44 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 gas proceed during the making of beer in a 
 liquid which is essentially a mixture of very 
 many substances ; nevertheless the chemical 
 changes take place under certain conditions, 
 the chief of which are the presence of diastase in 
 one of the changes, the presence of yeast in the 
 other, the presence of water in both, and the 
 maintenance of a suitable range of temperature. 
 The amount of water added for each ton of malt 
 varies in different breweries ; one specimen of 
 barley is richer in starch than another j one kind 
 of hops contains more of the bitter substances 
 that flavour beer than another kind ; and the 
 German brewer carries on the fermentation pro- 
 cess at a lower temperature, and more slowly. 
 than the English brewer. Variations in the 
 quantities of the main constituents of beer are 
 allowable, provided these variations are kept 
 within limits ; and variations in the conditions 
 under which the chemical changes proceed may 
 also be made, but only within a limited range. 
 Beer is a mixture ; but it is a mixture of not 
 very varying quantities of certain definite kinds 
 of matter, the presence of each of which is essen- 
 tial, in quantities that vary considerably of other 
 kinds of matter, some of which are definite sub- 
 stances and all of which can be resolved into 
 definite substances. The art of brewing is not 
 accomplished by mixing certain things and calling 
 the mixture beer. It is a much more difficult 
 business than that. Certain things must be 
 mixed, and the conditions must be arranged so 
 that some, but not all, of the constituents of the 
 mixture shall react to produce fairly fixed quan- 
 tities of certain other perfectly definite things; 
 

 CHEMICAL CLASSIFICATION OF MATTER. 45 
 
 and if the proper limits of the conditions are 
 passed, the new things, by the production of 
 which in due quantities wort is transmuted into 
 beer, will not be formed, but other things will be 
 formed instead, and the final result will be a 
 liquid which, whatever it may be called, is em- 
 phatically not beer. 
 
 Many people like to drink their beer from 
 pewter. As they pour the beer into the pewter 
 mug, they are pouring a liquid which is the 
 product of many chemical changes into a vessel 
 made by mixing, not chemically combining, two 
 metals. Pewter is made by melting a mixture 
 of about* four parts of tin with one part of lead. 
 There is no change into new kinds of matter. 
 The properties of the pewter are, on the whole, 
 the mean of the properties of the lead and tin 
 of which it is composed. The colour is brighter, 
 than the colour of lead, but duller than that of 
 tin ; pewter is harder than lead, but softer than 
 tin. From a knowledge of the properties of lead 
 and tin one could predict, very fairly well, the 
 properties of the mixture of these metals called 
 pewter. Moreover different specimens of pewter 
 contain different relative quantities of lead and 
 tin; one specimen may contain as much as 90 
 parts of tin per hundred of pewter, while another 
 contains less than 70 per cent, of tin. Pewter 
 does not belong to the class of ' definite kinds of 
 matter 7 or 'homogeneous bodies'; by processes 
 which are essentially sorting or disentangling 
 processes, the constituents of pewter, lead and 
 tin, could be obtained from pewter. 
 
 Very many manufacturing processes are based 
 on definite chemical changes ; but the products 
 
46 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 turned out in almost every branch of the chemical 
 industries are mixtures of several definite kinds 
 of matter. For this reason it is generally easy 
 for a chemist to lay down what may be called 
 the theory of a chemical manufacture ; but it is 
 very difficult to apply the theory to the compli- 
 cated, semi-chemical, conditions that prevail in 
 the factory. 
 
 In trying to understand chemical occurrences, 
 and to arrive at general statements which shall 
 express what holds good in all those transforma- 
 tions of matter wherein composition and pro- 
 perties change together, we must confine our 
 attention to the homogeneous bodies that are 
 concerned in the events we study; we must 
 simplify the natural occurrences by deliberately 
 cutting out all those parts of them which have 
 not to do with definite kinds of matter. 
 
 It was said (p. 34) that all material things 
 may be arranged by the chemist in two classes : 
 they are either things like iron or things like 
 wood ; that is to say, they are things that, like 
 iron, have not been separated into unlike parts, 
 or they are things which, like wood, have been 
 separated into dissimilar portions. We now 
 know that the class of wood-like things must be 
 divided into two sub-classes : things, the com- 
 position of which is variable within limits, and 
 the separation of which into unlike constituents 
 is effected by sorting processes which do not 
 involve the complete disintegration of the things 
 themselves ; and things whose composition is 
 always absolutely the same, and from which 
 unlike portions can be obtained only by breaking 
 
CHEMICAL CLASSIFICATION OF MATTER. 47 
 
 up and destroying the things themselves. Woocl 
 is an example of the first of those sub-classes, 
 and sugar is an example of the second. Wooc| 
 is a mixture of many homogeneous bodies, anc( 
 different specimens of wood differ in the relative 
 quantities of their component substances ; sugar 
 is a particular homogeneous body, and all speci- 
 mens of pure sugar have exactly the same com- 
 position. Chemistry is concerned with certain 
 changes occurring among homogeneous bodies : 
 and these bodies are either like iron, in that 
 however much they are divided the particular 
 properties of all the parts are identical ; or they 
 resemble sugar, in that they can be separated 
 into simpler homogeneous bodies whose special 
 properties are different from those of the more 
 complex homogeneous bodies out of which they 
 have been taken. 
 
 It is convenient to give class-names to the two 
 kinds of homogeneous bodies. Those that are 
 like iron are called Elements; those which re- 
 semble sugar are named Compounds. 
 
 We have now arrived at this point. All 
 material substances either have, or have not, 
 been resolved, or separated, into unlike portions ; 
 that is to say, every kind of matter belongs to 
 one or other of the classes 
 
 ELEMENTS. NOT-ELEMENTS. 
 
 But some of those kinds of matter which are 
 not-elements, which have been separated into 
 unlike portions, are perfectly definite homo- 
 geneous bodies that cannot be sorted out into 
 their dissimilar constituents ; and other kinds of 
 
48 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 matter belonging to the class not-elements are 
 mixtures of homogeneous bodies, and they can 
 be sorted out, or disentangled, into their dis- 
 similar constituents. We now express our 
 scheme of classification in this way; 
 
 ELEMENTS NOT-ELEMENTS. 
 
 Mixtures Compounds 
 
 It is evident that the same result is arrived at 
 by the following arrangement of all material 
 substances. 
 
 MIXTURES NOT-MIXTURES 
 
 Elements Compounds 
 
 Chemistry deals with certain changes that 
 take place among Elements and Compounds ; 
 and these changes are those in which both com- 
 position and properties are affected. 
 
 Turn back for a moment to the illustration, 
 used in last chapter, of the game with a box 
 containing a great many different substances 
 (see p. 26). Nothing is to be brought into, or 
 to be taken out of, the box ; but the contents of 
 any special parcel in the box may be changed. 
 One of the players examines the contents of a 
 parcel ; the other player then takes something 
 out of the parcel, or puts something into it, or 
 does both of these acts, or does not change the 
 contents of the parcel ; and the first player has 
 to find out which of the four possible moves has 
 been made by his opponent. All the substances 
 in the chemist's box are either elements or com- 
 pounds : as his box is the earth, or a portion of 
 the earth, it contains a vast number of things 
 
CHEMICAL CLASSIFICATION OF MATTER. 49 
 
 which are mixtures of elements or compounds; 
 but before the game really begins we must sup- 
 pose that these mixtures have been sorted into 
 the compounds, and in some cases into the 
 elements, of which they are composed. The 
 first thing to be done by the chemist would be 
 to determine the quantity of each individual 
 element in the box, and also in the selected 
 parcel ; and the next thing would be to resolve 
 every compound in the box and every compound 
 in the parcel, into its elements, and to determine 
 the quantity of each one of the various elements 
 that could be obtained from a definite quantity 
 of each particular compound. To do these things 
 would involve the constant and accurate use of 
 the balance. Having done these things the 
 chemist would know the quantity of each element 
 that was present, uncombined with other elements, 
 in the box, and in the parcel ; and he would also 
 know the quantity, and the quantitative com- 
 position in terms of elements, of every com- 
 pound in the box and in the parcel. When the 
 opponent had made his move, the chemist would 
 again determine the quantity of each uncombined 
 element, and the quantity of each compound in 
 the parcel, and in the box ; and he would follow 
 this by making a determination of the quanti- 
 tative composition of every compound, and ex- 
 pressing the results in terms of the elements 
 that compose the compounds. He would then 
 be able to state exactly what the opponent's 
 action had been. He might say, for instance, 
 " Here are the contents of the parcel before, and 
 after, your play." (See over page.) 
 D 
 
50 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 81 
 
 
 
 w^ 
 
 irHco 
 
 1 
 
 r 
 
 CO 
 
 
 B g .5 
 ml g 
 
 I 
 
CHEMICAL CLASSIFICATION OF MATTER. 51 
 
 " These results show that you have rearranged 
 the combinations of some of the elements ; you 
 have caused element A to enter into combina- 
 tion with other elements that were present 
 originally in compounds which you have split 
 up ; you have taken the element D away from 
 those elements with which it was combined; 
 and you have brought a new element, H, into 
 one of the new compounds that have been 
 made. It is evident that the contents of the 
 box outside the parcel have been changed, 
 by removing 19 '59 5 grains of the element H, 
 and bringing that inside the parcel. Now I 
 know that there was none of that element 
 H uncombined with other elements in the 
 box; therefore, you have broken up some com- 
 pound or compounds of H. What you have 
 done I can find out, exactly, by an examination 
 of the contents of the box similar to the exam- 
 ination I have made of the contents of the 
 parcel." 
 
 This illustration conveys some notion of the 
 kind of work chemists have done, and are do- 
 ing, in elucidating the compositions of material 
 things. The labour is very great, and it is made 
 a little tedious by the sameness of many of the 
 operations. But new difficulties are met at 
 every step ; and the efforts and ingenuity that 
 are needed to overcome these difficulties give 
 freshness and interest to this part of the work 
 of chemists. It should be remembered that the 
 study of composition is only a part of the study 
 of chemistry; for the object of chemistry is 
 to connect definite changes of composition with 
 
52 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 definite changes of properties ; and to generalise 
 the results of that twofold study in statements 
 which express the essential characteristics of 
 every case of chemical change, in statements, 
 that is to sa}^ which are general laws. 
 
 About seventy different kinds of matter are 
 known at present, no one of which has been 
 separated into unlike parts by any process to 
 which it has been subjected; these are the 
 elements by the combination of which an in- 
 numerable array of compounds is formed in 
 nature and in the laboratory. Considering that 
 the list of elements is much larger to-day than 
 it was thirty years ago, it is almost certain that 
 new elements will be discovered as the instru- 
 ments of chemical research become keener and 
 more delicate. But at present it may be said 
 that the compounds which we find in rocks, 
 soils, and waters, in living things, in the air, 
 and generally, in or on the earth, are formed 
 by the union of two or more of these seventy 
 elements. And by bringing together elements 
 and compounds in the laboratory under con- 
 ditions different from those which prevail in 
 ordinary nature, the chemist has built up thou- 
 sands of compounds the study of which has 
 thrown floods of light on the problems of what 
 may perhaps be called natural chemistry. 
 
 We cannot say of the elements that they 
 are unchangeable ; for we know that some sub- 
 stances which the chemists of half a century 
 ago called elements, because they could not 
 get out of them anything different from them- 
 selves, are now classed among compounds, be- 
 cause more powerful instruments of research 
 
CHEMICAL CLASSIFICATION OF MATTER 53 
 
 have enabled the chemists of to day to separate 
 these substances into parts, each unlike the 
 original, and one unlike another. A similar 
 fate may befall the kinds of matter we call 
 elements to-day. But until a homogeneous body 
 has been disintegrated into dissimilar portions it 
 must remain in the list of elements. Although 
 we cannot apply the expression unchangeable 
 to the elements, yet we may speak of them 
 as unchanging] for, so far as our experimental 
 evidence goes, none of them is ever broken or 
 shattered. They are the building stones of the 
 chemical architect ; the stones wherewith he 
 fashions many strange edifices that are totally 
 unlike the materials used in their erection. 
 You do not see the individual stones in the 
 structures which the chemist builds; because 
 the buildings are so unlike what you would 
 expect from looking at the unbuilt materials, 
 and because the joints are so delicately finished, 
 and the stones so truly fitted, that your eyesight 
 is not keen enough to detect the separating 
 lines. But all the buildings can be taken to 
 pieces; and when this is done the original 
 building stones are recovered absolutely un- 
 changed. 
 
 In chapter I. I quoted from an alchemist who 
 said; " There abides in nature a certain form 
 of matter which, being discovered and brought 
 by art to perfection, converts to itself, propor- 
 tionally, all imperfect bodies that it touches." 
 I shall conclude this chapter by quoting from 
 a great living chemist. " In chemistry we 
 recognise how changes take place in combina- 
 tions of the unchanging." 
 
CHAPTER IV. 
 
 THE COMPOSITION OF HOMOGENEOUS BODIES. 
 
 CHEMICAL changes are those wherein elements 
 combine with elements to form compounds; or 
 elements interact with compounds, or compounds 
 react with other compounds, to produce new 
 arrangements of elements. In every one of 
 these changes, in every material change indeed 
 whether it be chemical or not, one thing remains 
 unchanged ; the total quantity of matter that 
 is concerned in the change is the same at the 
 end, as it was at the beginning, of the process. 
 When sulphur is burnt in a stream of air, and 
 the gaseous oxide of sulphur that is produced 
 is brought into contact with a sufficient quantity 
 of another gaseous oxide called nitrogen dioxide, 
 and a sufficient quantity of vapour of water, and 
 the acid liquid formed by these processes is 
 evaporated and then frozen, the quantity of 
 pure sulphuric acid that is obtained is greater 
 than the quantity of sulphur that was burnt; 
 but it is greater by exactly the sum of the 
 quantities of oxygen and hydrogen that have 
 been removed from the nitrogen dioxide and 
 the steam which took part in the completed 
 change. Other compounds besides sulphuric acid 
 are formed in this chemical process ; and the sum 
 of the weights of all these compounds is exactly 
 equal to the sum of the weights of the elements 
 sulphur and oxygen and the various compounds 
 that reacted to produce the new compounds. 
 54 
 
THE COMPOSITION OF HOMOGENEOUS BODIES. 55 
 
 When salt is heated with sulphuric acid, and 
 the salt cake that is one of the products of 
 this reaction is mixed with coke and lime and 
 strongly heated, and the solid product so ob- 
 tained is lixiviated with water and evaporated, 
 crystals of washing soda are produced. Various 
 other compounds besides washing soda are 
 formed in this cycle of changes ; and the sum 
 of the weights of all the compounds that are 
 produced is exactly equal to the sum of the 
 weights of the elements and compounds that 
 have interacted to produce them. 
 
 The law of the conservation of mass is the name 
 given to the statement that the total quantity of 
 matter or, as it is generally said, the total mass 
 of matter concerned in any material change is 
 the same at the end as it was at the beginning 
 of the change. This law is the foundation of 
 the science of chemistry. The recognition of 
 this law has changed chemistry from an interest- 
 ing recreation into an exact, and much more 
 interesting, science. The law is one of the out- 
 comes of innumerable observations which, carried 
 on for centuries, have become gradually more 
 and more accurate. The truth of the law of the 
 conservation of mass is assumed to-day in every 
 chemical inquiry ; and the assumption is justified 
 by the results of every chemical inquiry. What- 
 ever new compounds are produced by rearrang- 
 ing collocations of elements, in whatever startling 
 and unlocked for ways the properties of the new 
 compounds differ from those of the compounds, 
 or the elements, which reacted to produce them, 
 the total quantity of matter in the products of 
 
56 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 these rearrangements is always found to be 
 exactly equal to the total quantity of matter in 
 the material system before the rearrangements 
 began. 
 
 It might be urged that the fact that chemical 
 change is almost invariably accompanied by 
 change of weight negatives the statement which 
 has just been asserted to be a natural law. For 
 instance, when iron rusts, the rust weighs more 
 than the iron did ; when sugar is heated, the 
 charred sugar weighs less than the sugar weighed 
 before it was heated ; less than a ton of pig-iron 
 is obtained from a ton of ironstone ; and so on. 
 But in every one of these changes something is 
 concerned besides the things which have been 
 weighed : iron rusts by taking oxygen from the 
 surrounding air, and rust is composed of iron 
 and oxygen ; some of the products of heating 
 sugar escape as gases ; when ironstone is reduced 
 in the blast-furnace some of the constituents of 
 the ore react with the fuel to form gases which 
 escape, and some enter into union with the lime 
 that is thrown into the furnace and form slag. 
 If every substance that takes part in a chemical 
 change is weighed, and everything that is pro- 
 duced in the change is weighed, then it is always 
 accurately true that no change occurs in the total 
 weight of the matter concerned. 
 
 We cannot destroy, nor can we create, a single 
 particle of matter ; we can only alter the distri- 
 bution of the various kinds of matter. 
 
 What kinds of things are those seventy elements 
 which can be put together so as to produce a 
 vast array of different compounds? All the 
 
THE COMPOSITION OF HOMOGENEOUS BODIES. 57 
 
 metals are elementary substances ; among the 
 most common we have copper, gold, iron, lead, 
 mercury, silver, tin, zinc, and aluminium which 
 is now coming into use; among rarer metals 
 may be mentioned arsenic, antimony, bismuth, 
 cobalt, nickel, and platinum ; and there are some 
 forty other elements which belong to the class 
 of metals. The four fairly common substances, 
 carbon, iodine, phosphorus, and sulphur, are 
 elements; the purest form of carbon found native 
 is the diamond. Seven elements are gases under 
 ordinary conditions of temperature and pressure. 
 Five of the seven are colourless and odourless : 
 one of them, hydrogen, is inflammable, and it is 
 the lightest kind of matter known to us ; another, 
 oxygen, is a gas in which any burning body burns 
 brilliantly and rapidly ; the third, nitrogen, is 
 neither inflammable nor a supporter of combus- 
 tion, it is sluggish and combines with other 
 elements only when greatly provoked ; the other 
 two colourless elementary gases, argon and 
 helium, were discovered but the other day; 
 these persistently refuse to combine with any 
 other kinds of matter. Two of the gaseous 
 elements are yellowish green ; they are chlorine, 
 which smells abominably, and fluorine, which is 
 the most eager of all the elements to enter into 
 union with others. Two elements are liquids at 
 ordinary temperatures ; these are mercury (one 
 of the metals), and bromine which is a red liquid 
 with a smell even more disagreeable and more 
 irritating than the smell of chlorine. Three 
 bodies which are solids complete the list of 
 elements known to-day : boron is an element 
 
58 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 separated from borax ; silicon is the characteristic 
 element of the clays ; and selenion is chemically 
 like sulphur. 
 
 Seven metals seem to have been known to the 
 ancients ; namely, copper, gold, iron, lead, silver, 
 tin, and zinc ; sulphur has also been known since 
 early times. The alchemists were acquainted 
 with antimony, arsenic, bismuth, and mercury ; 
 phosphorus was discovered towards the end of 
 the seventeenth century. The other elements 
 have been discovered within about the last 120 
 years, and most of them in the present century. 
 Great activity in separating new elements was 
 shown by Sir Humphry Davy in the early years 
 of this century; from 1808 to 1810 he iso- 
 lated half-a-dozen new metals. In the twenties 
 of the century five or six new elements were 
 isolated by the great Swedish chemist Berzelius ; 
 eight or nine were discovered between 1845 and 
 1865. About ten new elements have been added 
 to the list since 1865 ; four in the last ten years, 
 and two of these since the end of 1894. 
 
 The names of some elements have been given 
 to express a characteristic property of those 
 elements. For instance : hydrogen, the water- 
 producer; oxygen, the acid-producer; bromine, 
 the evil-smelling substance ; chlorine, the yellow 
 one; phosphorus, the light-bearer. The names 
 of other elements have been derived from the 
 names of the minerals from which those elements 
 were obtained, or from the names of the places 
 where these minerals were found. For instance : 
 beryllium, obtained from beryll; ytterbium, 
 separated from a mineral found at Yetterby, 
 
THE COMPOSITION OF HOMOGENEOUS BODIES. 59 
 
 in Sweden ; boron, the characteristic element of 
 borax. Two elements have been named to com- 
 memorate their discoveries by a Frenchman and 
 a German respectively ; these elements are gal- 
 lium and germanium. The names of several 
 elements are purely fanciful : for instance ; tel- 
 lurium and selenion, the earth and the moon ; 
 tantalum and niobium, from Tantalus and Niobe 
 his daughter ; mercury ; vanadium, from a Scan- 
 dinavian deity called Vanadis ; cobalt and nickel, 
 from "Kobold,"a sprite, and " Kupfer -nickel" 
 which was the name given by the mediaeval 
 German miners to a mineral like copper ore from 
 which no copper could be obtained. 
 
 Those sixty-five or seventy distinct kinds of 
 homogeneous bodies, no one of which has been 
 broken up into parts that are unlike one another, 
 or unlike the original substance, are the building 
 stones of the earth, and, so far as we can tell, 
 of the other stars also. Compounds, which are 
 themselves also definite substances, which have 
 definite properties, and the composition of each 
 of which is always exactly the same, are produced 
 by the intimate union of two or more elements ; 
 and mixtures of compounds, with here and there 
 mixtures of elements, or of elements and com- 
 pounds, form the solid crust of the earth, and 
 the waters that rest on this solid crust, and the 
 gaseous envelope that surrounds it, and the living 
 things that move on the earth, or fly in the air, 
 or swim in the waters. The number of compounds 
 that might be produced by combining different 
 quantities of two, three, four, or a larger number 
 of the elements is infinite. It has been calcu- 
 
60 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 lated that if the whole population of the world 
 were to devote their days and nights to dealing 
 cards for hands at whist, they would not have 
 exhausted one one-hundred-thousandth part of 
 the possible deals after occupying themselves 
 entirely at the task for one hundred million 
 years. The chemist's pack contains about seventy 
 cards ; how many deals will be made before the 
 combinations are exhausted'? To make all the 
 possible combinations would surely "exhaust 
 time and encroach upon eternity." 
 
 The study of chemistry is certainly not lacking 
 in variety. 
 
 Ke-arrangements of elements are taking place 
 every moment in our own bodies, and in animate 
 and inanimate objects outside ourselves. Every 
 breath of air drawn into the system supplies 
 oxygen, which interacts with compounds of 
 carbon, hydrogen, oxygen, and nitrogen in the 
 body to produce new compounds of these four 
 elements ; some of these new compounds react 
 with other compounds in the system to form 
 more complex, but perfectly definite, collocations 
 of elements, and some are removed from the 
 system in the breath we expire, and in other 
 ways, It is literally true that in the millionth 
 fraction of every second millions of millions 
 of living organisms are breaking down definite 
 groups of definite quantities of the four elements 
 carbon, hydrogen, oxygen, and nitrogen, and as 
 innumerable an array of living things is building 
 up definite arrangements of fixed quantities of 
 these same elements. To the sound of the 
 going of those silent changes the ear of the man 
 
THE COMPOSITION OF HOMOGENEOUS BODIES. 61 
 
 of science is attuned to listen ; and the sounds 
 are harmonious, and there is rhythm through 
 them all. The history of changes like those 
 infinitely minute ones which are conducted on a 
 vast scale can be traced, in a broad and general 
 way, in the crust of the earth. The general 
 tendency of the chemical changes that have 
 taken place as the earth has grown older has 
 been towards the formation of more complex, 
 from less complex, compounds. This process 
 seems to have been carried to completion, or 
 nearly to completion, in the moon. There, chemi- 
 cal action appears to have stopped, or, at anyrate, 
 to have become very sluggish ; there, the elements 
 have probably all combined into complex collo- 
 cations ; and the cessation of chemical changes 
 seems to be accompanied by the absence of life. 
 
 Whether the manifold transmutations which 
 take place in the combinations of the elements 
 occur in the earth's crust, or in living things 
 on the earth, or whether they are brought 
 about by human agents, on a small scale in 
 the laboratory, or on a large scale in manu- 
 facturing industries, all these changes follow 
 certain general lines, and the results of all of 
 them can be expressed, so far as the composi- 
 tions of the thousands of compounds are con- 
 cerned, in general statements which we call laws. 
 
 One law has been stated already ; the law of 
 the conservation of mass. This law in no 
 material change, or cycle of changes, is there 
 any change in the total quantity of matter 
 concerned in the operations is taken as true 
 in all chemical and physical investigations. 
 
62 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 I have compared the processes whereby com- 
 pounds are produced to building operations, the 
 elements being the building stones and the com- 
 pounds the finished buildings. The illustration 
 may be carried further. Suppose that a chemical 
 builder is trying to construct edifices with certain 
 elements. He soon discovers that he must work 
 under limitations. He cannot use any quantities 
 he chooses of the different elements ; the blocks 
 of each building stone with which alone he can 
 build are all exactly the same weight, and there 
 is a fixed and unalterable relation between the 
 weights of those pieces of the different building 
 stones wherewith he is allowed to construct his 
 buildings. It is as if a builder found each 
 variety of stone he intended to use cut into 
 blocks, any one of which weighed exactly the 
 same as any other, but the blocks of one kind of 
 stone weighed less, or more, than those of any 
 other kind ; and as if, when the builder wished 
 to use a portion of a block he found that not 
 one of the blocks of any variety of stone could 
 be broken. The chemical builder must use 
 whole blocks of each elementary building stone ; 
 if one block of this or that element is not enough, 
 he may try two, or three, or any number of, 
 blocks; but he cannot divide even a single 
 block into pieces, nor can he find even one 
 block of any element which differs by the 
 fraction of a grain in weight from the other 
 blocks of the same element. Suppose the 
 chemical buildings are constructed from the 
 three elements hydrogen, carbon, and oxygen : 
 a vast number of different compounds has been 
 
THE COMPOSITION OF HOMOGENEOUS BODIES. 63 
 
 produced by changes in the combinations of 
 these three unchanging things; but the quan- 
 tities of the three elements that are combined 
 in any one of these compounds are always 
 related to one another in the proportion of 
 one part by weight of hydrogen to twelve parts 
 by weight of carbon to sixteen parts by weight 
 of oxygen, or in the proportion of whole mul- 
 tiples of these three numbers (I am using round 
 numbers to obviate very small decimals ; the 
 more correct numbers are 1 : 11'97 : 15-88). 
 
 Suppose the chemical builder gets annoyed at 
 these limitations. He heaps together large, 
 indefinite, masses of the three elements he is 
 working with ; and subjects the mixture to the 
 action of very energetic reagents ; for he is 
 determined to bring in art to correct nature. 
 But his art is a part of nature, and nature wins. 
 Suppose a dozen compounds of the three elements 
 are produced ; the weights of hydrogen, carbon, 
 and oxygen in each compound are still related 
 to one another in the proportion of 1 : 12 : 16, or 
 in a proportion expressible by whole multiples 
 of these numbers. Suppose the result of the 
 impatient chemist's manipulations has been a 
 mixture of several compounds of hydrogen, 
 carbon, and oxygen, with two compounds of 
 hydrogen and oxygen, two of carbon and 
 oxygen, and half-a-dozen compounds of hydrogen 
 and carbon : every compound of the three 
 elements contains weights of hydrogen, carbon, 
 and oxygen related as represented by the 
 expression n 1: m 12 : p 16, when n, m, and 
 p are whole numbers (generally small whole 
 
64 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 numbers varying from 1 to about 8) ; each of the 
 two compounds of hydrogen and oxygen contains 
 these elements united in the ratio of one part 
 by weight of hydrogen to sixteen parts by 
 weight of oxygen, or of a whole multiple of 
 one part by weight of hydrogen to a whole 
 multiple of sixteen parts by weight of oxygen ; 
 the ratio of the weights of carbon and oxygen 
 in both the compounds that have been formed 
 of these two elements is that of 12 to 16, 
 or of a whole multiple of 12 to a whole multiple 
 of 16 ; and the weights of hydrogen and carbon 
 in each of the several compounds of these two 
 elements that have been formed are related to 
 one another in the ratio n 1 : ml 2, where n and 
 m are again whole numbers. Some portions of 
 the elements which the eager experimenter 
 threw into his mixing pot may remain uncom- 
 bined, and some portions of them may be 
 driven away from the other portions and sent 
 wandering through space. 
 
 The impetuous chemical builder finds that 
 his edifices cannot be constructed entirely ac- 
 cording to his own fancy ; he must work under 
 the limitations imposed on him by the nature of 
 his materials. 
 
 The illustrations given in the preceding para- 
 graphs contain in themselves all the laws of 
 chemical combination by mass ; but it may be 
 well to express these laws in more precise form. 
 Those statements regarding the quantities of 
 elements that react chemically which express 
 what holds good in all chemical processes are 
 generally divided into three parts; one part is 
 
THE COMPOSITION OF HOMOGENEOUS BODIES. 65 
 
 called the law of fixity of composition, another the 
 law of multiple proportions, and the third the law of 
 reciprocal proportions. It is better however for 
 our purpose to express these three laws in one 
 statement, and to word that statement so that 
 it may include what is universally true regarding 
 the quantities of compounds, as well as regard- 
 ing the quantities of elements, that interact 
 chemically. 
 
 The quantity by weight of each element which 
 takes part in any chemical reaction between elements, 
 or between elements, and combinations of elements, can 
 be expressed by a certain fixed number, or by a whole 
 multiple of that number. 
 
 And the quantity by weight of each compound 
 which takes part in any chemical reaction between 
 compounds, or between elements and compounds, can 
 be expressed by a certain fixed number, or by a whole 
 multiple of that number. 
 
 This fundamental law of chemical combination 
 may be put in other words, as follows. 
 
 To every element there is attached a fixed num- 
 ber ; and ,these fixed numbers express the smallest 
 quantities by weight of the elements that interact 
 chemically. If the quantities by weight of the 
 elements that react in any special case are not 
 expressed by the fixed numbers of those elements, 
 then the reacting quantities are expressed by whole 
 multiples of those fixed numbers. And what is 
 true of elements holds good also for compounds. 
 The quantities by weight of all compounds that 
 interact chemically are expressed by certain fixed 
 numbers ; and if more than this quantity of any 
 compound reacts, then the reacting quantity by 
 E 
 
66 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 weight is twice, or three times, or four times, 
 or to put it generally, a whole number of times, 
 more than the quantity exuressed by the fixed 
 number of that compound. 
 
 The law which has been stated is generally 
 called the law of combining weights, in so far as it 
 applies to elements ; because it is customary to 
 speak of the quantities by weight of elements 
 which, or whole multiples of which, take part in 
 chemical changes, that is, the quantities by weight 
 that are expressed by what I have called the 
 fixed numbers attached to the elements it is cus- 
 tomary to speak of these quantities as the combin- 
 ing weights of the elements. If this expression is 
 used the statement of the law as far as elements 
 are concerned may be put in fewer words : for it 
 will run thus : 
 
 Elements read in the ratios of their combining 
 weights, or of whole multiples of their combining weights. 
 
 Inasmuch as the quantity by weight of a com- 
 pound that is expressed by the fixed number 
 attached to that compound that is, the quantity 
 which, or a whole multiple of which, reacts chemi- 
 cally with other compounds or with elements is 
 often referred to as the reacting weight of the com- 
 pound, the law which has been stated may be 
 called the law of reacting weights, in so far as it 
 applies to compounds ; and with the help of the 
 term * reacting weight' the expression of the law 
 as far as compounds are concerned may be put in 
 fewer words, as follows : Compounds react in the 
 ratios of their reacting weights or of whole multiples 
 of their reacting weights. 
 
 The system of shorthand used by chemists is 
 
THE COMPOSITION OF HOMOGENEOUS BODIES. 67 
 
 based on the law of combining and reacting 
 weights. Instead of always writing the name of 
 an element, the first letter of the name, or the 
 first and another letter, is used as a symbol for 
 that element ; thus C stands for carbon, Cr for 
 chromium, B for boron, Be for beryllium, and so 
 on. The symbols of seven of the metals that 
 have been known for many centuries are derived 
 from the Latin names of these metals ; Sb = 
 antimony (stibium), Cu = copper (cuprum), Fe = 
 iron ( ferrum), Pb = lead (plumbum), Hg = mercury 
 (hydrargyrum), Ag = silver (argentum), and Sn = 
 tin (stannum). The symbols K and Na are used 
 for the metals potassium and sodium, respectively. 
 The symbol K seems to be derived from the word 
 al-kali (the ashes) applied for many centuries to 
 the ashes of plants, from which substances the 
 metal was obtained; (these ashes were also called 
 pot-ashes, hence the name given to the metal). 
 The symbol Na is probably derived from the 
 word natron, which was applied in the Middle 
 Ages to what seems to have been impure soda. 
 In this case, as in that of potassium, the name (at 
 least the English name) of the element has been 
 derived from the name commonly given to the 
 substance from which the element was isolated, 
 and the symbol has been taken from an older 
 name for the same substance. 
 
 Each symbol has a quantitative signification : 
 thus C represents one combining weight of carbon, 
 that is 12 parts by weight of carbon; Fe means 
 56 parts by weight of iron, that is one combining 
 weight of iron ; and so on. The compositions of 
 compounds, both the qualitative and the quanti- 
 
68 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 tative compositions, are expressed by writing side 
 by side the symbols of the elements that compose 
 the compounds, and attaching small figures to 
 these symbols to indicate how many combining 
 weights of each element are combined in that 
 weight of the compound which is expressed by the 
 formula ; it is customary to speak of the formula 
 of a compound and the symbol of an element. For 
 instance, water is composed of hydrogen and 
 oxygen, and always of weights of these elements 
 united in the ratio of 1 : 8. ^"ow the combining 
 weight of hydrogen is one (it is the standard 
 substance to which other combining weights are 
 referred), and the combining weight of oxygen is 
 16 ; the symbol H means one part by weight of 
 hydrogen, and the symbol means 16 parts by 
 weight of oxygen. The composition of water then 
 is expressed .by the formula H 2 0. This formula 
 tells that the compound represented is composed 
 of hydrogen and oxygen, and that 18 parts by 
 weight of the compound are composed of 2 parts 
 by weight of hydrogen and 16 parts by weight 
 of oxygen. Take the formula for sulphuric acid, 
 H 2 S0 4 . If a table of combining weights is con- 
 sulted it is found that the combining weights of 
 sulphur and oxygen are 32 and 16 respectively, 
 that of hydrogen being unity. Now (1 x 2) + 32 
 + (16 x 4) = 98 ; hence we are told that 98 parts 
 by weight of sulphuric acid are composed of two 
 parts by weight of hydrogen, 32 parts by weight 
 of sulphur, and 64 parts by weight of oxygen. 
 One more instance. The formula which expresses 
 the composition of cane sugar is C 12 H 22 O n . In- 
 asmuch as 0= 12, H = l, and O = 16 (that is, 
 
THE COMPOSITION OF HOMOGENEOUS BODIES, 69 
 
 the combining weights of the elements carbon, 
 hydrogen, and oxygen are 12, 1, and 16, respec- 
 tively), it is easy to calculate the weight of cane 
 sugar represented by the formula ; this weight is 
 evidently (12x12) + (22x1) + (11x16) = 3*2. 
 The formula expresses the composition of cane 
 sugar in terms of the elements of which this 
 compound is composed, for it tells that 342 parts 
 by weight of cane sugar are composed of 144 
 parts by weight of carbon, 22 parts by weight of 
 hydrogen, and 176 parts by weight of oxygen. 
 It is of course easy to calculate the percentage 
 composition of sugar, if it is desired to state the 
 quantities of the three elements that compose it 
 in that form. 
 
 The law of combining and reacting weights 
 is the most important outcome of the investiga- 
 tion of the compositions of homogeneous bodies, 
 considered apart from the examination of the 
 properties of these bodies. To John Dalton 
 belongs the signal honour of being the first to 
 enunciate this fundamental law of chemistry. 
 And it is most interesting to know that Dalton 
 formulated the law (in the earliest years of this 
 century) as a necessary deduction from the results 
 of inquiries he was making into the physical 
 properties of gases. He formed a theory of the 
 constitution of matter the Daltonian atomic 
 theory, which will be discussed briefly in a later 
 part of this book (see Chapter VIII.) to account 
 for some of the physical properties of gases $ he 
 then deduced the law of the composition of homo- 
 geneous bodies from the postulates of that theory, 
 stating the law in the language of the theory ; 
 
70 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 and then he performed a few experiments, and 
 obtained results in keeping with those demanded 
 by the theoretical deduction he had made. When 
 the law had been announced, and its accuracy 
 tested in a few cases by experiment, chemists 
 soon recognised that a flood of light was thrown 
 by the law on all the questions of composition 
 with which they were especially concerned. 
 Order and meaning were introduced into the 
 accumulations of figures which represented the 
 results of the analyses of compounds. It was 
 discovered that many experimental verifications 
 of the law had been made before the law was 
 known, and that sufficient analytical results had 
 been amassed to place the law on a fairly firm 
 experimental basis. 
 
 The methods of quantitative analyses have 
 become much more accurate since Dal ton's time ; 
 and the results of all the hundreds of thousands 
 of carefully executed analyses of compounds made 
 since then have proved that the law of combining 
 and reacting weights holds good accurately, and 
 without any limitations or modifications, in every 
 special instance of change in the compositions of 
 systems of homogeneous bodies. This law is 
 undoubtedly a law of nature. 
 
 The fact that the quantity by weight of an 
 element, or of a compound, which takes part in 
 any chemical change is always expressible by a 
 certain fixed number, or by a whole multiple of 
 that number, serves to emphasize, and to make 
 more definite, the difference between elements 
 and compounds on the one hand, and mixtures 
 on the other hand. The composition of every 
 
THE COMPOSITION OF HOMOGENEOUS feOfclES. 71 
 
 specimen of any compound, stated in terms of the 
 elements that have combined to form it, is always 
 exactly the same ; and although a pair of elements 
 may combine to form more than a single com- 
 pound, yet, not only is the composition of each 
 compound different from that of any other, 
 and not only is the composition of each com- 
 pound perfectly definite and quite unalter- 
 able, but it is also true that the quantities by 
 weight of each of the two elements in the various 
 compounds formed by the union of these elements 
 are related in such a simple way that it is pos- 
 sible to express all these quantities as whole mul- 
 tiples of one and the same quantity. There is 
 nothing even distantly approaching these facts 
 to be found in the compositions of mixtures. 
 Who shall determine how much sand, how much 
 lime, and how much water must be mixed in 
 order that the mixture may be used as mortar] 
 The most superficial examination of the behaviours 
 of different workmen engaged in making mortar 
 convinces one that the makers of the mortars 
 have no scruples in neglecting the law of com- 
 bining and reacting weights. But neglect it as 
 they like, the law holds good all the same. And 
 it would be better for the builder, and much 
 better for the tenant of the house, if this law, and 
 other laws of chemistry, were consciously recog- 
 nised and intelligently observed. For mortar- 
 making, like nine-tenths of our manufactures, is 
 the carrying out of several chemical changes in a 
 medium that is itself a mixture. Perfectly defi- 
 nite quantities of lime and water combine to form 
 slaked lime, itself a definite chemical compound ; 
 
72 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 and after the mortar is set in the walls, carbonic 
 acid gas is gradually absorbed by it from the air, 
 and, combining with the slaked lime, slowly forms 
 chalk; moreover, processes of chemical change 
 go on very gradually for many years between 
 the silica, which is a definite compound in the 
 sand, and the slaked lime, and these changes pro- 
 duce silicate of lime which helps to bind the 
 mortar into a compact mass. 
 
 Although the distinction between elements and 
 compounds, on the one hand, and mixtures, on 
 the other hand, is very marked, nevertheless the 
 fact remains that most of the chemical changes 
 that occur in our daily life and there is no 
 science that so " comes home to the bosoms and 
 the business of men " as chemistry occur be- 
 tween compounds, or between elements and com- 
 pounds, that are so wrapped up and hidden in 
 enveloping mixtures, that much detailed know- 
 ledge of chemical occurrences, and a considerable 
 share of imagination, are required to remove the 
 outer covering, and to see the essential changes 
 that are taking place inside. 
 
 Now that we have attained a fairly definite 
 conception of certain features of chemical change, 
 let us, for a moment, contrast that conception 
 with the picture which the transmutations of 
 matter presented to the alchemist. The notion 
 of distinct and definite kinds of matter, each with 
 its own characteristic properties, was impossible 
 in alchemical days, for the alchemists had no in- 
 struments for detecting small changes in the 
 quantities of the matter which exhibited changes 
 of properties. And until the changes which take 
 
THE COMPOSITION OF HOMOGENEOUS BODIES. 73 
 
 place in the quantity of any portion of matter 
 that is changing its properties have not only been 
 detected, but also measured, it is not possible to 
 say whether the compositions of portions of 
 matter which show different properties are, or 
 are not, identical. It is probably correct to say 
 that the alchemists could not detect distinct 
 differences in the compositions of portions of 
 matter with different properties, because they had 
 no instrument for determining, with accuracy, 
 differences in the quantities of matter in two, or 
 more, portions of matter ; and it is also probably 
 correct to say that the alchemists could not in- 
 vent an instrument for detecting and measuring 
 differences in the quantities of matter in various 
 portions thereof, because they had no clearly- 
 defined theory of the composition of material 
 things. The construction of accurate balances 
 led to great advances in chemical theory ; and 
 advances in chemical theory demanded, and 
 called forth, great improvements in the art of the 
 balance-maker. The alchemical conception of 
 the transformations of matter was radically 
 wrong, because the alchemists did not attain, 
 one might almost say that on account of their 
 intellectual environment they could not attain, 
 any clear notions regarding the compositions of 
 different portions of matter. Divorced from the 
 study of composition, the study of properties was 
 obliged to base itself on vague analogies. Rest- 
 ing on that foundation, the determination and 
 elucidation of properties seemed to grow apace, 
 but there was no reality in the growth ; the 
 solid buildings, of which the vague alchemical 
 
74 THE STORY Otf THE CHEMICAL ELEMENTS. 
 
 vision was nothing but a mirage, lay far away, 
 and many miles of dreary desert had to be passed 
 before the gates of the city were reached. But a 
 mirage means reality somewhere ; and if the 
 alchemists had not seen the vision, and followed 
 it, we might have been wandering in the desert 
 still. 
 
 Granting that the conception of definite com- 
 position could not be attained in alchemical 
 times, we must surely admire the ingenuity, and 
 applaud the subtlety, of the argument of the 
 alchemists, whereby they thought they had 
 established the possibility of transforming any 
 material thing into any other thing, while at the 
 same time leaving themselves an unchanging 
 resting place amid the phantasmagoria of appear- 
 ances. Matter was regarded by the alche- 
 mists merely as a vehicle for showing forth 
 properties ; they examined and recorded 
 most extraordinary changes of properties ; 
 therefore they concluded that properties could 
 be taken off, or put on to, any kind of 
 matter, like clothes. A king in rags would 
 receive no homage from his subjects, but clothe 
 him in purple and fine raiment and he would 
 again be recognised as the monarch. Lead, and 
 copper, and iron, and tin were but different 
 varieties of rags that hid the regal gold ; but the 
 rags might be removed in the furnace, or the 
 alembic, and the poor naked king might be 
 bathed in the " Water of Paradise," and the 
 royal robes, brought on the wings of the phoenix, 
 a bird of a most deep colour with a shining 
 fiery hue, might be put upon him, and then he 
 would appear in proper state and all would bow 
 
THE COMPOSITION OF HOMOGENEOUS BODIES. 75 
 
 the knee before the king. But the alchemists 
 felt that there must surely be an unchanging 
 substratum beneath the ever- varying phenomena. 
 Why was the thing they called gold sometimes 
 so hidden by false appearances that ordinary 
 people named it iron, or lead, or mercury, or 
 zinc ? The alchemists replied : because the gold 
 has become contaminated, and rendered feeble 
 and decrepit. Now their argument proceeded 
 as impure things shrink from the touch of 
 purity, and imperfect things recoil from things 
 which are perfect, so it will suffice to touch the 
 imperfect gold, that in his base estate is called 
 by some other name, with the One perfect matter, 
 and all the impurities will vanish and the gold 
 will be seen to be gold indeed. This "One 
 perfect matter, 7 ' they said, is immutable ; it is 
 the essence that is hidden under thick coverings 
 of imperfect matter. Some forms of matter 
 will doubtless contain more of it than others, 
 but as we do not know where it is chiefly con- 
 cealed we must search for it everywhere. And 
 the search is worth infinite pains ; for when we 
 have found The one thing, the smallest piece of it 
 will suffice, not only to drag off the unworthy 
 coverings from the regal gold, but also to re- 
 move all the trouble and sorrow from the world, 
 and to transform this earth into a veritable 
 paradise. For the alchemical quest was not 
 merely the quest for gold. At its best it was 
 more noble than that. It was an attempt, mis- 
 taken and hopeless, but not unworthy, to pass 
 at one bound from trouble to peace, from unrest 
 to quietness. In that quest alchemy failed ; that 
 pursuit chemistry has not attempted. 
 
CHAPTER V. 
 
 THE STUDY OF PROPERTIES. 
 
 IN chemistry we have to deal with the changes 
 that take place when elements or compounds 
 interact to produce other elements or other 
 compounds. The composition of an element is 
 expressed by the name of the element, for, so 
 far as our experimental results have extended, 
 any portion of an element is absolutely identical 
 in characteristic properties with any other por- 
 tion of that element. The composition of a 
 compound is expressed in terms of the elements 
 which by their union form the compound, and 
 which can be obtained by breaking up the com- 
 pound. The law of combining and reacting 
 weights generalises the results of the study of 
 composition, and expresses these results in a 
 perfectly accurate form, which is applicable, 
 without limitations or modifications, to every 
 special case. 
 
 We must now turn to the second main divi- 
 sion of chemical inquiry, and try to find out 
 something about the properties of the elements 
 and their compounds. We found that chem- 
 istry omits from the scope of its proper inquiry 
 those kinds of matter that are neither elements 
 nor compounds ; we shall find that the strictly 
 chemical examination of the properties of ele- 
 ments and compounds is limited to those pro- 
 perties which are exhibited when changes of 
 composition take place. Speaking strictly, iron, 
 or tin, or water, or salt, considered apart from 
 
 76 
 
THE STUDY OF PROPERTIES. 77 
 
 other things or from the action of external 
 agencies, presents no properties for the chemist 
 to study ; it is when the iron, or the tin, or the 
 water, or the salt, acts on, and is acted on by, 
 other kinds of matter (or is acted on by an 
 external agency) so as to bring about changes 
 of composition, it is only then that the pro- 
 perties of the iron, tin, water, or salt become 
 interesting to the chemist. Chemistry studies 
 elements and compounds in the act of changing. 
 Inquiring into the properties of an element or 
 compound, the chemist demands : What will it 
 do when it reacts with other bodies ? He must 
 always ask, not only : What is this substance ? 
 but also : What has it been, and what will it be 1 
 Chemistry is interested in this or that definite 
 kind of matter, not so much because of what it 
 is now, as because its present condition is the 
 result of changes in the past, and is the promise 
 of changes in the future. 
 
 It is more correct to speak of the chemical 
 reactions, rather than the chemical properties, of 
 elements and compounds; for the word " reac- 
 tions " emphasises that feature of the properties 
 of these bodies which is essentially chemical. 
 
 As an example of the kind of inquiries that 
 are made into the chemical reactions of com- 
 pounds, let us consider, briefly, some of the 
 reactions of water When hydrogen is burnt in 
 air, or in oxygen, water is produced ; when an 
 electric current is passed through a mixture of 
 water and a little sulphuric acid, hydrogen and 
 oxygen are obtained. When hydrogen is passed 
 over heated red lead, or heated copper scales, or 
 
78 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 heated iron rust, water is formed together with 
 lead, or copper, or iron. These qualitative facts 
 furnish the bases of methods for determining the 
 composition of water; and the results of such 
 determinations are expressed by saying that 
 important reactions of water are that, when 
 hydrogen is burnt in oxygen, or when hydrogen 
 is caused to withdraw oxygen from a metallic 
 oxide, one part by weight of hydrogen combines 
 with eight parts by weight of oxygen to form 
 nine parts by weight of water, and that when 
 water is acted on by an external agency which 
 decomposes the water, nine parts by weight 
 of water always yield one part by weight of 
 hydrogen and eight parts of oxygen. AVhen 
 steam is passed over red-hot iron filings, the 
 steam and the iron interact to produce hydrogen 
 (one part by weight of hydrogen for every nine 
 parts of steam that have reacted) and an oxide 
 of iron ; and steam reacts with several other 
 metals similarly to the way in which it reacts 
 with iron, that is to say, hydrogen and an oxide 
 of the metal are produced. Water reacts at the 
 ordinary temperature with sodium and potas- 
 sium the metals obtained from soda and potash 
 respectively hydrogen is given off, and com- 
 pounds of oxygen with hydrogen and sodium, 
 or hydrogen and potassium, dissolve in the 
 water which has not been used to react with 
 the quantity of sodium or potassium employed. 
 When a mixture of steam and chlorine (the evil- 
 smelling yellowish gas got from common salt) is 
 passed through a porcelain tube kept at bright 
 redness, the steam and chlorine slowly interact 
 
THE STUDY OF PROPERTIES. 79 
 
 to produce oxygen (eight parts by weight for 
 every nine parts of steam that react) and hydro- 
 chloric acid which is dissolved in the excess of 
 steam. When soda crystals are heated, water is 
 given off in the form of steam ; when the residue 
 of calcined soda is dissolved in water and the 
 solution is evaporated somewhat, crystals of 
 washing soda form as the liquid cools. It is a 
 reaction of water to combine with calcined soda 
 to form washing soda ; and it is a reaction of 
 water to be driven out of this combination by 
 heat. When cane sugar is heated, one of the 
 things given off is water ; but the production of 
 this water is accompanied by the complete trans- 
 formation of the cane sugar into other compounds. 
 Water enters into the composition of oil of 
 vitriol, but the properties of the water are 
 entirely hidden in those of the acid, and water 
 cannot be obtained from oil of vitriol except by 
 processes which completely break up the acid. 
 
 As the study of the reactions of water pro- 
 ceeds, we find that very many of these reactions 
 are similar to one or other of those which have 
 been briefly described. These are : (1) reactions 
 wherein water is produced by the combining of 
 hydrogen and oxygen, one or both of these 
 elements being in many cases first removed from 
 combinations with other elements ; (2) reactions 
 wherein water is decomposed, hydrogen coming 
 off, and oxygen remaining combined with the 
 substance which has reacted with the water ; (3) 
 reactions wherein water is decomposed with 
 evolution of oxygen, the hydrogen of the water 
 remaining combined with the substance which 
 
80 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 has taken part in the reaction ; (4) reactions 
 wherein water combines with other compounds, 
 or is obtained from combinations with other com- 
 pounds without the water undergoing decompo- 
 sition, and without the bringing a.bout of any 
 very profound change in the compound where- 
 with the water combines, or from which the 
 water is withdrawn ; and (5) reactions wherein 
 water enters into such intimate chemical union 
 with other compounds that water cannot be 
 obtained again from the products of the reaction 
 without completely breaking up these products ; 
 in these reactions, although neither hydrogen nor 
 oxygen is given off, it is likely that a complete 
 rearrangement of the elements of the water and 
 the elements of the other compound takes place. 
 A short consideration will convince one that 
 many of those reactions which have been asserted 
 to be reactions of water might be described with 
 equal accuracy as reactions of hydrogen ; and 
 that many might be described very correctly as 
 reactions of oxygen. For instance, it is surely 
 a reaction of hydrogen that one grain of it com- 
 bines, under certain conditions, with eight grains 
 of oxygen to produce nine grains of water, and 
 it is a reaction of oxygen that this element com- 
 bines, under proper conditions, with hydrogen, in 
 the ratio of 8 to 1 by weight, to produce water. 
 Again, the facts that hydrogen is obtained when 
 steam is passed over red hot-iron, and that 
 oxygen is produced when a mixture of steam and 
 chlorine is passed slowly through a very hot 
 tube, might be asserted with perfect accuracy to 
 be reactions of hydrogen and oxygen, respec- 
 
THE STUDY OF PROPERTIES. 81 
 
 tively, or these same reactions might be claimed 
 to be reactions of iron on the one hand, and 
 of chlorine on the other hand. 
 
 It is this mariysidedness of the simplest 
 chemical reactions that gives both its great 
 charm and its great difficulty to the study of 
 chemistry. Every reaction must be regarded 
 from several points of view before it can be 
 placed with those reactions that are most like it, 
 and separated from those which do not resemble 
 it. And it must always be remembered that the 
 study of reactions ought to be connected with 
 the study of composition ; that the reactions of 
 every definite substance ought to be compared 
 with those of other homogeneous bodies, and 
 that when elements and compounds have been 
 classified on the basis of their reactions further 
 reasons for the classification should be found by 
 tracing similarities between the compositions of 
 those bodies which exhibit similar reactions. 
 
 Let us take another instance of the study of 
 reactions to illustrate the methods used, and the 
 kind of results obtained, in this department of 
 chemical inquiry. And this time we shall choose 
 an element the element hydrogen. The reactions 
 which are first examined when the chemical 
 properties of an element or a compound are 
 being studied are those wherein the body in 
 question is prepared or isolated. Investigation 
 shows that the colourless, odourless, very light 
 gas which is given off when iron is thrown into 
 diluted oil of vitriol is the same gas which is 
 obtained when zinc, or tin, or one of several 
 other metals, is substituted for iron. Then it is 
 
 F 
 
82 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 found that the same gas can be obtained from 
 water by passing it, in the form of steam, over 
 hot iron, or hot zinc, or hot magnesium, or one 
 of several other metals. Further investigation 
 shows that compounds of the gas in question, 
 from which the gas can be obtained, exist in 
 almost every variety of living things, both 
 animal and vegetable. 
 
 These results suggest further inquiries; this 
 gas is obtainable by the interaction of iron, zinc, 
 or several other metals, with diluted oil of vitriol ; 
 will the gas be produced if some other acid 
 liquid is substituted for oil of vitriol ? Experi- 
 ments shew that almost anyone of those sour, 
 acrid liquids that are commonly called adds may 
 be used instead of oil of vitriol ; thus the same 
 gas is produced by the reaction of iron and con- 
 centrated vinegar, or iron and spirits of salt, or 
 iron and citric acid dissolved in water, as is pro- 
 duced by the reaction of iron and diluted oil of 
 vitriol. Further suggestions arise from these 
 results. An answer is sought, by experiment, to 
 the question : Is this gas always obtained when 
 iron, or zinc, reacts with an acid 1 The common 
 acid called a qua fortis is tried ; and a reddish gas 
 is produced quite different from that obtained 
 when one of the other acids was used. 
 
 The reactions between the light colourless gas 
 and other homogeneous bodies are now studied. 
 The gas is found to be inflammable ; and water 
 is discovered to be the product of the burning. 
 Accurate quantitative experiments would then be 
 required to find out whether water is the only 
 product, or whether other compounds are pro- 
 
THE STUDY OF PROPERTIES. 83 
 
 duced also. As water is formed when the gas is 
 burnt, the name hydrogen (water-producer) is 
 given to the gas. The change that proceeds 
 when hydrogen is burnt to water is then studied 
 in detail and quantitatively ; and when the pro- 
 cess has been elucidated, the suggestion at once 
 presents itself, that if hydrogen were brought 
 into contact with compounds of oxygen with 
 other elements it would probably rob these com- 
 pounds of their oxygen and form water. Experi- 
 ments are tried, and it is found that many com- 
 pounds of oxygen with the metals are deoxidised 
 when heated in hydrogen, with the production 
 of water, and the metal which was before com- 
 bined with oxygen. These results, incidentally, 
 suggest a method for obtaining metals from their 
 compounds with oxygen, and this method is 
 found to be applicable in many cases. 
 
 Attempts are then made to combine hydrogen- 
 with other elements, and results are obtained 
 which suggest an enormous field of inquiry. 
 For instance, hydrogen and chlorine (the badly- 
 smelling gas from common salt) combine easily 
 in the sunshine ; the product is a gas which dis- 
 solves readily in water, producing thereby an 
 exceedingly acid liquid, which is known in daily 
 life as spirits of salt, or sometimes as muriatic acid 
 (the name hydrochloric acid is more descriptive, 
 suggesting its production from hydrogen and 
 chlorine). Then hydrogen and nitrogen, one of 
 the inert gases in the atmosphere, combine with 
 difficulty under- the influence of a succession of 
 electric discharges : but when they have com- 
 bined they produce a gas with a most pene- 
 
84 THE STORY OF THE CHEMICAL CLEMENTS. 
 
 trating smell, which dissolves very easily in 
 water, producing thereby what is commonly 
 known as spirits of hartshorn (because the sub- 
 stance was once made by heating the horns of 
 harts), but is better called ammonia. These 
 two compounds, hydrochloric acid and ammonia, 
 both compounds of hydrogen, are examined 
 and compared with other compounds, and they 
 are found to be representatives of two classes of 
 compounds which are much opposed in their 
 chemical characters ; to these two classes of 
 compounds are given the names adds arid alkalis 
 respectively. Ammonia is an alkali, hydrochloric 
 acid is an acid. 
 
 These results lead to a study of the reactions 
 and the compositions of a number of compounds 
 which, like hydrochloric acid, are sour, which 
 change the hues of various colouring matters, 
 which burn the skin and corrode cotton, wool, 
 wood, and the like ; all these compounds are 
 found to be compounds of hydrogen. Attention 
 is turned to the alkalis, potash and soda, slaked 
 lime and baryta, all of which have properties 
 resembling those of ammonia ; these compounds 
 are found to be also compounds of hydrogen. 
 Then the chemist naturally inquires what change 
 takes place when I mix an acid and an alkali ? 
 He tries, and, after much laborious quantitative 
 work, he finds that a compound is produced 
 which has the properties neither of the acid nor 
 of the alkali ; the compound is a mild, neutral 
 substance, unlike the burning acid, and different 
 from the caustic alkali which corroded the flesh 
 so easily. The study of the composition of a 
 
THE STUDY OF PROPERTIES. 85 
 
 number of the compounds produced by what is 
 called neutralising acids by alkalis shew that in 
 many cases these compounds which are known 
 as salts do not contain hydrogen, and that if 
 hydrogen is one of the components of a salt, 
 there is always less hydrogen than was contained 
 in the acid from which the salt was produced. 
 What, then, has become of some part, or in cer- 
 tain cases of all, of the hydrogen of the acid ] 
 Experiments shew that in almost every case 
 water has been produced when an acid has been 
 neutralised by an alkali to form a salt. Then it 
 is remembered that hydrogen was given off when 
 iron, or zinc, or other metal reacted with certain 
 acids; it is now necessary to find out what 
 becomes of the iron or the other metal which 
 disappears in the acid. The results of a great 
 many quantitative experiments is to prove that 
 the metal combines with the elements of the acid, 
 except the hydrogen, or part of the hydrogen, 
 and that the compound so formed is a salt. 
 
 But if both acids and alkalis are compounds of 
 hydrogen, why are some of these compounds so 
 very different in their properties from the others ? 
 Acids and alkalis are opposed most markedly in 
 their chemical reactions. Why are acids acidic ? 
 Why are alkalis alkaline ? To such wide ques- 
 tions as these an examination of the reactions of 
 hydrogen has led us. 
 
 The answer given in the older days to such a 
 question as that Why are acids acidic ? was, 
 Acids are acidic because they all contain more or 
 less of the principle of acidity. The form of the 
 answer was sometimes varied a little, and all 
 
86 THE STORY OF THE- CHEMICAL ELEMENTS. 
 
 acids were declared to have in them some portion 
 of the primordial acid. But a principle of acidity, 
 and a primordial acid, are phrases, neither of 
 which helps to advance accurate knowledge. 
 Alter seeking for many centuries to express the 
 properties of similar particular kinds of matter 
 by assigning to them all the possession of a 
 common principle, or essence, naturalists have 
 found that nothing comes of this method of 
 classifying, that real knowledge is not advanced 
 by it, that, indeed, when a man has loudly 
 asserted the existence of a principle of acidity 
 in certain bodies, and a principle of alkalinity 
 in others, he is exactly where he was before. 
 The attempts which the chemist makes to find 
 the cause of the acidity of acids and of the 
 alkalinity of alkalis take a different form. The 
 chemist asks What is the composition of those 
 compounds which have certain reactions in 
 common, and which we call acids ? What is the 
 composition of those other compounds we call 
 alkalis, which also have certain common reactions 1 
 The only answers chemistry can give to such 
 questions as those we are asking now are answers 
 which shew the connexions between properties 
 and composition. Chemistry ignores the more 
 fine-sounding questions that begin with Why? 
 and contents herself with trying to find answers 
 to those that begin with How? And the first 
 result of the determination of the compositions 
 of a number of acids is that the acids are all 
 found to be compounds of hydrogen, and that 
 this is the only element which is common to 
 them all The next question to be answered by 
 
THE STUDY OF PROPERTIES. 87 
 
 experiment evidently is this : Are all compounds 
 of hydrogen acids 1 A short investigation suf- 
 fices to answer this question in the negative; it 
 is found, for instance, that alkalis the antipodes 
 of acids are compounds of hydrogen. Evi- 
 dently, then, whether a compound of hydrogen 
 shall or shall not be an acid must be connected 
 with the other elements wherewith the hydrogen 
 is combined. Here is a rich field of research. 
 I cannot tell of the diggings and toilings in this 
 field, of the controversies between rival labourers, 
 of the hopes and disappointments ; I must notice 
 only the general result. When hydrogen is com- 
 bined with a relatively large quantity of one or 
 more oxygen-like elements, of which oxygen 
 itself may or may not be one, the compound is 
 an acid. By "oxygen-like elements " are meant 
 elements which broadly resemble oxygen in their 
 typical chemical reactions ; the most common of 
 these elements are chlorine, bromine, iodine, 
 nitrogen, sulphur, and phosphorus. Of course a 
 large amount of accurate study would be required 
 before one could attach a definite meaning to 
 the phrases I have used " oxygen-like elements," 
 or " elements which broadly resemble oxygen in 
 their typical chemical reactions/ But these 
 phrases present subjects for definite inquiry, 
 they are science-producing expressions; and in 
 this respect they are in complete contrast with 
 such alchemical phrases as the principle of acidity 
 or the primordial acid. 
 
 But which among the various reactions that 
 have been mentioned as belonging to acids is the 
 characteiistic reaction of acids? Prolonged in- 
 
88 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 vestigation has enabled chemists to assert that 
 all acids interact in presence of water with metals 
 (the common metals are iron, zinc, copper, mer- 
 cury, tin, and lead) to exchange the whole or 
 part of their hydrogen for metal, and so to pro- 
 duce salts, that is, compounds of the metal used 
 with the elements of the acid except the hydro- 
 gen, or that part of it which has been turned 
 out of the acid by the metal. In other words, 
 either the whole, or some portion, of the hydro- 
 gen in certain compounds is more or less readily 
 displaced from these compounds by causing them 
 to interact with metals in the presence of water. 
 Compounds which have this common property 
 are called acids, and these compounds are all 
 formed of hydrogen in union with relatively 
 large quantities of one or more oxygen-like ele- 
 ments. It is then a chemical reaction of hydro- 
 gen that, when this element is united with con- 
 siderable quantities of certain other elements, all 
 of which broadly resemble oxygen in their 
 chemical reactions, the hydrogen leaves its com- 
 bination with these other elements when an 
 aqueous solution of the compound of which it 
 forms a part interacts with a metal. 
 
 What, then, about alkalis 1 The outcome of 
 very extended experiments is to shew that alkalis 
 are compounds of hydrogen and oxygen with 
 relatively large quantities of elements which in 
 their chemical reactions are the very opposi'es of 
 oxygen. The hydrogen in alkalis is not turned 
 out by causing them to react with metals. 
 
 Summing up, we see that the chemical reactions 
 of hydrogen certainly depend on the kind of 
 
THE STUDY OF PROPERTIES. 89 
 
 elements wherewith the hydrogen is combined. 
 When it is combined with a large quantity of 
 oxygen-like elements it is acidic hydrogen ; it 
 can be turned out of these compounds by metals: 
 when it is combined with a large quantity of 
 certain other elements which are chemically very 
 unlike oxygen, then, even although some oxygen 
 also is in these compounds, the hydrogen cannot 
 be turned out by metals ; it is chemically quite 
 different from the hydrogen of acids. It is evi- 
 dently impossible to study the chemical reactions 
 of hydrogen without studying the chemical re- 
 actions of many other elements, and many com- 
 pounds, and the compositions also of many com- 
 pounds. It is, indeed, literally true that an 
 exhaustive study of the chemical reactions of 
 hydrogen involves an exhaustive study of many 
 chemical reactions of every one of the elements, 
 and of a very large number of compounds also. 
 It may, I think, even be said that a full investi- 
 gation of any chemical reaction would lead to the 
 investigation of all other chemical reactions ; for 
 the reactions of any element or compound are 
 the changes that take place when that body 
 reacts with others to produce certain new sub- 
 stances, and the reactions cannot be regarded as 
 properly investigated when the properties of 
 the new substances only are studied, but the 
 investigation requires the examination of the 
 changes that occur when the new substances react 
 with other substances ; and so the study proceeds, 
 until the whole of chemistry is embraced in the 
 scope of the inquiry. And, moreover, the chemical 
 reactions which are known to-day are not so many 
 
90 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 as those that will be known to-morrow ; every 
 day adds new substances, and, therefore, new 
 reactions, to the science. Lack of variety, lack 
 of subjects of inquiry, are impossible in chemistry; 
 the study always draws one on to new fields ; it 
 is satisfying, but never gorging; it is difficult, 
 but always fascinating ; there is no branch of 
 natural science wherein such a constant demand 
 is made on that accurate and imaginative common 
 sense which is called scientific method. 
 
 CHAPTEE YI. 
 
 CONNEXIONS BETWEEN COMPOSITION AND 
 PROPERTIES. 
 
 THE brief examination of certain chemical pro- 
 perties of water and of hydrogen whicli was 
 made in the last chapter necessarily involved the 
 recognition of definite connexions between the 
 chemical reactions and the compositions of the 
 substances examined. But as the study of 
 the connexions between reactions and composi- 
 tion is the very essence of chemistry, it is 
 advisable to devote a chapter to the illustration 
 of the methods employed and of the results 
 obtained. 
 
 It is sixty years since Talbot laid the founda- 
 tions of photography. He dipped paper into a 
 solution of lunar caustic (nitrate of silver), and 
 then brushed over tfris paper a solution of common 
 salt ; on this prepared paper he laid a trans- 
 
COMPOSITION AND PROPERTIES. 91 
 
 parent drawing, and placed the paper and drawing 
 in sunlight; after a little time he removed the 
 drawing, and at once he took the paper, whereon 
 the sun had imprinted a copy of the drawing 
 with the lights and shadows reversed, into a dark 
 room and immersed it in a hoiling solution of 
 common salt. Talbot thus obtained a copy of 
 his drawing on which the sunlight was without 
 action ; but the lights and shadows of the original 
 drawing were reversed in the copy. He now 
 laid the copy on another piece of paper, prepared 
 by dipping in solution of silver nitrate and brush- 
 ing over with common salt solution, exposed the 
 arrangement to sunshine, and passed the paper 
 through a boiling solution of salt ; and thus he 
 had a copy of the original drawing with the 
 lights and shadows represented as in the original. 
 These processes present an interesting series of 
 changes of properties in certain definite sub- 
 stances, Talbot's process was not quite satisfac- 
 tory : it was tedious ; the first result was apt to 
 fade after being exposed for some time to the 
 light ; the copy was not quite so sharp as might 
 be desired, and so on. It was evidently neces- 
 sary to elucidate, as far as possible, the changes 
 of composition that accompanied the observed 
 changes of properties, and then to compare both 
 series of changes with other changes like them, 
 in order that, reasoning on this basis of well- 
 establish el facts, remedies might be suggested 
 and modifications might be tried. Now the paper 
 which had been soaked in silver nitrate solution 
 and then brushed over with a solution of Fait con- 
 tained in its pores a deposit of a white solid called 
 
92 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 chloride of silver. The formation of chloride of 
 silver, by bringing together solutions of silver 
 nitrate and common salt, had been known for a 
 long time before Talbot's day, and the fact was 
 discovered fully two hundred years before Talbot 
 made his experiments that chloride of silver is 
 blackened by sunlight. But what change is 
 brought about in the composition of chloride of 
 silver by the action of the sun 1 It might well 
 be supposed that a definite answer to this ques- 
 tion must be found before photography could 
 make any marked advance; nevertheless it is 
 strange that a definite and undoubtedly correct 
 answer has not yet been given. When chloride 
 of silver is exposed to sunshine there is a very 
 slight decrease in weight, a minute trace of chlo- 
 rine is removed (chloride of silver is a compound 
 of chlorine and silver), and perhaps an extremely 
 small quantity of oxygen enters into chemical 
 union with portions of the silver and chlorine. 
 Whatever the change of composition may be it 
 affects only a very thin layer on the surface of 
 the chloride of silver. 
 
 Talbot found that chloride of silver dissolves 
 In a boiling solution of common salt a definite 
 compound of silver, sodium, and chlorine is pro- 
 duced but that the dark-coloured substance 
 produced by the action of sunlight on chloride 
 of silver does not dissolve in this solution. It is 
 inconvenient to work with boiling liquids. After 
 trying various substances, Talbot found that 
 chloride of silver dissolved easily in a cold solu- 
 tion of hyposulphite of soda a double hyposul- 
 phite of silver and soda is produced and that 
 
COMPOSITION AND PROPERTIES. 93 
 
 the product of the darkening of chloride of silver 
 by sunshine was insoluble in this liquid. Here, 
 then, was a distinct step forward. Talbot now 
 tried another compound of silver called iodide of 
 silver in place of the chloride, and he found that a 
 change was brought about by the sunshine in the 
 composition of a minutely thin layer of the iodide 
 of silver, but that this change did not make 
 itself visible until another change was induced 
 by dipping the paper coated with iodide of silver, 
 after exposure to sunshine, into some reagent 
 which reacted with the very small portion of 
 changed iodide of silver to produce a visible 
 image. This was the beginning of new modifica- 
 tions ; it was no longer necessary to expose the 
 sensitive paper to the action of sunlight until a 
 picture was imprinted on it visibly, but a very 
 short exposure sufficed so to change a minute 
 quantity of the sensitive salt that another change 
 was brought about by the reaction of some other 
 reagent on this altered trace of the sensitive salt, 
 and a visible picture was the final result. Those re- 
 agents which act on chloride or iodide of silver, and 
 also on bromide of silver, after exposure of these 
 salts of silver to sunlight, are called developers, 
 for they develop the picture by causing changes 
 in composition and properties in those portions 
 of the salts of silver that have already been 
 changed, although only to an extremely small 
 extent, by the action of sunlight. 
 
 The process of producing pictures of objects 
 by concentrating the rays of light coming from 
 those objects on to one spot, by means of lenses, 
 was introduced by Daguerre, following the lines 
 
94 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 of Ni6pce, towards the end of the thirties of this 
 century. Since that time lenses and cameras 
 have been very greatly improved, but the prin- 
 ciples of this, the physical, part of photography 
 remain unchanged. 
 
 The sensitive surface on which the picture of 
 an object is to be drawn by the sun is generally 
 prepared nowadays by dissolving bromide of 
 potassium, or bromide of ammonium, in water, 
 adding gelatin, and then an exactly sufficient 
 quantity of silver nitrate solution to react 
 with the whole of the bromide of potassium, 
 so as to produce solid silver bromide, and 
 potassium nitrate which remains in solution ; 
 an emulsion is formed by shaking until the solid 
 bromide of silver is disseminated, in extremely 
 small particles, through the gelatin solution. 
 This emulsion is then heated for a time these 
 operations are, of course, carried on in the dark, 
 or in a room lighted by orange-red light and 
 allowed to cool, and the semi-solid gelatin, with 
 the minute particles of silver bromide dissemi 
 nated through it, is washed with cold water 
 until everything that will dissolve in water has 
 been removeed. The washed gelatin emulsion is 
 thoroughly mixed with enough gelatin solution 
 to make it the proper consistency ; and this semi- 
 liquid emulsion is poured evenly over clean glass 
 plates, which are then allowed to dry. When a 
 prepared plate is exposed in the camera for a 
 fraction of a second to the rays of light which 
 come from an object outside the camera and are 
 collected and concentrated on the plate by the 
 lenses, a change of composition, accompanied by 
 
COMPOSITION AND PROPERTIES 95 
 
 a change of properties, is brought about in an 
 extremely thin film on the surface of fhe bromide 
 of silver on the plate. The change of composition 
 is certainly one wherein a minute trace of bromine 
 is .removed from silver bromide ; and it is prob- 
 able, but not certain, that a compound, or com- 
 pounds, of silver, bromine, and oxygen (from the 
 air) is formed. The trace of bromine removed 
 from the silver bromide that has been affected 
 by the rays of light is absorbed by the gelatin, 
 wherein it produces one or more compounds. 
 The amount of change on the sensitive surface of 
 the plate depends on the intensity of the light 
 that falls on it ; more light comes from the lighter 
 parts than from the darker parts of the objects ; 
 and, therefore, the amount of chemical change is 
 greatest in those parts of the silver bromide on 
 which the rays from the more lighted portions of 
 the objects have impinged. 
 
 At this stage nothing is to be seen on the 
 plate ; the picture must now be developed. For 
 this purpose the plate is immersed in a solution 
 of some reagent which removes bromine, and 
 perhaps oxygen also, from the compound (or 
 compounds) that have been formed by the action 
 of light on the silver bromide, and by doing this 
 produces silver in a state of very fine division. 
 The action of the developer generally used seems 
 to extend beyond the minute quantity of photo- 
 chemically changed silver bromide ; these de- 
 velopers probably remove bromine, and hence 
 produce silver, from some of the silver bromide 
 which has not been changed by light. As the 
 process of development proceeds a picture of the 
 
96 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 objects appears, with the lights and shadows 
 inverted, the picture being built up of very finely 
 divided black particles of silver. As most ligh t has 
 reached the plate from those parts of the objects 
 which were most highly lighted, it follows that 
 the decomposition of the silver bromide has been 
 carried furthest in those parts of the plate, and 
 that the deposit of silver, produced by the 
 developer, is thickest on those parts ; that is to 
 say, the highest lights of the objects produce the 
 darkest picture on the plate. When the develop- 
 ing process is finished the unchanged silver 
 bromide is dissolved off the plate by immersing 
 the plate in a solution of hyposulphite of soda, 
 and then washing very thoroughly in running 
 water ; the picture is now said to be fixed. 
 
 I need not go into the processes whereby the 
 picture on the plate is reproduced on sensitised 
 paper with the lights and shadows in the same 
 order of brilliancy as they were in the original 
 objects. Suffice it to say that the plate is covered 
 with a piece of paper which is coated with a 
 silver salt (generally silver chloride), and light is 
 allowed to fall through the plate on to the sensi- 
 tised paper, wherein changes are brought about 
 more or less similar to those which occurred on 
 the plate when it was exposed in the camera. 
 The light reduces the silver salts in the paper, 
 and the compounds thus produced are reddish- 
 brown ; reduction occurs to the least extent on 
 those parts of the paper which are covered with 
 the thickest silver films on the plate, so that 
 those parts of the original objects which were 
 most lighted appear also most lighted in the final 
 
COMPOSITION AND PROPERTIES. 97 
 
 print. The print is now toned by immersion in a 
 solution of chloride of gold ; the various com- 
 pounds which form the picture on the print 
 break up this gold salt, and a small quantity of 
 gold is deposited on the picture ; the object of 
 this process is to impart a pleasing colour to the 
 print. The silver chloride which remains in the 
 paper unchanged is now removed by immersing 
 the paper in a solution of hyposulphite of soda ; 
 the paper is then thoroughly washed and allowed 
 to dry. 
 
 Parts of the processes used in photography are 
 certainly physical, and part of the success de- 
 pends on using skilfully made instruments ; 
 but the changes whereon the essential portions 
 of photographic processes rest are changes in 
 the compositions and properties of definite chemi- 
 cal compounds. The change, the almost in- 
 finitely minute but still definite change, brought 
 about by the light in the sensitive silver 
 bromide produces a compound (or compounds) 
 whose most marked property is its power of 
 reacting, gradually, with other definite com- 
 pounds, that are called developers, to produce 
 silver. Then there is the fact that silver 
 bromide reacts with hyposulphite of soda to 
 form a compound which is soluble in water, 
 whereas no action occurs between hyposulphite 
 of soda and either silver or the compound (or 
 compounds) formed by the action of light on 
 silver bromide. This gradual development of 
 the wonderful art of photography is a striking 
 instance of what may be done by studying the 
 connexions between definite changes of com- 
 
 G 
 
98 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 position and definite changes of properties. It 
 is true that many improvements have been made 
 in photographic processes by people who neither 
 knew nor cared much about the exact study of 
 material changes; but it is also true that all 
 the deep-seated changes in the art, all the far- 
 reaching discoveries, have come from accurate 
 and painstaking attempts to find out what 
 changes in composition actually take place 
 during the different parts of photographic 
 processes, and in what ways these are con- 
 nected with changes of properties. 
 
 The next example I give of the results of the 
 study of the connexions between changes of 
 composition and changes of properties will be 
 taken from the manufacturing industries. There 
 are great differences between the properties of 
 iron and those of ironstone ; the properties of 
 the former make it the most useful of the metals, 
 whereas ironstone is of no use in itself, but only 
 in that it is a source of iron, Corresponding 
 with these differences in properties, there must 
 be some difference in composition ; if the latter 
 difference is known it will doubtless be possible 
 to arrange reactions whereby the passage from 
 the one substance to the other may be accom- 
 plished. Analyses of ironstones shew that most 
 of them contain compounds of iron and oxygen, 
 mixed with other compounds of other elements. 
 The problem then is to remove oxygen from its 
 combination with iron. Now it is known that 
 charcoal or coke burns fairly easily, and that 
 while burning it combines with oxygen ; more- 
 over, experiments in the laboratory shew that 
 
COMPOSITION AND PROPERTIES. 99 
 
 when a mixture of finely powdered charcoal and 
 oxide of lead, or oxide of copper, is heated, the 
 glowing charcoal takes oxygen away from the 
 metallic oxide, and produces carbonic acid gas 
 and lead, or copper. Further experiments on 
 the burning of charcoal, or coke, shew that when 
 a layer of one of these materials is lighted at the 
 bottom, and is fed with sufficient air from be- 
 neath, the carbonic acid gas which is produced 
 is deprived of half its oxygen as it passes through 
 the upper layers of heated charcoal, or coke, and 
 a gas is produced which is called carbon mon- 
 oxide. This carbon monoxide is easily in- 
 flammable in the air ; it burns with a pale blue 
 flame and produces carbonic acid gas ; it also 
 easily removes oxygen from various metallic 
 oxides when heated with them. 
 
 Here, then, are chemical reactions on which 
 may be reared a process for converting ironstone 
 into iron, If the ironstone is mixed with char- 
 coal, or coke, and strongly heated, air being 
 admitted from beneath, some of the charcoal, or 
 coke, will be burnt to carbonic acid by the 
 oxygen which is admitted in the air, but some 
 of it will probably be burnt by oxygen which it 
 withdraws from the oxide of iron in the iron- 
 stone. Moreover, as the carbonic acid gas 
 passes upwards it will most likely be robbed of 
 half its oxygen by the hot charcoal, or coke, 
 through which it passes; and if this happens, 
 a gas carbon monoxide will be formed which 
 is eager to combine with oxygen wherever it 
 can get it, arid which will, therefore, in all 
 likelihood, take away oxygen from the oxide of 
 
100 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 iron in the ironstone. By two different re- 
 actions, then, will the wished-for change be 
 accomplished ; oxygen will be taken away from 
 the oxide of iron by the reaction between that 
 oxide and charcoal, or coke, at a high tempera- 
 ture, and oxygen will be removed from the same 
 oxide of iron by the reaction between it and the 
 carbon monoxide which is produced by burning 
 the charcoal, or coke, under special conditions. 
 
 These two reactions are those which take 
 place, along with other chemical changes, in the 
 blast furnaces where ironstone is changed into 
 iron. 
 
 The reactions which occur between oxides of 
 metals and carbon at high temperatures have been 
 turned to account in the preparation of many 
 other metals besides iron from their ores. Many 
 metallic ores contain compounds of the metals 
 with oxygen ; many others contain compounds 
 with sulphur ; and some, called carbonates, 
 contain compounds with carbon and oxygen. 
 Now, as almost all metallic sulphides can be 
 converted into oxides by roasting in a current 
 of air the sulphur is burnt off, being changed 
 to gaseous sulphur dioxide, and oxygen from 
 the air combines with the metal that is freed 
 from sulphur and as most, if not all, metallic 
 carbonates can be changed into oxides, and 
 carbonic acid which goes off as gas, by heating 
 strongly, it is evidently no difficult task to 
 convert the sulphide, or carbonate, of a metal 
 in an ore of that metal into the oxide. And 
 when the oxide has been obtained, the metal 
 itself can very frequently be produced by 
 
COMPOSITION A&D WSLQPiKfr&R. 101 
 
 strongly heating, in a suitable apparatus, a 
 mixture of the oxide with charcoal or coke. 
 
 An examination of the change of composition 
 that accompanies the change of properties when 
 a naturally occurring metallic ore is converted 
 into a metal, has enabled the processes which 
 are likely to prove effective in the extrac- 
 tion of this or that particular metal from 
 its ores to be arranged together, so that one 
 who has a general, and at the same time accu- 
 rate, knowledge of this class of reactions knows 
 which processes are worth trying should he have 
 to deal with some new problem in the isolation 
 of a metal from those compounds of it that are 
 found native. And more than this, a know- 
 ledge of the kind I have spoken of enables a 
 man to suggest new methods when all the 
 ordinary and commonly used methods have 
 failed. He knows the general character of 
 the only methods he need attempt; he cuts 
 out a great many processes which the mere 
 rule-of-thumb man might be likely to waste 
 time and energy in trying ; he goes at once 
 to the root of the matter. 
 
 Take, for instance, the preparation of alu- 
 minium, a metal which has come into considerable 
 use within the last few years. Compounds of 
 the oxide of aluminium with silica occur in very 
 large quantities in the earth ; these compounds 
 form the clays. Judging from the reactions 
 that occur when many metallic oxides are heated 
 with charcoal, it seemed possible that aluminium 
 might be obtained by heating a mixture of the 
 oxide of that metal obtainable from clay with- 
 
1G2 ?,H^ H;COBY aF 1-HE CHEMICAL ELEMENTS. 
 
 out much difficulty and charcoal. But when 
 this reaction was tried, the oxide of aluminium 
 was not deprived of its oxygen. The compounds 
 which metals form with chlorine, bromine, or 
 fluorine always shew distinct analogies in their 
 chemical reactions with the metallic oxides. As 
 oxygen can very often be torn away from a 
 metallic oxide by heating that oxide with car- 
 bon, or in a stream of gaseous carbon monoxide, 
 or with the extremely poisonous compound 
 called potassium Cyanide, so chlorine, bromine, 
 or fluorine can generally be removed from a 
 metallic chloride, bromide, or fluoride by the 
 reaction of the metal sodium, or the metal zinc, 
 with the melted chloride, bromide, or iodide. 
 These reactions vvere applied, and applied suc- 
 cessfully, to the isolation of aluminium. Alu- 
 minium chloride, or, rather, a compound of 
 aluminium and sodium chlorides, was prepared 
 from clay ; this compound was melted, and 
 pieces of the metal sodium were pressed into 
 the semi-liquid mass; an exceedingly violent 
 reaction occurred; common salt was formed by 
 the union of the sodium with the chlorine which 
 was torn away from its combination with alu- 
 minium ; and the melted aluminium, now re- 
 moved from its union with chlorine, was poured 
 out into iron moulds. 
 
 But this method for maKing aluminium was 
 not commercially successful ; the preparation 
 of the compound of aluminium and sodium 
 chlorides from clay was a tedious and some- 
 what difficult business, and the preparation of 
 the metal sodium was expensive. Now it 
 
COMPOSITION AND PROPERTIES. 103 
 
 was known that many chlorides, bromides, or 
 fluorides of metals are torn asunder into their 
 constituents when these salts are melted and 
 a current of electricity is sent through the 
 melted salts. This method was tried with 
 aluminium fluoride, and was successful. A 
 compound of aluminium fluoride with sodium 
 fluoride occurs native ; it is called cryolite. All 
 that need be done to get aluminium from 
 cryolite is to mix the mineral with a certain 
 amount of common salt the mixture melts 
 more easily than cryolite alone to melt this 
 mixture, and to pass a current of electricity 
 through the molten substance. 
 
 Turn back for a moment to the iron made by 
 removing oxygen from ironstone. Iron made in 
 that way is not pure iron ; it contains several 
 per cents, of carbon, which makes the iron fusible 
 and capable of being run into moulds, or pigs as 
 they are called in the iron trade. Steel is 
 nearly pure iron ; at least, it is iron containing 
 considerably less carbon than pig-iron. As steel 
 is much harder than pig-iron, and as it can be 
 tempered, a process which pig-iron refuses to 
 undergo, it is an exceedingly useful variety of 
 iron. How, then, can pig iron be changed into 
 steel ? The problem is to remove from pig-iron 
 somewhat more than half the quantity of carbon 
 it contains, but to leave about 1 to 1^ per cent. 
 of carbon in the product. 
 
 If air is blown into melted pig-iron, the whole 
 of the carbon is burnt to carbonic acid, and at 
 the same time a little of the iron is burnt to iron 
 rust (iron oxide). It is not possible to stop the 
 
104 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 burning process at the point whereat just enough 
 carbon has been burnt to insure the formation 
 of steel ; but it is possible to stop the process 
 exactly when the last trace of carbon has been 
 burnt away ; and it is then easy to stop the 
 inrush of air, and to add exactly enough carbon 
 to convert the contents of the vessel into steel. 
 The Bessemer process for making steel takes advan- 
 tage of the different behaviours of iron and 
 carbon towards air at a high temperature. Huge 
 pear-shaped vessels, made of plates of wrought 
 iron and lined with fireclay, are employed ; 
 several tubes pass through the bottoms of the 
 vessels, arid air can be blown in through these 
 tubes. The vessel is charged with molten pig- 
 iron ; air is blown in, and at once the burning 
 processes begin. The carbon is burnt rapidly to 
 carbonic acid, which escapes as gas, and at the 
 same time a little iron is converted into oxide. 
 When the last trace of carbon is burnt away 
 the exact point is indicated by a sudden change 
 in the appearance of the flame issuing from the 
 vessel the air-blast is stopped ; enough carbon 
 is added to convert the almost pure iron in the 
 vessel into steel (of course, the weight of mater- 
 ial in the vessei is known), and the molten con- 
 tents are run into proper vessels wherein the 
 steel cools. The small quantity of iron oxide 
 produced in the burning process remains un- 
 melted as a dross or cinder which does not mix 
 with the molten steel. 
 
 These cases the art of photography, the pro- 
 duction of iron from iron ore, and steel from 
 iron, and the production of aluminium from com- 
 
COMPOSITION AND PROPERTIES. 105 
 
 pounds of that metal which are found in the 
 earth are illustrations of the results that have 
 been obtained by the careful study of the con- 
 nexions between changes of properties and changes 
 of composition, that is, by the study of chemical 
 transformations. The illustrations have been 
 taken from the sphere of technical chemistry. 
 I should now like to give some examples both of 
 the results, and of the methods used, of a more 
 general character than any that can be selected 
 from those chemical transformations, all more or 
 less special, which are turned to account in so 
 many manufacturing industries. 
 
 Oxygen combines with every one of the, ele- 
 ments except bromine and fluorine ; these com- 
 pounds, each of oxygen and one other element, 
 are called oxides. Here, then, is a large number 
 of compounds with a similar composition ; they 
 are all binary compounds of oxygen. The study 
 of the chemical reactions of this class of com- 
 pounds, related so definitely in their composition, 
 cannot but be of interest. Some oxides dissolve 
 easily in water, others dissolve with difficulty, 
 and some do not dissolve at all. Aqueous solu- 
 tions of certain oxides are acrid, corrosive, and 
 biting ; they dissolve many metals with the 
 evolution of a gas which is often hydrogen, and 
 they dissolve washing soda, or chalk, giving off 
 carbonic acid gas. Aqueous solutions of certain 
 other oxides have a soap- like feel, but are not 
 acrid nor biting ; these solutions do not dissolve 
 metals, washing soda, or chalk, with evolution 
 of gas. When definite quantities of a pair of 
 oxides, one belonging to the first, and the other 
 
106 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 to the second, of these classes, are mixed in 
 aqueous solutions, the characteristic properties of 
 both oxides vanish ; a liquid is obtained which 
 is neither acrid nor corrosive, nor soapy to the 
 touch ; this liquid does not dissolve metals, nor 
 chalk, nor washing-soda with evolution of a gas. 
 The prominent properties of the liquid are best 
 described by the word neutral. When this 
 neutral liquid is evaporated, and then allowed to 
 cool, a solid separates, generally in crystals, 
 and an analysis of this solid shews that it is 
 composed of the two oxides aqueous solutions of 
 which were mixed. The two oxides have com- 
 bined to form a definite substance whose pro- 
 perties are different from those of either of the 
 oxides ; the characteristic properties of the 
 oxides have been lost in the properties of the 
 new substance. 
 
 It is customary to call those oxides which 
 dissolve in water to form acrid liquids that 
 dissolve metals, acidic oxides; and to give the 
 name basic oxides to those whose aqueous solu- 
 tions are not acrid and do not dissolve metals. 
 The compound that is formed by mixing definite 
 quantities of aqueous solutions of two oxides, 
 one of which is acidic and the other basic, and 
 evaporating, is called a salt. It is found that 
 many oxides which do not dissolve in water 
 nevertheless dissolve in aqueous solutions of 
 acidic oxides, combining with these acidic oxides 
 to form salts ; oxides which react in this way are 
 included among the basic oxides. Again, there 
 are several oxides which do not dissolve in 
 water, but which combine with basic oxides to 
 
COMPOSITION AND PROPERTIES. 107 
 
 form salts when heated with them ; such oxides 
 are included among the acidic oxides. In all 
 these reactions, of course, definite quantities of 
 the various oxides react, and the weight of 
 each oxide which takes part in any one of the 
 reactions is always expressible as a whole mul- 
 tiple of the reacting weight of that oxide (see 
 back, p. 66); moreover, the total weight of all 
 the products of each reaction is exactly equal to 
 the total weight of all the definite bodies that 
 have reacted. 
 
 There are some oxides which do not belong to 
 either of those divisions we are considering ; but 
 the two classes of acidic and basic oxides contain 
 more than three-fourths of all the oxides. 
 
 Let me illustrate the reactions of basic and 
 acidic oxides by describing three cases in a little 
 detail. When carbon is burnt in plenty of 
 oxygen, or of air, the carbon gradually dis- 
 appears ; if the burning is carried on in a jar, 
 and if some water is poured into the jar when 
 the burning is done, and the jar is shaken, the 
 water is found to have become sour to the taste; 
 it is able to change the hues of various colouring 
 matters, and to dissolve some metals (not by 
 any means all -metals) slowly with evolution of 
 hydrogen. Carbonic acid gas has been pro- 
 duced and has dissolved in the water. If a 
 solution in water of a little of another oxide, 
 namely, oxide of sodium, is prepared, that solu- 
 tion has the properties already enumerated as 
 being characteristic of basic oxides ; the liquid is, 
 indeed, weak soda lye. Carbonic acid gas can 
 also be made by pouring hydrochloric acid on to 
 
108 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 lumps of marble ; if the gas is passed into an 
 aqueous solution of the basic oxide of sodium 
 until the bubbles of the gas cease to be absorbed 
 by the liquid, and if the liquid is then evaporated 
 considerably and allowed to cool, a white crystal- 
 line powder separates, and this powder is proved 
 by quantitative experiments to be bicarbonate of 
 soda. The salt bicarbonate of soda has been pro- 
 duced by the interaction of definite quantities of 
 carbonic acid gas, which is an acidic oxide, and 
 sodium oxide, which is a basic oxide. 
 
 A liquid having all the properties, to a very 
 marked degree, of solutions of acidic oxides is 
 obtained by dissolving a certain white crystalline 
 compound called sulphur trioxide in water. The 
 liquid is very sour, very acrid and corrosive, it 
 burns holes in one's clothes, it irritates the flesh, 
 it dissolves many metals, even without heating, 
 and it rapidly evolves carbonic acid gas from 
 washing-soda or chalk. If some of this liquid is 
 heated, and iron rust, which is composed of iron 
 and oxygen, is thrown into the hot liquid, the 
 rust gradually dissolves ; if the addition of rust 
 is continued till a little of it refuses to dissolve, 
 if the liquid is then strained off through a filter 
 from the undissolved rust, evaporated until it is 
 nearly dry, and allowed to cool, a pale yellowish 
 brown solid is obtained which shews none of the 
 characteristic reactions of either oxide by the 
 reaction of which it has been produced, Quan- 
 titative analyses prove that this solid is composed 
 of ferric oxide (rust) and sulphur trioxide, and 
 its reactions cause it to be placed in the class of 
 salts. 
 
COMPOSITION AND PROPERTIES. 109 
 
 There is a yellow salt called chromate of 
 potash, which is largely used as a colouring 
 material, and from which many other colouring 
 materials are obtained. This salt is a compound 
 of the acidic red oxide of chromium and the basic 
 oxide of potassium. The commonest mineral 
 which contains compounds of chromium is chrome- 
 ironstone, the main constituents whereof are 
 oxide of iron arid an oxide of chromium called 
 the green oxide ; this oxide of chromium is basic, 
 it contains less oxygen, relatively to a fixed 
 weight of chromium, than the red, acidic, oxide. 
 One result of a long series of examinations is 
 that although the red, acidic, oxide of chromium 
 cannot be prepared by directly adding oxygen to 
 the green, basic, oxide, nevertheless if the green 
 oxide is heated for a long time in the air in the 
 presence of solid oxide of potassium (solid potash, 
 which is composed of potassium, hydrogen, and 
 oxygen, is used), and of a flux which serves to 
 keep the whole mass melted, the green oxide 
 combines with oxygen which it takes out of the 
 air and forms the red, acidic, oxide, and simul- 
 taneously the red oxide of chromium combines 
 with the basic oxide of potassium that is present, 
 and the wished-for salt, chromate of potash, is 
 produced. The process cannot be stopped at 
 the half-way house marked by the formation of 
 the red, acidic, oxide of chromium ; but this 
 does not matter, as the object of the manu- 
 facturer is, not to isolate this oxide, but to 
 apply the typical reaction which occurs between 
 an acidic and a basic oxide to this oxide and the 
 basic oxide of potassium. 
 
110 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 The study of the reactions of oxides, that large 
 class of compounds which are so far like one 
 another in composition that each is composed of 
 oxygen and another element, has led to the 
 division of the greater number of the oxides into 
 two classes, acidic oxides and basic oxides. Even a 
 cursory examination of the reactions of these two 
 classes of oxides has carried us away from oxides 
 to another class of compounds called salts. And 
 in our attempts to elucidate the reactions of 
 those compounds of oxygen, the oxides, we have 
 reached results which had already been obtained 
 in our attempts to elucidate the reactions of cer- 
 tain compounds of hydrogen. In studying the 
 connexions between the compositions and pro- 
 perties of a great class of compounds of hydrogen, 
 the acids, we found that a salt is always one of 
 the products of the reaction between an aqueous 
 solution of an acid and a metal. We now know 
 that a salt is also produced when an aqueous 
 solution of an acidic oxide interacts with a basic 
 oxide. This, of course, obliges an inquiry to be 
 made into the compositions of those oxides that 
 are basic; the result is, as might have been 
 expected, that basic oxides are oxides of metals, 
 that is, of elements which resemble iron, copper, 
 lead, tin, silver, and gold (compare what was said 
 about metals in the last chapter, p. 88). 
 
 But what kinds of elements are those whose 
 oxides are acidic ] The answer given by ex- 
 periment is most acidic oxides are oxides of 
 elements which are chemically and physically 
 unlike the metals, and which elements chemi- 
 cally resemble sulphur, phosphorus, nitrogen, 
 
COMPOSITION AND PROPERTIES. Ill 
 
 and oxygen. We have already (p. 87) called 
 these elements oxygen-like elements ; they are 
 more commonly classed together under the name 
 non-metals. 
 
 Two questions remain, directly suggested by 
 the results we have obtained, and a comparison 
 of these with those which the study of hydrogen 
 taught us regarding acids and salts. A metal 
 interacts with an acid to form a salt, and a gas 
 which is generally hydrogen ; the salt is com- 
 posed of the metal and the elements of the acid 
 save the hydrogen, or a portion of the hydrogen, 
 (see p. 85) ; but a basic oxide, which is an oxide 
 of a metal, reacts with an aqueous solution of an 
 acidic oxide to form a salt without the evolution 
 of hydrogen or any other gas. Does an aqueous 
 solution of an acidic oxide then contain an acid ? 
 And when a basic and an acidic oxide react in 
 solution is anything formed besides the salt, and 
 if so, what is formed ? These questions open a 
 very great field of inquiry, which cannot be gone 
 into here. Suffice it to say that an acid is 
 formed in almost every case when an acidic 
 oxide dissolves in water ; the solution is accom- 
 panied by an interaction between the oxide and 
 the water ; and the product of this interaction is 
 an acid, which is a compound of hydrogen (from 
 the water), oxygen, and the non-metallic ele- 
 ment the acidic oxide whereof was dissolved in 
 the water. And the most probable explanation 
 of the changes of composition that occur when 
 an aqueous solution of an acidic oxide that is, 
 an aqueous solution of an acid interacts with a 
 basic oxide, is to the effect that the metal of the 
 
112 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 basic oxide turns out the hydrogen, or a portion 
 of the hydrogen, of the acid, and. combining with 
 the other elements that formed the acid, pro- 
 duces a salt, while the hydrogen that was re- 
 moved from its combination enters into union 
 with the oxygen of the basic oxide and forms 
 water. 
 
 One general result of the very cursory exami- 
 nation we have been able to make of the re- 
 actions of oxides is that the chemical properties 
 of these compounds depend, in some measure, on 
 the general chemical character of the elements that 
 are combined with oxygen. This result is exactly 
 similar to that we arrived at from the study of 
 certain compounds of hydrogen ; for we found 
 that the chemical properties of hydrogen depend 
 (there may, of course, be other conditioning cir- 
 cumstances) upon the general chemical character 
 of the elements, and also upon the relative quan- 
 tities of the elements, wherewith the hydrogen is 
 combined (compare p. 89). Perhaps the relative 
 quantities of oxygen combined with another ele- 
 ment may affect the basic or acidic reactions of 
 the oxides of that element. Experiments shew 
 that the basic or acidic characters of oxides are 
 undoubtedly conditioned by the relative quan- 
 tities of oxygen they contain. To give only two 
 examples. There is an oxide of the metal man- 
 ganese which contains weights of manganese and 
 oxygen combined in the ratio (expressed in 
 round numbers) of 1 to J, and that oxide is 
 decidedly basic ; there is another oxide of the 
 same metal which is distinctly acidic, and that 
 oxide contains weights of manganese and oxygen 
 
COMPOSITION AND PROPERTIES. 113 
 
 combined in the ratio (using round numbers) of 
 1:1. There are two oxides of chromium, one 
 of which contains just twice as much oxygen as 
 the other combined with the same weight of the 
 metal ; that oxide with the smaller quantity of 
 oxygen is basic, and that with the larger quan- 
 tity of oxygen is acidic. 
 
 The study of the reactions of oxides leads bo 
 the study of acids, salts, metals, and non-metals. 
 This study illustrates what I said before that 
 every chemical interaction contains in itself the 
 whole science of chemistry. I have tried in this 
 chapter to indicate the nature of the problems 
 the chemist has to deal with, to outline some of 
 the methods he uses in trying to solve these 
 problems, and to notice a few of the results he 
 has obtained. I would conclude the chapter by 
 insisting on the fact that the study of the con- 
 nexions between composition and reactions, and 
 as a necessary consequence the examination of 
 the connexions between changes of composition 
 and changes of reactions is the very essence of 
 chemistry. To examine a thing as it is, apart 
 from other things, is not the business of the 
 chemist. When change begins chemistry begins. 
 And as the feature of human life is variability, 
 and as nine- tenths of our manufactures are con- 
 cerned with changes in material things, chemistry 
 is the science which enters most intimately into 
 the daily life of humanity. 
 
CHAPTER VII. 
 
 THE COMPOSITIONS AND PROPERTIES OF SOME 
 COMPOUNDS OF CARBON. 
 
 THERE is one element the number of whose com- 
 pounds is greater than the sum of all the com- 
 pounds of all the other elements ; that element 
 is carbon. Diamond is almost pure carbon ; 
 charcoal, graphite, lampblack, coal, and coke are 
 mixtures of carbon with various other substances. 
 The flesh, blood, bones, tissues, and all the other 
 parts of animals contain large quantities of 
 compounds of carbon ; and compounds of this 
 element form the greater part of every plant. 
 Vast quantities of carbon compounds are found 
 in the crust of the earth ; about 3 Ibs. of every 
 10,000 Ibs. of air consist of a compound of 
 carbon; and the same gaseous compound is 
 found dissolved in almost every specimen of 
 water. And besides the great array of com- 
 pounds of this element that is distributed through 
 nature, many thousands of other compounds of 
 the same element have been prepared in the 
 laboratory. 
 
 The range of properties of the compounds of 
 carbon is very great. Some are solids, some 
 liquids, and some gases, under ordinary con- 
 ditions of temperature and pressure ; and the 
 colours and appearances of these compounds are 
 as varied as their number is large. All the 
 aniline dyes are compounds of carbon ; gun- 
 cotton, nitro-glycerin, and the constituent com- 
 pounds of all smokeless powders are compounds 
 114 
 
STUDY OF SOME COMPOUNDS OF CARBON. 115 
 
 of carbon ; alizarin, indigo, and the colouring 
 compounds of saffron, brazil-wood, logwood, and 
 cochineal belong to this class ; all ordinary kinds 
 of fuel coal, wood, peat, coal-gas, and oils 
 are mixtures of compounds of carbon ; almost 
 every kind of food we consume is a mixture of 
 compounds of this element ; the characteristic 
 constituents of almost all our beverages are 
 carbon compounds ; the greater number of the 
 drugs that are used in illness are mixtures of 
 this class of compounds; all, or almost all, the 
 evil-smelling gases that are produced when 
 animal or vegetable matter decays are compounds 
 of carbon ; and all the odours of sweet essences, 
 and the delicate bouquets of choice wines are the 
 odours and the bouquets of compounds of the 
 same element carbon. The chemical reactions 
 of these compounds are surprisingly numerous 
 and surprisingly different : some compounds are 
 oxides, many are acids, and many are salts ; and 
 there are also representatives of dozens of classes 
 of compounds which are not to be found among 
 the compounds of any of the other elements. The 
 study of the compounds of carbon is eminently 
 fitted to throw light on the central problem of 
 chemistry, the problem, that is to say, of the 
 connexions between composition and reactions. 
 
 Suppose a person with some general knowledge 
 of chemical changes let us say, one of those 
 who have followed the course of this book to the 
 present chapter had put before him a statement 
 of the qualitative composition of each one of the 
 compounds of carbon known to-day, I am certain 
 his first remark would be : "How vast a number 
 
116 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 of different compounds is formed by the union ot 
 so small a number of different elements." For 
 the statement would reveal the fact that by far 
 the gr-ater number of the compounds of carbon 
 I suppose there would be at least 20,000 of 
 them are compounds of that element with the 
 three other elements hydrogen, oxygen, and 
 nitrogen, or with two of these three elements. 
 The statement would include a very fair number 
 of compounds containing chlorine, or bromine, 
 or iodine, or sulphur, in addition to carbon and 
 one or more of the three elements just mentioned ; 
 and there would be a tolerable show of com- 
 pounds containing metals, besides carbon, and 
 hydrogen, oxygen, and nitrogen, or two of these 
 three elements. Nevertheless the great bulk of 
 this array of compounds would be found to be 
 compounds of carbon with two, or at most with 
 three, other elements. We have evidently rich 
 material in the compounds of carbon for studying 
 the connexions between composition and reactions. 
 
 Although there is such a great number of 
 carbon compounds, and although the reactions 
 of them differ much, yet the classification of 
 these compounds on the basis of reactions is 
 more definite, descriptive, and suggestive than 
 the classification of the comparatively small 
 number of compounds of the other elements. 
 
 Each of these many compounds has its own 
 perfectly definite and quite unchangeable com- 
 position. The weights of the elements united in 
 any one of the compounds are expressible by the 
 combining weights, or by whole multiples (not 
 generally very large whole multiples) of the 
 
STUDY OF SOME COMPOUNDS OF CARBON. 117 
 
 combining weights, of the elements in question. 
 Take, for instance, compounds of the four ele- 
 ments carbon, hydrogen, oxygen, and nitrogen. 
 The symbol for the first of these elements is 0, 
 for the second H, for the third 0, and for the 
 fourth N ; and each of these symbols represents 
 one combining weight of the element symbolised 
 (the values of the four combining weights, in 
 round numbers, are C = 12, H=l, = 16, 
 N = 14). The compositions of all the compounds 
 which have been formed, or which can be formed, 
 by the union of these four elements is expressed 
 by the single formula 
 
 C.B.O.N,;' 
 
 where a, ft, c, and d are whole numbers. If any 
 values not greater than 10 were given to each of 
 the indices a, 6, c, and d, the symbols would 
 express the compositions of a large number (but by 
 no means all) of the compounds that have been 
 isolated of carbon with the three other elements. 
 
 Each of the many compounds of carbon has 
 its own peculiar reactions. which mark it off from 
 every other definite kind of matter; neverthe- 
 less, the reactions of many of the compounds are 
 so much alike that these compounds may be 
 put together in one class ; the reactions of another 
 considerable number are so similar that these are 
 put into another class ; and so on. 
 
 All I can do is to trace in merest outline some 
 of the results obtained by the study of the two- 
 fold aspect of a few compounds of carbon. I 
 shall try to state the results so as to suggest the 
 methods by which they have been attained. 
 
118 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 For the purpose of gathering together the 
 compounds of carbon under a common name, it 
 is customary to speak of them as organic com- 
 pounds. But these two expressions, organic 
 compounds and compounds of carbon, have not 
 always had the same meaning. Many of the 
 compounds known to the earlier chemists were 
 obtained from substances of animal or vegetable 
 origin ; these compounds, coming from organic 
 sources, were naturally called organic compounds. 
 As it was deemed very important about a cen- 
 tury ago to emphasise every distinction between 
 living and non-living things, chemists used to 
 assert that the elements of which organic com- 
 pounds were composed must be different from 
 the elements that build up the compounds con- 
 tained in minerals. But analyses of organic 
 compounds were amassed ; these compounds 
 were found to be composed of elements well 
 known to the student of mineral chemistry. 
 Then it was said that the arrangement of the 
 elements in organic compounds must be differ- 
 ent from the arrangement of the elements in 
 inorganic compounds, that is, compounds obtained 
 from inorganic sources. If one asked ; Why 
 must there be some essential difference between 
 organic and inorganic compounds ? one was 
 told that organic compounds were built up in 
 the plant or the animal by the agency of "vital 
 force," an agency unknown in the inorganic 
 world; and that where " vital force" came in it 
 produced results radically unlike those brought 
 about by any other cause. 
 
 The controversy, kept aflame, like so many 
 
STUDY OF SOME COMPOUNDS OF CARBON. 119 
 
 others, by giving different, and always indefinite, 
 meanings to the same terms, was stilled and then 
 extinguished by the results of a series of experi- 
 ments published in 1828 by a great German 
 chemist named Wdhler. If there be any com- 
 pound which is a typical product of the working 
 of living organisms that compound is urea. In 
 1828 Wohler prepared urea from compounds of 
 mineral origin ; that is, in the language of the 
 " vital force " school, Wohler prepared, without 
 the aid of ."vital force," a compound which till 
 then had been prepared only by the action of 
 "vital force." Many other compounds regarded 
 as distinctive products of the action of the " vital 
 force" were prepared soon afterwards from 
 inorganic sources ; and when twenty or thirty 
 years had passed "vital force" had disappeared 
 from the domain of chemistry. 
 
 Organic chemistry has then to aim at the 
 expression of the connexions between the proper- 
 ties and the compositions of compounds of carbon 
 in general statements, which shall hold good in 
 all particular cases, and which shall shew forth 
 the essential characteristics of the relations that 
 have been ascertained. Organic chemistry has 
 advanced some way towards this goal, but she is 
 yet far from the end of the journey. 
 
 Among the earliest, the most indefatigable, 
 and the most daring, workers in organic chemistry 
 are to be mentioned Liebig and Dumas, two great 
 men of science whose names are ever to be held 
 in renown by all students of nature. In 1867 
 Liebig was asked by Dumas why he had ceased 
 to work at the chemistry of carbon compounds ; 
 
~120 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 Liebig replied that "with the theory of substitu- 
 tion as a foundation, the edifice may now be built 
 ))y workmen ; masters are no longer needed. " In 
 the opinion of one of the great masters of the 
 science, the theory of substitution was the founda- 
 tion of organic chemistry. 
 
 The conception of substitution must, then, be 
 one of the most important main results of the 
 study of organic chemistry. Let us try to under- 
 stand what this conception is. 
 
 The mental picture corresponding to the word 
 substitution, as that word is used in organic 
 chemistry, is a complex one. In the words I 
 have quoted, Liebig likened substitution to the 
 foundation whereon the edifice of organic chem- 
 istry was to be built. It is essentially a builder's 
 image ; we shall best understand it by think- 
 ing of some series of organic compounds as 
 buildings. 
 
 During one of the royal soirees at the Tuileries, 
 when Charles X. was King of France, the wax 
 candles used to light the rooms gave off irritat- 
 ing vapours which annoyed the visitors. Dumas 
 was summoned and requested to find the reason, 
 and the cure, for the trouble. How had the wax 
 been made so white ? asked Dumas. By bleach- 
 ing it. What was the bleaching agent ? Chlorine. 
 Wax, Dumas knew to be a mixture of compounds 
 of carbon, hydrogen, and oxygen ; hydrogen com- 
 bines easily with chlorine to form the extremely 
 irritating gas, hydrochloric acid. Here was a 
 possible cause of the fumes which came from the 
 candles. If some of the chlorine used for bleach- 
 ing the wax were retained in the wax, then the 
 
STUDY OF SOME COMPOUNDS OF CARBON. 121 
 
 heat produced by the burning candles might well 
 cause chemical changes to proceed and hydro- 
 chloric acid gas to be given off. Dumas found 
 that the fumes which annoyed the guests at the 
 Tuileries contained hydrochloric acid ; and he 
 found that this was produced by changes in com- 
 pounds of carbon, hydrogen, and chlorine formed 
 in the wax during the bleaching of it by chlorine. 
 The cause and the cure were found. The wax 
 must no longer be bleached by chlorine, because 
 if chlorine is used some of it will always enter 
 into chemical union with compounds in the wax, 
 and when a candle made of this wax is lighted 
 other chemical reactions, wherein hydrochloric 
 acid gas is produced, will occur. 
 
 But Dumas was not content to stop here. He 
 investigated the reactions that take place when 
 chlorine is passed over wax ; and from that he 
 passed to the examination of interactions between 
 chlorine and various organic compounds. He 
 found that in many of these reactions hydrogen 
 was removed from the compound used, and 
 chlorine entered into union with the elements 
 which remained, producing in this way a new 
 compound whose general chemical reactions were 
 very often extremely like those of the original 
 compound. Take, for example, some of the 
 results of Dumas' investigation of the reactions 
 between chlorine and acetic acid (the acid of vine- 
 gar), an acid composed of carbon, hydrogen, and 
 oxygen. Dumas found that the reactions pro- 
 ceeded in three stages, and that the products of 
 each stage were a new acid, resembling acetic 
 acid in its general chemical behaviour, and 
 
122 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 hydrochloric acid gas. He found that each of 
 the new acids was composed of all the carbon, all 
 the oxygen, and a portion of the hydrogen, of 
 the acetic acid, combined with a definite weight 
 of chlorine. And he found that the relations 
 between the weights of the hydrogen in the 
 parent acid, the hydrogen removed from that 
 acid in each stage of the reaction, and the 
 chlorine which entered into combination with 
 the carbon, oxygen, and hydrogen that remained, 
 could be most simply expressed by saying that, 
 in each stage one-fourth of the total hydrogen in 
 the acetic acid was taken away, and chlorine was 
 put in place of it, in the proportion of 35 -5 parts 
 by weight of chlorine for one part by weight of 
 hydrogen removed. 
 
 Dumas' results may be pictured crudely in the 
 following diagrams. 
 
 (1) (2) (3) (4) 
 
 D O 
 
 D O 
 
 These diagrams represent the compositions of 
 acetic acid and the three acids obtained by the 
 reactions of chlorine with it ; in each, the symbol 
 
 I \J is meant to represent the sum of the weights 
 
 of carbon and oxygen in the reacting weight of 
 one of the acids ; this symbol is unchanged 
 throughout. The small squares represent com- 
 bining weights of hydrogen, and the small circles 
 stand for combining weights of chlorine, a combin- 
 
STUDY OF SOME COMPOUNDS OF CARBON. 123 
 
 ing weight of chlorine being 35*5 times heavier 
 than a combining weight of hydrogen. 
 
 The compositions of the four acids are repre- 
 sented in ordinary chemical formulae as follows : 
 
 C 2 H 4 2 = acetic acid. 
 (1)C 2 H 3 C10 2 ; (2) C 2 H 2 C1 2 2 ; (3) C 2 HC1 3 2 
 
 (1), (2), and (3) are the new acids produced from 
 the parent acetic acid. 
 
 In these reactions Dumas said he had substi- 
 tuted chlorine for hydrogen without destroying the 
 general chemical character of the acetic acid. 
 The three new acids were proved by him to 
 be very like acetic acid in their chemical be- 
 haviours ; he called them monochloracetic, dichlor- 
 acetiCj and trichloracetic acid, respectively. These 
 four compounds, said Dumas, belong to the same 
 type or class. The removal of a definite weight 
 of hydrogen, and the simultaneous substitution 
 for it of a definite weight of chlorine, has not been 
 accompanied by a falling to pieces of the whole 
 building. This process of substitution is like 
 that of removing two or three wooden beams, or 
 it may be a number of stones, from a house, and 
 replacing them by a single beam of iron ; the 
 house does not fall to pieces if the operation is 
 done with care. But as there are limits beyond 
 which the substitution of one kind of material 
 for another cannot be carried in a building 
 without imminent risk of collapse, so chemical 
 substitution cannot be repeated indefinitely. In 
 the case we are considering there seems room for 
 one other repetition of the process. The last of 
 
124 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 the three acids still contains hydrogen (see the 
 diagrams, p. 122, or the formulae, p. 123) ; cannot 
 this, too, be taken away and a compensating 
 weight of chlorine be put in its place 1 Experi- 
 ments shew that if the reaction with chlorine is 
 pushed further than the third stage, by raising 
 the temperature, new compounds are produced 
 which are totally unlike acetic acid ; that, in 
 other words, there is a general breaking up of 
 the whole building. 
 
 In a process of substitution, an element, or a 
 group of elements, is taken out of a compound, 
 a part of a chemical building is removed, and 
 another element, or group of elements, is put 
 into the vacant place, another kind of building 
 material is put into the building in the place of 
 that which was removed. The new compound is 
 not the same as the original compound, the new 
 building differs somewhat from the old building ; 
 but both compounds are much alike, both build- 
 ings are of the same class or type. As the 
 removal of some wooden supports, with perhaps 
 a couple of cartloads of bricks and rubble, from 
 above the windows of a shop, and the substitution 
 of a single iron beam in the place of the material 
 removed, and a single large window for the 
 several smaller windows taken away, improves 
 the shop, but does not change it into something 
 which is not a shop, so the process of substituting 
 one element for another, or one group of elements 
 for another, does not necessarily destroy the 
 general chemical character of the compound that 
 undergoes the process, for the compound pro- 
 duced by the substituting process may belong to 
 
STUDY OF SOME COMPOUNDS OF CARBON. 125 
 
 the same type as the original compound. But if 
 too large a hole is made in the supporting wall of 
 the shop that is undergoing restoration, or if the 
 eradication of the rubble is carried too far, the 
 whole edifice may fall about the ears of the 
 workmen. So, too, processes of chemical substi- 
 tution may change into processes of complete 
 disintegration ; the original compound may be 
 broken up, and compounds may be produced 
 which differ radically from it. And, finally, 
 when the hole has been made in the wall of the 
 shop, and the bricks have been cleared out, and 
 the place has been prepared for the new beam, 
 a strong and rigid beam must be put in, else the 
 weight pressing on it will crush the new support, 
 and the purpose of the substitution will be 
 defeated. Similarly, not any element, and not 
 any group of elements, in a compound can be 
 substituted by any other element or group ; that 
 which is substituted must be chemically like that 
 whose place it occupies, else the whole edifice 
 will topple over. 
 
 Let us consider some cases of substitution, 
 with the object of attaining a clearer understand- 
 ing of this conception which was declared by 
 Liebig to be the foundation of organic chemistry. 
 If a mixture of alcohol and oil of vitriol is heated 
 in a flask connected with a condensing apparatus, 
 and alcohol is allowed to trickle very slowly into 
 the hot mixture, ether, which is a liquid with a 
 pleasant odour, distils over. If ether is digested 
 with water alcohol is reproduced. If alcohol is 
 heated for some time with acetic acid (the acid of 
 vinegar) a liquid with a markedly ethereal odour 
 
126 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 is produced. And, finally, it is possible by 
 various reactions to pass from alcohol to several 
 other liquid compounds all of which have more 
 or less ethereal odours. Liquids which smell 
 alike are not necessarily chemically alike ; but 
 similarity of odour may go with similarity of 
 other properties and of composition. The various 
 pleasantly smelling liquids obtained by reactions 
 between alcohol and different compounds are 
 purified, by distilling each until it boils at a con- 
 stant temperature, and are then analysed quanti- 
 tatively, and the results are expressed by indi- 
 cating the number of combining weights of each 
 element that are united in each compound. 
 
 It may be well to indicate by an example the 
 method whereby the results of the analysis of a 
 compound are expressed in terms of the numbers 
 of combining weights of the different elements in 
 the compound. Take alcohol ; analyses shew that 
 the percentage composition of this compound is 
 
 Carbon 52 -2 
 
 Oxygen 3 4 '8 
 
 Hydrogen 13'0 
 
 100-0 
 
 The combining weights of carbon, oxygen, and 
 hydrogen are 12, 16, and 1 respectively. Alcohol 
 is composed of a certain, whole, number of com- 
 bining weights of each of these elements ; how 
 many combining weights of each must there be 
 that the following equation may be satisfied ? 
 
STUDY OF SOME COMPOUNDS OF CARBON. 127 
 
 (12 xn): (16 xm): (lx^) = 52'2: 34'8 : 13. 
 
 n = number of combining weights of carbon ; 
 
 m= oxygen; 
 
 P= ,, hydrogen. 
 
 The first thing to be done is to divide the num- 
 ber expressing the weight of each element in 100 
 parts by weight of the compound by the com- 
 bining weight of the element. The results of 
 doing this are : 
 
 52-2 
 
 - = 4*35 combining weights of carbon ; 
 
 34-8 
 
 jg- = 2-17 oxygen; 
 
 13 
 
 =13 hydrogen. 
 
 These results must now be expressed in whole 
 numbers. Dividing the smallest number, 2 -17 
 (the number belonging to oxygen), into each of 
 the others, we have 
 
 M ,Q K 1 O 
 
 ^Tj-y = 2 ; and ^rp? = 6 (almost exactly). 
 
 That is, whatever be the number of combining 
 weights of oxygen, there are twice as many com- 
 bining weights of carbon, and six times as many 
 combining weights of hydrogen. In other words, 
 the elements are combined in the ratio of one 
 combining weight of oxygen to two combining 
 weights of carbon, to six combining weights of 
 hydrogen ; and the simplest formula which ex- 
 presses the composition of the compound is 
 OC 2 H 6 . 
 
128 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 The results of the analyses of the other plea- 
 santly smelling liquid compounds obtained from 
 alcohol are treated in this way ; the following 
 formula express some of the results : OC 2 H 6 
 alcohol ; OC 4 H 10 ether ; O 2 C 4 H 8 by heating 
 alcohol with acetic acid; NC 2 3 H 5 by heating 
 alcohol with nitric acid; C1C 2 EL by heating 
 alcohol with hydrochloric acid ; IC 2 H 5 by heat- 
 ing alcohol with hydriodic acid. 
 
 The reactions of formation, and many other 
 reactions, of these compounds establish close 
 similarities between the chemical properties of 
 the conipounds ; their compositions are, there- 
 fore, probably similar also. But the formulae 
 given above do not suggest definite resemblances 
 of composition. Let us employ the conception 
 of substitution. It is no good making fanciful 
 guesses about the substitution of this element 
 for that, or the substitution of one group of 
 elements for some other group ; the conception 
 is useful only when the chemical reactions of 
 a series of compounds have been thoroughly 
 examined and classified. In the present cases 
 we must assume the main results of such an 
 examination. 
 
 The reactions of alcohol shew great resem- 
 blances to those of potash and soda. Now 
 potash and soda are compounds of hydrogen, 
 oxygen, and potassium, and hydrogen, oxygen, 
 and sodium, respectively ; these compounds are, 
 therefore, called hydroxide of potassium arid 
 hydroxide of sodium. Potash and soda react 
 with acids to form salts ; for instance, potash and 
 hydrochloric acid produce potassium chloride, 
 
STUDY OF SOME COMPOUNDS OF CARBON. 129 
 
 and soda and nitric acid produce sodium nitrate. 
 Again, there is an oxide of potassium which 
 reacts with water to form the hydroxide (potash), 
 and there is an oxide of sodium which reacts 
 with water to form the hydroxide (soda). Since 
 alcohol is like potash and soda, in its chemical 
 reactions, it is probably a compound of hydrogen 
 and oxygen with some element, or elements, 
 chemically like potassium or sodium ; it is prob- 
 ably a hydroxide. And if alcohol is a hydroxide, 
 it will likely react with acids to form salts ; 
 moreover an oxide may exist related to alcohol 
 in a way like that wherein oxide of potassium is 
 related to hydroxide of potassium, and oxide of 
 sodium is related to hydroxide of sodium. The 
 formulae which express the compositions of potash 
 arid soda, respectively, are KOH and NaOH. 
 If alcohol is a hydroxide, analogous in com- 
 position to these compounds, the formula for 
 alcohol, OC 2 H 6 , must be expressed so as to 
 indicate the composition of a hydroxide ; this 
 may be done by writing the formula (C 2 H 5 )OH. 
 Look at the two formulae, KOH and (C 2 H ? )OH ; 
 the bracket enclosing the symbols C 2 H 5 in the 
 second formula is meant to imply that two com- 
 bining weights of carbon and five combining 
 weights of hydrogen are held firmly together. 
 The two formulae are comparable ; one is the 
 expression, in a certain language, which may be 
 called the language of the law of combining 
 weights, of the composition of a hydroxide of a 
 metal (potassium) ; the other is the expression, in 
 the same language, of the composition of a hydr- 
 oxide of a group of elements itself composed of 
 I 
 
130 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 two combining weights of carbon and five com- 
 bining weights of hydrogen. When potassium 
 hydroxide reacts with acetic acid the salt 
 potassium acetate is produced ; if the reaction 
 of alcohol with acetic acid is chemically like the 
 reaction of potassium hydroxide, we should expect 
 a salt to be produced differing from potassium 
 acetate in having the group C 2 H 5 in place of the 
 metal potassium. Here are the formulae which 
 express the compositions of the two products : 
 
 Potassium acetate Acetate produced from alcohol 
 KC 2 H 3 2 . (C 2 H 6 )C 2 H 3 2 . 
 
 The composition of the compound obtained 
 by heating alcohol, and acetic acid was already 
 expressed by the formula O 2 C 4 H 8 (see p. 128); 
 the composition of the same compound is now 
 expressed as (C 2 H 5 )C 2 H 3 2 . But the second of 
 these formulae is merely what may be called a 
 dissection of the first, adopted to bring out the 
 similarity between the reactions of this compound 
 and those of potassium acetate. 
 
 Sodium nitrate is produced by the reaction 
 between sodium hydroxide (soda) and nitric 
 acid ; the formulas which express the composi- 
 tion of this salt, and that of the product of 
 reacting on nitric acid with alcohol, are 
 
 Sodium nitrate Nitrate produced from alcohol 
 
 NaNO s . (C 2 H 5 )N0 8 . 
 
 The compositions of sodium chloride and potas- 
 sium iodide, on the one hand, and the com- 
 positions of the products of the interaction be- 
 tween hydrochloric, and hydriodic, acid 
 
STUDY OF SOME COMPOUNDS OF CARBON. 131 
 
 alcohol, on the other hand, are expressed by 
 the formulae ; 
 
 Sodium Potassium Chloride from Iodide from 
 chloride iodide alcohol alcohol 
 
 NaCl. KI. (C 2 H 5 )C1. (C 2 H 6 )I. 
 
 Finally, the compositions of sodium (or potas- 
 sium) oxide, and ether, are expressed thus ; 
 
 Sodium oxide Potassium oxide Ether 
 
 Na 2 0. K 2 0. (C 2 H5) 2 0. 
 
 Gathering together the formulae which express 
 the compositions of various compounds derived 
 from alcohol, when the relations between these 
 compounds and alcohol are looked on as similar 
 to the relations between potash and various 
 compounds obtained therefrom, and placing these 
 formula side by side with those of the corre- 
 sponding potash compounds, we have the follow- 
 ing results. 
 
 (C 2 H 5 )OH A Icohol. KOH potash. 
 
 (CsHs^O Ether. E^O potassium oxide. 
 
 (C 2 H 5 )C1 Ethyl chloride. KC1 potassium chloride. 
 
 (C2H5JI Ethyl iodide. KI potassium iodide. 
 
 iCsfisiNOs Ethyl nitrate. KNOs potassium nitrate. 
 
 (C2H5J C2Hs02 Ethyl acetate. KC 2 H 3 02 potassium acetate. 
 
 Not only are the two compounds in any one of 
 these lines similar in chemical properties, they 
 are also represented as having similar com- 
 positions. And the similarities of composition 
 are brought out by supposing that the quantity 
 by weight of a group of two elements expressed 
 by the formula C 2 H 5 can be substituted for one 
 combining weight of the element potassium. 
 The group C^3- 5 is called Ethyl ; it is composed 
 of two combining weights of carbon in chemical 
 union with five combining weights of hydrogen. 
 
132 THE STORY OF THfe CHEMICAL ELEMENTS. 
 
 This group can take the place of one combining 
 weight of potassium in various compounds with- 
 out altering the general chemical characters of 
 these compounds. The compounds of potassium 
 formulated above are very unlike the correspond- 
 ing ethyl compounds in their physical properties ; 
 for instance, the potassium compounds are all 
 white, odourless, solids, while the ethyl com- 
 pounds are all colourless, sweet-smelling, liquids. 
 But corresponding compounds in the two series 
 shew such distinct chemical similarities that 
 they must be regarded as belonging to the same 
 chemical type. 
 
 The considerations glanced at in the foregoing 
 paragraphs lead to the recognition of the possi- 
 bility of a group of elements taking the place of 
 a single element without destroying the general 
 chemical character of the compound wherein this 
 substitution occurs. This recognition has been 
 gained by applying the principle of substitution ; 
 it is indeed a part of the conception of substitu- 
 tion, nevertheless it is a most important advance 
 from the earlier notion of substitution being 
 effected by putting one element in the place of 
 another element. The name compound radicle is 
 given to a group of elements which can be, 
 theoretically, moved from compound to com- 
 pound without the breaking up of the group. 
 A moment's consideration will show, I think, 
 that the word theoretically, or some equivalent 
 term, must be introduced into the description of 
 the compound radicle. Compare the compounds 
 of the element potassium with those of the com- 
 pound radicle ethyl (C 2 H 5 ). The compounds of 
 
STUDY OF SOME COMPOUNDS OF CARBON. 133 
 
 potassium can be broken up and potassium can 
 be isolated from every one of them ; further, 
 various elements can be added on to potassium, 
 and the diffeient compounds of that element can 
 thus be produced. But the compounds of ethyl 
 cannot be broken up in such a way that the com- 
 pound radicle ethyl may be isolated ; this group 
 of elements has no existence by itself ; nor can 
 compounds of ethyl be produced by actually join- 
 ing various elements on to ethyl, for ethyl can- 
 not, so to speak, be held in the hand. The 
 compound radicle ethyl is a theoretical con- 
 ception introduced for the purpose of enabling 
 chemists to represent similarities of properties 
 as accompanied by similarities of composition. 
 Grant the hypothesis, and order is palpably 
 introduced, where, without the hypothesis, order 
 cannot be expressed. The facts shew that order 
 is there ; but we require a language wherein the 
 orderly facts may be expressed, and after many 
 trials we have hit upon the language that uses 
 the conception of the compound radicle, and the 
 conception of the possibility of substituting com- 
 pound radicles, not only for other compound 
 radicles, but also for single elements. 
 
 When we come to speak of the theory that 
 gives a mechanical conception of the mechanism 
 of chemical reactions, we shall find that the 
 notion of the compound radicle becomes clearer 
 and more precise. 
 
 Organic chemistry is built on the foundation of 
 the conception of substitution, and that founda- 
 tion is strengthened by working in with it the 
 subsidiary conception of the compound radicle. 
 
134 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 It is not only in the domain of chemical science 
 that great advances have been made by employ- 
 ing the notions of substitution and the com- 
 pound radicle. Large and flourishing chemical 
 industries are based on the practical applications 
 of these conceptions; among these industries 
 there is perhaps none so important as the manu- 
 facture of aniline colours. Because of the name 
 " coal-tar colours," by which these dye-stuffs are 
 often known, there seems to be a fairly wide- 
 spread belief that the aniline colours are hidden 
 in coal-tar, and that they are got out of that 
 ugly black stuff by processes of purification. 
 But this is quite a mistake. A colourless liquid 
 called benzene is obtained by distilling coal-tar ; 
 when benzene is treated with concentrated aqua 
 fortis (nitric acid), mixed with some oil of vitriol, 
 an oily liquid, smelling like oil of almonds, is 
 formed ; and when this nitro-benzene is warmed 
 with iron filings and acetic acid, aniline distils 
 over. Aniline is a colourless liquid ; its com- 
 position is expressed by the formula (C 6 H 5 )NH 2 . 
 The compound radicle C 6 H 5 is called phenyl. 
 A very large number of compounds, many 
 of which are dyes, is prepared by substi- 
 tuting some of the hydrogen in the phenyl 
 radicle of aniline by other compound radicles ; 
 compounds with dyeing properties are also pro- 
 duced by substituting various compound radicles 
 for one or two of the combining weights of 
 hydrogen represented in the formula of aniline 
 as not forming part of the phenyl radicle. Then 
 some of the derivatives of aniline obtained by 
 the substitution of compound radicles for hydro- 
 
STUDY OF SOME COMPOUNDS OF CARBON. 135 
 
 gen react and produce more complex compounds, 
 from which again definite numbers of combining 
 weights of hydrogen, or of other elements, are 
 removed, and their place is taken by definite 
 quantities of more or less complicated compound 
 radicles. And so the preparation of new com- 
 pounds proceeds, one may say in every case by 
 processes that are essentially substitutions ; the 
 relations of the compounds to one another are 
 often extremely complex, but the principle of 
 substitution always remembering that a group 
 of elements may be put in the place of a single 
 element without changing the essential chemical 
 character of the compound is a guide through 
 all the complicated wanderings. 
 
 Of course, as the number of what may be 
 called substituted anilines increases, various sub- 
 sidiary hypotheses are used as helps in deter- 
 mining the relations of these compounds to one 
 another; but the guiding principle is substitu- 
 tion ; not haphazard attempts to put this or that 
 group in place of some other group, but regu- 
 lated, orderly, substitutions based on the results 
 of accurate quantitative experiments, wherein 
 properties are constantly connected with com- 
 position. The performance of various processes 
 of substitution, and the comparisons of the dye- 
 ing properties of the products with those of the 
 parent substances, lead to suggestions that if 
 the substituting radicle is altered, or perhaps if 
 substitution by the same radicle is not carried 
 quite so far, or, it may be, is carried a little 
 farther, the products so obtained will probably 
 dye a richer tint than any of those dyes that 
 
136 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 have been tried already ; and the experiment is 
 made, and the special tint which the new com- 
 pound dyes forms the basis of other experi- 
 ments. 
 
 It is evident that processes of this kind cannot 
 be conducted except by those who have made a 
 minute, experimental, critical study of a very 
 large field of chemical inquiry. Unless a man is 
 thoroughly at home in the carrying out of com- 
 plicated processes of substitution among the 
 derivatives of aniline and hundreds of these 
 derivatives are known to-day it would be 
 absurd for him to attempt the preparation of 
 a new colouring matter by first comparing the 
 dyeing properties and the compositions of the 
 known compounds, then predicting, theoretically, 
 the composition of the compound with the special 
 colour he wants, and then setting about the prac- 
 tical fulfilment of his prediction. To set a man 
 to such a task without making sure that his 
 training fitted him for it would be like setting 
 one who had dabbled a little with bricks and 
 mortar to raise the roof of a large building, to 
 substitute for the present upper row of windows 
 in the side wall a smaller number of much 
 larger windows, to raise the wall some feet, and 
 then to set the roof on again. Any one who 
 asked a man not very especially skilled in 
 such work to undertake the change I have 
 indicated in the building would be called a 
 fool. And yet hundreds of attempts have 
 been made to carry out the more difficult tasks 
 of the chemical builder by men who profess 
 that a little practice is worth a pound of theory 
 
STUDY OF SOME COMPOUNDS OF CARBON. 137 
 
 There was a time when most of the trade in 
 aniline colours, and the dye stuffs related to 
 them, was in the hands of English manufac- 
 turers ; the trade has now become a German 
 industry, simply because the German manufac- 
 turer has either been a trained chemist or he 
 has had the sense to understand that the prob- 
 lems at the root of the industry are too com- 
 plicated to be attacked by anyone who is not 
 a trained, and a truly scientific, chemist. To 
 attempt to gain, even in outline, any grasp of 
 the principles on which the aniline industry 
 rests, means to make a thorough and accurate 
 study, in the laboratory and in the lecture-room, 
 first of chemistry as a whole, then of organic 
 chemistry as an especial branch of the science, 
 and then of aniline and its derivatives and allied 
 compounds as a peculiar department of organic 
 chemistry. For the aniline industry is truly a 
 chemical industry, and the note of all chemistry 
 is change. The rules that hold good to-day 
 will not suffice to meet the demands of the 
 trade to-morrow. Fashion in colours changes ; 
 and the manufacturer who can supply only his 
 few stock colours soon falls out of the running. 
 
 There is another department of applied 
 chemistry in which the regulated use of the 
 principle of substitution has led to astonish- 
 ing results, and that is the making of new 
 drugs with specific properties. Take, for in- 
 stance, some of the more recently introduced 
 febrifuges. The study of the reactions of 
 quinine, and of the reactions of compounds 
 obtained from quinine under definite condi- 
 
138 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 tions, has led to the establishment of certain 
 connexions between the presence of definite 
 compound radicles, related to one another in 
 definite ways, and the existence of fever-abating 
 properties. Although these studies have not 
 yet enabled the chemist to build up quinine 
 itself, they have enabled him to put together 
 chemical structures, by arranging definite com- 
 pound radicles in definite ways, which have 
 marked effects on the bodily temperature of 
 human beings. Antipyrine is one of these com- 
 pounds, and phenacetin is another. It is im- 
 possible in such a book as this to attempt to 
 describe the constitutions of these compounds ; 
 for the descriptions must be recorded in language 
 that is meaningless to anyone other than a 
 trained chemist. Suffice it to say that both 
 compounds are constructed by building up cer- 
 tain compound radicles on the basis of nitrogen, 
 and that the radicles are arranged in a perfectly 
 definite manner relatively to one another. Why 
 the building processes should be conducted in 
 this or that particular direction ; why this 
 building stone that is, this compound radicle 
 should be used rather than that ; reasons for 
 these things have been found in the study of 
 the reactions whereby bodies with properties 
 like those it is desired to impart to the new 
 compounds have been broken down into their 
 constituent compound radicles. Buildings with 
 properties of the desired kind have been taken 
 to pieces : the nature, that is, the compositions 
 and the properties, of the pieces have been 
 examined ; the positions of the pieces in the 
 
STUDY OF SOME COMPOUNDS OF CARBON. 139 
 
 original buildings have been noted; and then 
 similar buildings have been constructed ; and 
 in many cases the new buildings have possessed 
 the wished-for properties. 
 
 Now it has been said that those complex 
 chemical building stones which are called 
 compound radicles cannot be isolated (refer 
 back to p. 133) ; and yet I have spoken of dis- 
 secting a compound into the compound radicles 
 that compose it. Both statements are accurate. 
 Only in the processes of dissection the radicles 
 are not obtained wholly apart from combination 
 with other things ; compounds of the compound 
 radicles are obtained, and the existence of the 
 radicles in these compounds is inferred from 
 the reactions of the compounds. It is this de- 
 termination of the compound radicles to remain 
 hidden that makes the conduct of definite 
 building operations with these radicles so difficult. 
 The chemist who is endeavouring to construct 
 such a complicated structure as antipyrine need 
 have a clear and penetrating imagination ; he 
 has constantly to see the inner structure of the 
 buildings he is trying to copy, and yet he cannot 
 isolate those building stones which it is essential 
 for him to work with. He has, as it were, to 
 lift his building stones with a stick, which has 
 the peculiar property that the moment a stone is 
 poised on it the stone and the stick become one, 
 neither the stick nor the stone has any longer an 
 independent existence. And when he puts the 
 stone into its place, feeling about as best he 
 can, the stone vanishes into the building ; he 
 only knows that the stone is there by constantly 
 
140 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 testing the whole building, perhaps a knock with 
 his mallet gives a different sound from what he 
 heard before the last stone was added to the 
 building. And then, when he looks for the stick 
 that he may lift another stone into its place, 
 very often the stick has vanished too, and the 
 .chemist must set about making a new one. 
 
 The chemical building trade, at least in its 
 more refined branches, is even more difficult than 
 the illustration I have used would lead one to 
 expect. The chemist is all the time working 
 with hypothetical "building materials. The ele- 
 ments are definite kinds of matter that can be 
 touched and handled, and put together, and 
 separated ; but comparatively little is done by 
 working with the elements in the erection of 
 the complicated buildings of organic chemistry. 
 There, groups of elements are the units of the 
 edifices ; and on scarcely one of these building 
 units is the chemist able confidently and definitely 
 to lay his hands. He can only conjecture that 
 they are in the buildings, because by no other 
 hypothesis has he been able to picture to himself 
 the existence of an orderly arrangement of the 
 parts of these buildings ; but the orderly pro- 
 perties of the buildings force him to postulate 
 for them an orderly composition also. 
 
 The chemist has to guide his operations by the 
 light of an hyp >thesis ; mere facts are quite 
 insufficient for him. And he has to state that 
 hypothesis in an exceedingly symbolic language, 
 a language which makes constant use of such ab- 
 stract conceptions as substitution and compound 
 radicle. It is true that the language gains in 
 
THE ATOM AND THE MOLECULE. 141 
 
 clearness and accuracy as it developes ; but it 
 gains in these qualities by founding itself on 
 another hypothesis, that, namely, of the atom and 
 the molecule. It is the laboratory which saves 
 chemistry from artificiality. It is the constant 
 contact with acids, and alkalis, and salts, and evil 
 smelling and sweetly odorous definite substances ; 
 it is the constant handling of distilling appa- 
 ratuses, and flasks, and beakers, and measuring 
 vessels, and balances ; it is this blessed walking 
 on the solid earth that gives an intense reality to 
 the science which is concerned with abstract con- 
 ceptions of composition and properties, and which 
 is obliged to clothe its conceptions in the imagery 
 of a symbolic language. 
 
 CHAPTER VIII. 
 
 THE ATOM AND THE MOLECULE. 
 
 THE especial problem of chemistry is to trace 
 definite connexions between the compositions 
 and the properties of compounds. The composi- 
 tions of compounds are stated in terms of the 
 elements which unite to form these compounds, 
 and which can be obtained by disintegrating the 
 compounds ; and the quantities of the elements 
 which are united in the compounds are expressed 
 as numbers of combining weights of the ele- 
 ments. We found in the last chapter that cer- 
 tain connexions between some of the chemical 
 properties of many compounds can be suggested 
 by the formulae which express the compositions 
 
142 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 of these compounds, only on the assumption that 
 certain groups of elements behave like single 
 elements, and that such groups, or compound 
 radicles as they are called, can be substituted for 
 one another, or for single elements, \vithout 
 destroying the general chemical type to which 
 the compounds belong. This result might be 
 stated in general terms by saying that the 
 primary notion of composition, as expressed in 
 quantities of individual elements, must be modi- 
 fied to include the notion, of the compound 
 radicle, in order that the observed properties of 
 compounds, and especially of carbon compounds, 
 may be represented as connected in definite 
 ways with the compositions of the same com- 
 pounds. 
 
 Let us now turn back to the first building up 
 of a compound of peculiarly organic origin from 
 materials derived from inorganic sources ; let 
 us revert to the synthesis of urea by Wohler in 
 1828 (see p. 119). The substance from which 
 urea was immediately produced by Wohler was 
 a salt called ammonium cyanate. There is one 
 very peculiar thing about these two compounds, 
 urea and ammonium cyanate; both have abso- 
 lutely the same elementary composition. Each 
 compound contains 46*66 per cent, of nitrogen, 
 26*67 percent, of oxygen, 20*0 percent, of carbon, 
 and 6 67 per cent, of hydrogen: the composition 
 of each is expressed by the formula N 2 OCH 4 . 
 But the properties of the two compounds differ 
 in the most marked way ; they belong to wholly 
 different classes of compounds. The direction 
 wherein some explanation of this phenomenon 
 
THE ATOM AND THE MOLECULE. 143 
 
 seems likely to be found is the application to 
 it of the conception of the compound radicle. 
 Is ammonium cyanate constructed by putting 
 together groups of elements different from ^ those 
 which form urea 1 Before attempting to find an 
 answer to this question we must have some 
 clearer notions than we now possess about the 
 putting together of elements and groups of 
 elements ; and the only way in which such 
 clearer notions can be gained is by framing a 
 consistent hypothesis regarding the mechanism 
 of chemical reactions. 
 
 If the expressions that have been used in pre- 
 ceding chapters about chemical buildings, the 
 construction of chemical edifices, and the like 
 occurrences, are considered carefully, a tacit 
 assumption will be found running through them 
 to the effect that each element is composed of 
 small, but definite, portions of matter, all of 
 which have the properties of that element. The 
 illustrations that have been employed to throw 
 light on processes of chemical construction, as 
 well as the language that has been used to 
 express these processes have almost taken for 
 granted, although they have not expressly 
 stated, that all kinds of homogeneous substances 
 have a grained, and not a jelly-like, structure. 
 It is into this subject, the structure of definite 
 kinds of matter, that we must now inquire. 
 
 About the beginning of the century a Quaker 
 gentleman, called John Dalton, living in Man- 
 chester, who had been a schoolmaster but at 
 that time supported himself by giving lessons in 
 mathematics and natural philosophy, was think- 
 
144 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 ing a great deal about the pressure of gases, the 
 expansion of gases by heat, and the solubilities 
 of gases in different liquids. Being a man of 
 vivid imagination in all that pertained to the 
 scientific questions which interested him most, 
 Dalton was in the habit of seeing, with his mind's 
 eye, the ultimate structure of the ga-es that 
 formed the subject of his inquiries. He says, " A 
 vessel full of any pure elastic fluid [that is, any 
 gas] presents to the imagination a picture like 
 one full of small shot/' But Dalton was not con- 
 tented with picturing to himself what he con- 
 ceived to be the structure of the gases he was 
 thinking about ; he tried to connect that struc- 
 ture with the properties of these gases. Now he 
 found, from his experiments and those of others, 
 that water dissolves different volumes of different 
 gases at the same temperature arid pressure ; he 
 thought of the particles of the various gases, like 
 particles of small shot, being pushed between the 
 particles of water, until there was no room for 
 more at that pressure and temperature ; but, he 
 asked, if this is what happens, why does not 
 water dissolve the same volume of all gases when 
 the temperature and pressure are kept the same ? 
 To this question Dalton gave an answer which 
 has been of enormous importance to chemistry : 
 "I am nearly persuaded that the circumstance 
 depends upon the weight and number of the 
 ultimate particles of the several gases, those 
 whose particles are lightest and single being least 
 absorbable', and the others more, according as 
 they increase in weight and complexity/' And 
 to that answer he added a statement which has 
 
THE ATOM AND THE MOLECULE. 145 
 
 been even more important in aiding the advance 
 of chemistry than the answer itself: "An in- 
 quiry into the relative weights of the ultimate 
 particles of bodies is a subject, so far as I know, 
 entirely new. I have lately been prosecuting 
 this inquiry vvith remarkable success. The prin- 
 ciple cannot be entered upon in this paper, but 
 I shall just subjoin the results, as far as they 
 appear to be ascertained by my experiments." 
 Dalton appends a " Table of the relative weights of 
 the ultimate particles of gaseous arid other bodies." 
 That list is the first example of what we should 
 now call a " Table of atomic weights." 
 
 When Dalton pictured to himself "a vessel 
 full of a gas as like one full of small shot," he 
 was doing what had been done by Greek philo- 
 sophers about 2500 years before him ; but when 
 he connected a definite and measurable pheno- 
 menon with differences in the weights of the 
 ultimate particles of different bodies, and, more 
 especially, when he shewed (in later publications 
 than that I have quoted from) how to determine 
 the relative weights of these hypothetical ulti- 
 mate particles, he converted the interesting 
 speculation of the Greeks into a scientific and 
 science-producing theory. 
 
 I cannot enter into the reasoning whereby 
 Dalton was led in the first instance to assign cer- 
 tain relative values to the weights of the ulti- 
 mate particles, or atoms, of various elements; 
 the reasoning, and the experiments which sup- 
 plied the data, were rather physical than chemi- 
 cal. But at a slightly later time Dalton enunciated 
 the law of combining weights as a necessary 
 
 K 
 
146 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 deduction from his theory of the structure of 
 matter; and then the reasoning whereby the 
 relative weights of the atoms of the elements 
 were determine 1 ran somewhat on the following 
 lines. 
 
 The theory began by assuming that every 
 definite kind of matter, that is, every element 
 and compound, has a grained structure ; of 
 course, if all elements and compounds have a 
 grained structure, mixtures must have a similar 
 structure. But what exactly is meant by a 
 grained structure ? Merely that any definite sub- 
 stance fills space as apples fill a barrel, and not 
 as jelly fills a mould; that if one could magnify 
 enormously a small portion of any definite sub- 
 stance, say water, one would see an immense 
 heap of extremely small particles of water piled 
 together with interstices between them ; a grained 
 structure means exactly what Dalton's picture 
 of a vessel full of small shot meahs Then the 
 theory asserted that every individual grain of 
 any definite kind of matter is exactly like every 
 other grain of the same kind of matter, alike in 
 weight and in all other properties. Dalton 
 called these minute grains of elements and com- 
 pounds atoms, but as he applied the term both to 
 elements and compounds it is evident that he 
 did not use the term in its strict, etymological, 
 meaning of something ivhich cannot be cut. We 
 are assured that Dalton was very fond of 
 clenching a chemical argument with the 
 statement : " Thou knowest thou canst not cut 
 an atom." He evidently meant, you can't 
 cut an atom without destroying the essential 
 
THE ATOM AND THE MOLECULE. 147 
 
 properties of the substance ; if you cut an atom 
 of water, for instance, you have no longer water, 
 but oxygen and hydrogen. Dalton evidently 
 thought of the atom of an element as, practically 
 at any rate, indivisible. It is well to notice here 
 that the Daltonian theory does not concern itself 
 with the question, which lies outside the domain 
 of physical science, of the finite or infinite divisi- 
 bility of matter. Granting, then, that matter has 
 a grained structure, that all the grains of any one 
 kind of matter are identical in weight and other 
 properties, and that these grains, or atoms, cannot 
 be cut without the disappearance of the special 
 kind of matter of which each grain is a minute 
 portion, the theory went on to assert that the 
 chemical union of elements consists in the join- 
 ing of definite numbers of atoms of elements into 
 more complex atoms, which are atoms of com- 
 pounds ; and that chemical interaction between 
 compounds consists in, first, the breaking up of 
 the atoms of the reacting compounds, and then 
 the rearrangement of the elementary atoms so 
 produced in new combinations, that is, so as to 
 form new compound atoms. 
 
 Having thus cleared the ground, let us see how 
 the relative weights of the atoms of the elements 
 were determined after the enunciation of the 
 law of combining weights. As hydrogen is the 
 lightest kind of matter known, that element was 
 taken as the standard in terms of which the 
 relative weights of atoms were to be expressed ; 
 the weight of an atom of hydrogen was called one, 
 and the object was to find out how many times 
 an atom of each element was heavier than an 
 
148 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 atom of hydrogen. Take the case of the union 
 of hydrogen and oxygen ; weights of these ele- 
 ments combine in the ratio of one part of hydro- 
 gen to eight parts of oxygen ; but the act of 
 combination consists, according to the theory, in 
 the joining of atoms of the two elements ; there- 
 fore, one atom of oxygen weighs eight times 
 more than one atom of hydrogen ; and, as the 
 atomic weight of an element is the number which 
 tells how many times an atom of that element is 
 heavier than an atom of hydrogen, the atomic 
 weight of oxygen is eight. Or take the case of 
 hydrogen and chlorine ; the weights of these 
 elements which combine stand to one another in 
 the ratio of 1 to 35*5; therefore, as the act of 
 combination is asserted by the theory to consist 
 in the joining of atoms of hydrogen, all of which 
 are identical in weight and other properties, with 
 atoms of chlorine, all of which are also identical 
 in weight and other properties, but all of which 
 differ in weight as well as in other properties 
 from atoms of hydrogen, it follows that one atom 
 of chlorine is 35*5 times heavier than one atom 
 of hydrogen, in other words, that the atomic 
 weight of chlorine is thirty-five and a half. Or 
 take the case of sulphur ; inasmuch as the weights 
 of sulphur and hydrogen which combine are in 
 the ration of 16 to 1, it follows, by reasoning 
 similar to that already used, that the atomic 
 weight of sulphur is sixteen. 
 
 The relative weights of the atoms of the ele- 
 ments, that is, the atomic weights of the elements, 
 were obtained by the use of the Daltonian 
 theory in exactly the same way as the relative 
 
THE ATOM AND THE MOLECULE. 149 
 
 values of the combining weights of the elements 
 were obtained ; both atomic and combining 
 weights were stated in terms of hydrogen as 
 unity, because hydrogen weighs less, bulk for 
 bulk, than any other element. Indeed, it is 
 worthy of note that the two conceptions expressed 
 by the terms atomic weight and combining 
 weight were introduced into chemistry at the 
 same time and by the same man. 
 
 A close examination of the reasoning whereby 
 it has been asserted in the preceding paragraphs 
 that values were found by Dalton for the atomic 
 weights of the elements, will disclose a large 
 assumption which was purposely slurred over in 
 these paragraphs. Turn again to the case of 
 oxygen ; the atom of oxygen was said by Dalton 
 to weigh 8 times more than the atom of hydro- 
 gen, because 8 parts by weight of oxygen com- 
 bine with 1 part by weight of hydrogen. In 
 drawing this conclusion the assumption has 
 evidently been made that when oxygen and 
 hydrogen combine a single atom of either 
 element is joined to a single atom of the other 
 element. Suppose Dalton had chosen to assume 
 that two atoms of hydrogen combine with one 
 atom of oxygen, then, as the weights of the 
 elements that combine are in the ratio of 8 to 1 
 (oxygen to hydrogen), and as it is asserted that 
 one atom of oxygen combines with two atoms of 
 hydrogen, it is evident that each atom of oxygen 
 must weigh 16 times as much as each atom of 
 hydrogen (16:2 = 8:1). On this assumption, 
 the atomic weight of oxygen would be 16, and 
 not 8 as determined by Dalton. 
 
150 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 Suppose, again, that Dal ton had assumed that 
 two atoms of oxygen combine with one atom of 
 hydrogen, then the conclusion must have fol- 
 lowed, from the fact that eight parts of oxygen by 
 weight combine with one part of hydrogen, that 
 the atom of oxygen is four times heavier than the 
 atom of hydrogen, and on this assumption the 
 atomic weight of oxygen would be four. To 
 arrive at the value 8 for the atomic weight 
 of oxygen, Dalton made the simplest of all the 
 possible assumptions regarding the numbers of 
 atoms of hydrogen and oxygen that combine, the 
 assumption, namely, that the atoms of these 
 elements combine in the ratio of one to one. 
 
 Look for a moment at the case of nitrogen. 
 Dalton said that because 4 -66 parts by weight 
 of nitrogen combine with one part by weight of 
 hydrogen, the atomic weight of nitrogen is 4 '6 6 ; 
 the assumption was implicitly made that atoms 
 of these two elements combine in the ratio of 
 equality. But at a later time the atomic weight of 
 nitrogen was changed to 14, that is, to 4 '66 x 3 ; 
 and it was then asserted that three atoms of 
 hydrogen combine with a single atom of nitrogen, 
 to form a compound atom of ammonia (14:3 = 
 4-66 : 1). 
 
 Why should the simplest possible assumption 
 regarding the numbers of combining atoms be 
 made in some cases, and a more complex assump- 
 tion be accepted in other cases ] Briefly put, 
 the principle which guided chemists was this ; 
 make the simplest assumption you can that is in 
 keeping with the observed reactions of the com- 
 pound you are dealing with. If it is assumed, 
 
THE ATOM AND THE MOLECULE. 151 
 
 for instance, that one atom of hydrogen combines 
 with one atom of oxygen to form a compound atom 
 of water, then it should not be possible to remove 
 part of the hydrogen, or part of the oxygen, from 
 the water-atom ; if there is only one atom of 
 each element in the compound atom of water, 
 then, if hydrogen is taken away, or if oxygen is 
 removed, from water, all the hydrogen must be 
 taken away, or all the oxygen must be removed ; 
 because an atom of an element is, by definition, 
 a quantity which cannot be divided into smaller 
 parts of the same substance. Now, they said, 
 experiments prove that the hydrogen in ammonia 
 can be removed in three equal portions, but that 
 if any nitrogen is taken away from ammonia 
 all the nitrogen must be taken ; therefore, as the 
 chemical reactions of ammonia consist, in the 
 first instance, in breaking up the atom of this 
 compound, it is evident that there must be three 
 atoms of hydrogen, but only a single atom of 
 nitrogen, in the compound atom of ammonia ; 
 and therefore as the weights of the two elements 
 that combine to form ammonia are in the ratio of 
 1 to 4 '66 (hydrogen to nitrogen), it follows that 
 the atomic weight of nitrogen must be fourteen. 
 
 The principle which guided chemists in select- 
 ing one value, from several possible values, for 
 the atomic weight of an element is a principle 
 that is constantly used in all branches of natural 
 science, and in all affairs of ordinary life ; it is, 
 make the simplest assumption that is in keeping 
 with the facts, and act on that assumption. 
 
 But this principle could not be applied to 
 the determination of atomic weights either with 
 
152 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 sufficient ease or sufficient exactness. Each case 
 had to be discussed elaborately ; numbers of 
 subsidiary facts had to be marshalled, and com- 
 pared with other similar facts, or contrasted 
 with other dissimilar facts. The theory cer- 
 tainly laid down a principle capable of universal 
 application; but the application did not go far 
 enough ; it left chemists in doubt as to which 
 of several possible values for the atomic weight 
 of an element was the correct value. What the 
 theory did was to enable a value to be found 
 which, or a whole multiple, or a whole sub- 
 multiple, of which, was the value for the atomic 
 weight. It was only necessary to determine the 
 weight of an element which combined with one 
 part by weight of hydrogen ; then this weight, 
 or a whole multiple of this weight, or it might 
 be a whole sub -multiple of this weight, was the 
 relative weight of the atom of the element in 
 question in terms of the weight of the atom of 
 hydrogen taken as unity. In the case of an 
 element which refused to combine with hydro- 
 gen, all that need be done was to use oxygen as 
 a go-between, and to find the weight of the 
 element which combined with 8 parts by weight 
 of oxygen; for 8 parts by weight of oxygen 
 combine with 1 part by weight of hydrogen. 
 
 But chemists wished to be able to decide 
 which of several possible values was the correct 
 value for the atomic weight of each element. 
 Turn again to the case of oxygen. If it is 
 assumed that one atom of oxygen combines 
 with one atom of hydrogen, then, from the 
 experimental data, the atomic weight of oxygen 
 
THE ATOM AND THE MOLECULE. 153 
 
 must be 8, and the compound atom of water 
 must weigh 9 times more than the atom of 
 hydrogen ; if it is assumed that two atoms of 
 oxygen combine with one atom of hydrogen, 
 then the atomic weight of oxygen must be 4, 
 and the compound atom of water must weigh 
 9 times more than the atom of hydrogen ; and 
 if it is assumed that one atom of oxygen com- 
 bines with two atoms of hydrogen, the atomic 
 weight of oxygen must be 16, and the com- 
 pound atom of water must weigh 18 times more 
 than the atom of hydrogen. If a method can be 
 found for determining the relative weight of the 
 compound atom of water, an important step will 
 nave been taken towards determining the rela- 
 tive weight of the atom of oxygen also ; and if 
 the relative weights of a number of compound 
 atoms, all of which contain oxygen, can be de- 
 termined, data will have been obtained for find- 
 ing the true value for the atomic weight of 
 oxygen. 
 
 The shortcoming in the theory he had enun- 
 ciated was felt, and plainly acknowledged, by 
 Dal ton. He says that his object was, not only 
 to ascertain "the relative weights of the ulti- 
 mate particles both of simple and compound 
 bodies," but also to determine "the number of 
 simple elementary particles which constitute one 
 compound particle, and the number of less com- 
 pound particles which enter into the formation 
 of one more compound particle." In order to 
 attain these objects Dalton laid down certain 
 rules regarding the complexities of compound 
 particles, which helped him, and others, to 
 
154 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 arrive at consistent conclusions regarding the 
 relative weights of the atoms of the elements. 
 
 Bat rules were not sufficient ; an all-embracing 
 general principle was required, whereby the rela- 
 tive weights of the atoms of compounds could be 
 determined. Such a principle was supplied in the 
 year 1811 by the Italian naturalist, Avogadro. 
 
 Avogadro remarked two things. First he said 
 there must be some reason for the fact that when 
 gaseous elements or gaseous compounds react to 
 produce new gases, there is an extremely simple 
 relation between the volumes of all the gaseous 
 bodies concerned ; for instance, a pint of oxygen 
 combines with two pints of hydrogen to form two 
 pints of steam, and two pints of hydrochloric acid 
 gas are formed by the union of a pint of hydro- 
 gen with an equal volume of chlorine. Then he 
 noticed the undesirability of applying the same 
 term, the term atom, to an extremely minute por- 
 tion of an element, and also to an extremely minute 
 portion of a compound ; for what was called an 
 atom of a compound could be, and constantly 
 was, shattered into parts, whereas an atom of an 
 element was, by definition, that extremely small 
 quantity of the element which was never broken 
 up in any of the reactions wherein the element 
 took part. Avogadro said in effect : Let us re- 
 cognise two orders of extremely small particles ; 
 let us speak of the molecule, and also of the atom; 
 let the molecule, either of an element or a com- 
 pound, be the smallest portion of the body which 
 exhibits those properties which we recognise 
 when the body is regarded apart from the ac'ion 
 on it of other bodies; let the term atom be 
 
THE ATOM AND THE MOLECULE. 155 
 
 applied to elements only, and let it mean one of 
 those parts of an elementary molecule into which 
 that molecule is shattered when it reacts chemi- 
 cally with other molecules. Avogadro pictured 
 the molecules of elements as themselves struc- 
 tures ; he thought of these structures as remain- 
 ing intact in all those changes we call physical ; 
 and he represented chemical action between 
 elements as consisting, in the first place, in the 
 falling to pieces of the molecules of those 
 elements, and then in the arrangement of the 
 parts of molecules so produced, that is, in the 
 arrangement of the atoms, in new combinations, 
 that is, new molecules. And he represented 
 chemical action between compounds by an ex- 
 actly similar mechanism ; he pictured compound 
 molecules clashing together, and falling to pieces, 
 and then he saw the elementary atoms re- 
 arranging themselves in new groups, and so 
 producing either the molecules of new com- 
 pounds, or, if atoms of the same kind should 
 coalesce, the molecules of elements. By his re- 
 cognition of the molecule, and the atom, Avo- 
 gadro removed the distinction between the mech- 
 anisms involved in chemical reactions between 
 elements on the one hand and compounds on the 
 other hand, which the Daltonian use of the term 
 atom had tended to foster. Then, to account for 
 the great simplicity which experiments had 
 proved to exist in the relations between the 
 volumes of gases that react chemically, Avogadro 
 said : Equal volumes of elementary or compound 
 gases, measured at the same temperature and pressure, 
 contain equal numbers of molecules. This state- 
 
156 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 ment has been known for many years as Aw- 
 gadro's Law ; it is of paramount importance in 
 chemistry. 
 
 Avogadro enunciated the words I have put 
 into italics as an empirical statement ; since his 
 day it has been raised to the rank of a necessary 
 deduction from a general dynamical theory of 
 the constitution of homogeneous gases. It is 
 quite impossible in this place to go into the 
 reasoning whereon this deduction from the 
 dynamical theory of gases rests ; all that can 
 be ^ said is that the reasoning is mathematical 
 and dynamical. The truth and universal ap- 
 plicability of Avogadro's law are really assumed 
 in chemistry. 
 
 Equal volumes of gases, equal numbers of molecules. 
 Here -is a method for doing what Dalton re- 
 cognised must be done, but which he failed to 
 do, a method for finding the relative weights of 
 the molecules, or, as Dalton would have said, the 
 compound atoms, of all compounds which can 
 be gasified without decomposition. And the 
 method is also applicable to elements ; pro- 
 vided an element can be gasified without under- 
 going chemical change, the molecular weight of 
 that element can be determined by applying the 
 law of Avogadro. 
 
 Let us see how the law is used to accomplish 
 this end. All the molecules of the same gas are 
 asserted to have the same weight and to be 
 identical in properties. Suppose a pint of 
 hydrogen, a pint of oxygen, and a pint of dry 
 steam, are weighed at the same temperature and 
 pressure; and the results are stated so as to 
 
THE ATOM AND THE MOLECULE. 157 
 
 express how many times heavier than the lightest 
 of the three the other two are, that is, how many 
 times the pint of oxygen, and how many times 
 the pint of steam, is heavier than the pint of 
 hydrogen. Those numbers which express the 
 relative weights of equal bulks of the three gases 
 must also express the relative weights of equal 
 numbers of the three kinds of molecules ; for 
 equal volumes of gases contain equal numbers 
 of molecules. Now, as all the molecules of 
 hydrogen weigh the same, and as all the mole- 
 cules of oxygen weigh the same, and as all the 
 molecules of steam weigh the same, the numbers 
 which express the relative weights of equal 
 numbers of molecules of hydrogen, oxygen, and 
 steam, respectively, must also express the re- 
 lative weights of single molecules of these three 
 kinds of matter. In other words, the molecular 
 weights of the three gases are expressed by the 
 numbers which tell the relative weights of equal 
 volumes of these gases, provided these volumes 
 are made equal at the same temperature and 
 pressure, for the volume of a gas changes very 
 much with changes of temperature or pressure. 
 
 Let us take the actual experimental results in 
 the cases we are examining. Experiments prove 
 that any volume of oxygen weighs sixteen times 
 more than the same volume of hydrogen, tem- 
 perature and pressure being the same ; and ex- 
 periments prove that any volume of dry steain 
 weighs nine times more than the same volume of 
 hydrogen, at the same temperature and pressure. 
 Therefore the molecule of oxygen is sixteen times 
 heavier than the molecule of hydrogen. In 
 
158 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 other words, if the molecular weight of hydrogen is 
 set down as one, then the molecular weight of 
 oxygen is sixteen, and the molecular weight of 
 steam, or water-gas as it may be called, is nine. 
 
 Hydrogen is chosen as the standard substance 
 because it is the lightest kind of matter known. 
 The molecular weight of a gas is the number 
 which tells how many times a molecule of that 
 gas is heavier than a molecule of hydrogen. 
 And the molecular weight of a gas is determined 
 by finding how many times a determinate volume 
 of that gas is heavier than the same volume of 
 hydrogen measured at the same temperature and 
 the same pressure. 
 
 Now supposing, let us say for reasons of con- 
 venience, that it is decided to represent the 
 molecular weight of hydrogen by the number 
 two, rather than by unity. Then, in the cases 
 we have examined, the molecular weight of 
 oxygen would be expressed by the number 32, 
 and the number 18 would express the molecular 
 weight of water-gas ; for 1 : 16 : 9 = 2 : 32 : 18. 
 Supposing that the molecular weight of hydro- 
 gen is taken to be three, then the molecular 
 weights of oxygen and water-gas must be ex- 
 pressed by the numbers 48 and 27 respectively. 
 If it should be found convenient to call the 
 molecular weight of hydrogen one half, it is evi- 
 dent that the number 8 must be used to repre- 
 sent the molecular weight of oxygen, and the 
 number 4| to represent the molecular weight 
 of water-gas. 
 
 As a matter of fact, the molecular weight of 
 hydrogen is always expressed by the number two ; 
 
THE ATOM AND THE MOLECULE. 159 
 
 and therefore the molecular weights of oxygen 
 and water-gas are expressed by the numbers 
 32 and 18 respectively. Now why should the 
 molecular weight of hydrogen, which it is agreed 
 shall be the standard element whereto all other 
 molecular weights shall be referred, be repre- 
 sented by the number two rather than by the 
 number one 1 It is to be remembered that we 
 have agreed to express the atomic weights of all 
 elements by numbers which tell how many times 
 heavier each atom is than the atom of hydrogen, 
 and that in assigning values to these atomic 
 weights we have agreed to take the atom of hy- 
 drogen as unity \ in other words, we have agreed 
 to express all atomic weights on the scale of 
 hydrogen, calling the weight of an atom of 
 hydrogen one. Were we then to express all 
 molecular weights on the scale of hydrogen, with 
 the molecular weight of hydrogen taken as one, 
 we should obtain the same numbers for the mole- 
 cular, as for the atomic, weights of the elements. 
 Now, granting Avogadro's law, it can be shewn 
 that the molecular weights of some elements 
 at any rate are not the same as the atomic 
 weights of these elements; it can be shewn 
 that when such elements react chemically their 
 molecules are shattered into smaller parts. In 
 the case of hydrogen, it can be shown that the 
 molecule of this element is broken into two 
 parts in many chemical reactions. But if the 
 molecule of hydrogen is composed of two parts, 
 and if the parts of the molecule of an element 
 are to be called atoms, then the molecular weight 
 of hydrogen must be represented by a number 
 
160 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 twice that which represents the atomic weight 
 of hydrogen ; now we have agreed that the 
 atomic weight of hydrogen shall be represented 
 by the number one ; hence the molecular weight 
 of hydrogen must be represented by the number 
 two. 
 
 The proof that the molecule of hydrogen 
 separates into two parts during many chemical 
 actions, rests upon, and assumes the validity of, 
 the law of Avogadro. Consider one especial 
 reaction of hydrogen; namely, its combination 
 with the gaseous element chlorine to form the 
 gaseous compound hydrochloric acid. Experi- 
 ments prove that equal volumes of hydrogen 
 and chlorine combine to form a volume of 
 hydrochloric acid gas that is equal to twice the 
 volume of the hydrogen, or twice the volume 
 of the chlorine, used ; and that hydrochloric acid 
 gas is composed of hydrogen and chlorine, and 
 of these elements only. Apply Avogadro's 
 law to these facts. Equal volumes, equal number 
 of molecules. Whatever may be the number of 
 molecu^s of hydrogen, twice as many molecules 
 of hydrochloric acid gas have been formed ; 
 therefore every molecule of hydrogen has pro- 
 duced, by uniting with chlorine, two molecules 
 of hydrochloric acid gas, for one volume of 
 hydrogen has produced two volumes of hydro- 
 chloric acid. Therefore every molecule of 
 hydrogen has separated into two parts (it is 
 just possible that each molecule may have 
 separated into more than two parts) ; and each 
 half molecule has combined with a half mole- 
 cule of chlorine to produce a molecule of hydro- 
 
THE ATOM AND THE MOLECULE. 161 
 
 chloric acid gas. Now these fractions of the 
 molecule of hydrogen are called atoms of hydro- 
 gen ; and as each molecule of hydrogen has 
 separated into two atoms, the molecular weight 
 of hydrogen is evidently double its atomic 
 weight ; but the atomic weight is, by agree- 
 ment, to be taken as unity, therefore the mole- 
 cular weight of hydrogen must be expressed by 
 the number two. 
 
 Of course the argument applies to chlorine 
 also ; had we fixed on chlorine as the standard 
 element for atomic and molecular weights we 
 should have called the atomic weight of that 
 element one, and, from the foregoing facts and 
 reasoning, we should have set down the molecular 
 weight as two. 
 
 Many other reactions between hydrogen and 
 other gaseous elements lead to the same conclu- 
 sion, namely, that the molecular weight of 
 hydrogen is twice its atomic weight ; in other 
 words, that the molecule of hydrogen is broken 
 into two parts, or atoms, during many chemical 
 reactions, and that these atoms enter into union 
 with other atoms to form new molecules. 
 
 The molecular weights of several elements, and 
 of many compounds, have been determined by 
 the use of Avogadro's law. And this law has 
 also enabled the atomic weights of the greater 
 number of the elements to be obtained. It is 
 not easy to follow the method whereby the 
 application of the law leads to determinations of 
 atomic weights ; but an illustration may put the 
 reader in the way of understanding the principle 
 of the method. Let us choose oxygen. The 
 
162 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 relative densities, referred to hydrogen, of a 
 number of compounds of oxygen in the gaseous 
 state have been determined ; when the number 
 expressing how many times each of these 
 gaseous compounds of oxygen is heavier than 
 an equal bulk of hydrogen is multiplied by two, 
 the result is the molecular weight of that com- 
 pound. All these compounds have been analysed, 
 and the quantity of each element has been ex- 
 pressed as so much of that element by weight in 
 one molecular weight of the compound. Then 
 the smallest quantity by weight of oxygen found 
 in a molecular weight of any one of these com- 
 pounds is taken as the atomic weight of 
 oxygen. For instance, carbonic acid gas is 
 found to be 22 times heavier than hydrogen, 
 bulk for bulk; therefore the molecular weight 
 of this gaseous compound is 44 : the analysis of 
 carbonic acid gas shows that it is composed of 
 27*27 per cent, of carbon, and 72- 73 per cent, of 
 oxygen ; if these numbers are sUted as parts by 
 weight of carbon and oxygen, respectively, per 
 44 parts by weight of the compound, the results 
 are, 12 parts of carbon and 32 parts of oxygen. 
 That is to say, in the quantity of carbonic acid 
 gas expressed by the molecular weight of that 
 compound, there are 32 parts by weight of oxygen 
 (and 12 parts by weight of carbon). Now, no 
 molecule of a compound of a certain element can 
 contain less than a single atom of that element, 
 although the molecule may contain several atoms 
 of the element ; this follows from the conception 
 of the atom as the minute portion of an element 
 the division of which into parts, if that could be 
 
THE ATOM AND THE MOLECULE 163 
 
 accomplished, would be attended by the produc- 
 tion of kinds of matter entirely different from the 
 element. Hence in the present case the atomic 
 weight of oxygen cannot be greater than 32. 
 Another compound of oxygen is dealt with in 
 the same way as carbonic acid gas. Suppose 
 water to be the compound chosen. Water-gas is 
 found to be nine times heavier than hydrogen ; 
 hence the molecular weight of water-gas is 18. 
 Analysis shows that 18 parts by weight of 
 water are composed of 16 parts of oxygen, and 
 two parts of hydrogen ; therefore, the atomic 
 weight of oxygen cannot be greater than 16, 
 although it may be a whole sub-multiple of 
 that number. Other compounds of oxygen are 
 examined in the same way ; and the final result 
 is that less than 16 parts by weight of oxygen 
 has never been found in the quantity of any 
 compound of oxygen which is expressed by the 
 molecular weight of. that compound. The atomic 
 weight of oxygen is therefore taken to be 16. 
 
 Sufficient has been said to indicate, not to ex- 
 plain thoroughly, the meanings of the terms atom 
 and molecule. The atomic and molecular theory 
 declares that matter has a grained structure ; that 
 were our eyesight keen enough we should see 
 that every piece of each definite kind of matter has 
 a structure, like a brick wall, or a company of 
 soldiers, or a heap of apples, or a roomful of 
 cricket balls, or a bottleful of small shot. The 
 theory calls these minute portions of homogene- 
 ous matter molecules ; it asserts that all the 
 molecules of any particular element or com- 
 pound are identical in weight and in all other 
 
164 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 properties, and that the properties of any 
 quantity of the element or compound are the 
 properties of the molecules of that element or 
 compound. The theory bids us think of the 
 molecules as always moving about ; moving fairly 
 freely, but knocking against each other now and 
 again, when the body is in a state of gas ; moving 
 less freely when the body is a liquid ; and packed 
 tightly, with but little freedom of motion, when 
 the body is a solid. As long as the molecules of 
 an element, or compound, merely change their 
 rates of motion, but are neither disintegrated, nor 
 enter into union with other molecules, the theory 
 says that physical action only is taking place. 
 But when a molecule of one kind collides with a 
 molecule of another kind in such a way that, either 
 the molecules are shattered and their parts pair off 
 in new combinations, or the molecules swing to- 
 gether as a single, more complex molecule, then, ac- 
 cording to the theory, chemical action has occurred. 
 The molecules of elements, as well as those of 
 compounds, may be disintegrated by the violence 
 of their collisions, or by some other cause : but 
 in most cases, at any rate, these parts, these frag- 
 ments of molecules tend to coalesce ; sometimes 
 the old portions join hands again, and the mole- 
 cules that were shattered are re-formed ; more 
 frequently new combinations of partners are 
 formed, and new kinds of molecules are pro- 
 duced. If the molecule of an element has parts, 
 these parts are all exactly alike in weight and all 
 other properties. The molecule of a compound 
 has as many kinds of different parts as the num- 
 ber of the elements that make up that compound. 
 
THE ATOM AND THE MOLECULE. 165 
 
 When molecules are completely disintegrated 
 the parts of the molecules are called atoms. The 
 properties of an element considered apart from 
 other kinds of matter for instance, the specific 
 gravity, the electrical properties, the expansion 
 by heat, the colour, and so on are the pro- 
 perties of the molecules of that element; but, 
 as the molecules are separate d into their atoms 
 when chemical change occurs, the chemical re- 
 actions of the element are conditioned by the 
 properties of the atoms of that element. More- 
 over, as the moment an elementary molecule is 
 divided, the atoms tend to join into groups, 
 either by uniting with atoms like themselves to 
 re-form the elementary molecules, or by combin- 
 ing with other kinds of atoms to produce com- 
 pound molecules, the chemical properties of an 
 element are only exhibited when that element 
 acts on, and is acted on by, other kinds of homo- 
 geneous matter. And what is true of the mole- 
 cule of an element is true also of the molecule 
 of a compound ; the chemical properties of the 
 compound are exhibited only when the molecule is 
 broken up, or when it unites with other molecules. 
 No atom has ever been divided; if an atom 
 were divided the theory does not say whether 
 the cutting of an atom is, or is not, possible 
 kinds of matter would be produced quite different 
 from the element whose atom was separated into 
 parts. The atom is the ultimate particle of 
 matter with which the chemist has to deal, at 
 present at any rate. Chemistry recognises how 
 changes take place in combinations of these un- 
 changing particles. 
 
166 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 It would not be difficult to show that the law 
 of combining and reacting weights, which is the 
 generalised result of the study of the compositions 
 of homogeneous kinds of matter (see p. 66), follows 
 necessarily from the postulates of the atomic and 
 molecular theory ; but I leave this deduction to 
 the imaginative common sense of the reader. 
 
 Some one may say : What of the other theory ; 
 the theory that any apparently homogeneous 
 matter is truly homogeneous throughout; that 
 matter has not a grained but a jelly-like struc- 
 ture 1 To such a person I would reply try the 
 jelly theory. It has been often tried ; attempts 
 have been made to explain the observed pro- 
 perties of matter by its aid ; but it breaks down 
 at once. The first test, and the final test, of 
 every scientific theory is : Does it work ? The 
 jelly theory of matter does not work. 
 
 The molecule then has itself a structure ; it is 
 built up of parts, of atoms ; and these parts, one 
 is forced to admit, are arranged in a definite way 
 relatively to one another. The conception of the 
 molecule, like so many chemical conceptions, is 
 essentially that of a building. And this is the 
 part of the subject which we must now go into 
 a little more deeply. Let us recall Wohler's 
 synthesis of urea (see p. 119). By heating am- 
 monium cyanate to 100 Wohler obtained urea. 
 But ammonium cyariate and urea have identical 
 compositions ; the composition of both is ex- 
 pressed by the formula N 2 OCH 4 . The molecule 
 of ammonium cyanate is composed of two atoms 
 of nitrogen, one atom of oxygen, one atom of 
 carbon, and four atoms of hydrogen ; arid the 
 
MOLECULAR ARCHITECTURE. 167 
 
 molecule of urea is composed of the same numbers 
 of the same atoms. How, then, can the proper- 
 ties of the two molecules be different from one 
 another ? It is easy to see that the properties of 
 a molecule must depend on the properties of the 
 atoms that compose it, and on the numbers of 
 each kind of atoms. But the case of ammonium 
 cyanate and urea shews that there is a third con- 
 ditioning circumstance. What can that circum- 
 stance be except the arrangement of the atoms 
 that compose the molecule ] But the considera- 
 tion of the structure of molecules demands a 
 chapter to itself. 
 
 CHAPTEK IX. 
 
 MOLECULAR ARCHITECTURE. 
 
 THE process of forming a chemical compound 
 from two or more elements has been likened in 
 these pages to the construction of a building. 
 We must now accustom ourselves to think of the 
 formation of a chemical building as the putting 
 together of definite numbers of exceedingly 
 minute pieces of definite kinds of matter, which 
 pieces are called atoms, and as resulting in the 
 production of a more complex, but still extremely 
 minute, piece of matter, which is called a mole- 
 cule. And although the properties of the build- 
 ing are very different from the properties of the 
 stones, or bricks, used in its construction, never- 
 theless we must regard the properties of the 
 molecule as conditioned by, firstly the properties, 
 secondly the numbers, and thirdly the arrange- 
 
168 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 ment, of the atoms which have been built into 
 that molecule. 
 
 The molecules which the chemist builds by 
 joining atom to atom are exceedingly minute. 
 We do not know the size of a molecule ; but 
 there are many lines of reasoning, on different 
 classes of facts, all of which point to the con- 
 clusion that the smallest portion of a definite 
 kind of matter, say of water or iron, that can be 
 seen by a very efficient microscope, is composed 
 of not less than sixty million, and not more than 
 one hundred million, molecules. 
 
 I have tried, in Chapter Y1IL, to indicate the 
 kind of facts, and the nature of the reasoning on 
 these facts, which lead chemists to definite con- 
 clusions regarding the number of atoms of each 
 kind that go to form the molecule of any parti- 
 cular sort of matter. But in what directions 
 shall chemists look for guidance when they 
 attempt to frame a consistent and satisfactory 
 picture of the arrangement of those surpassingly 
 small pieces of matter which, when put together, 
 produce a thing so minute that it must be 
 magnified from sixty to one hundred million 
 times before it is visible 1 The task may seem 
 impossible, but it has been accomplished. 
 
 It is to be remarked that the chemist's task is 
 not, at present, to discover what is the arrange- 
 ment of the atoms in this or that molecule ; that 
 problem he does not attempt, as yet, although 
 some day he expects to see the molecule with one 
 of the many eyes wherewith the accurate study 
 of nature supplies him : what he has to do now, 
 as I have said, is to form a satisfactory and 
 
MOLECULAR ARCHITECTURE. 169 
 
 consistent mental image of the arrangement of 
 the atoms in the molecules of the various elements 
 and compounds the study of which is the busi- 
 ness of chemistry. The image which the chemist 
 forms must be clear, self-consistent, in keeping 
 with the facts that are known, and suggestive of 
 facts to be investigated ; in a word, he must 
 construct a scientific theory of the structure of 
 molecules. 
 
 It would be absurd in such a book as this to 
 attempt anything beyond the slightest sketch 
 of the gradual building up of the theory of 
 molecular architecture. If I can incite a few 
 readers of this book to go more deeply into the 
 matter, and if I am able to start them on the right 
 path, I shall have done all I can legitimately hope 
 to do here. Consider the four compounds 
 hydrochloric acid gas, water gas, ammonia gas, 
 and marsh gas ; the compositions, the molecular 
 weights, and the number of atoms in the mole- 
 cules, of these four gaseous compounds are 
 expressed by the four formulse, HC1 (hydro- 
 chloric acid gas), H 2 (water gas), H 3 N (ammonia 
 gas), and H 4 C (marsh gas). An atom of the 
 element chlorine combines with one atom of the 
 element hydrogen to form the compound mole- 
 cule of hydrochloric acid gas ; an atom of the 
 element oxygen combines with two atoms of 
 hydrogen to form the compound molecule of 
 water gas; an atom of the element nitrogen 
 combines with three atoms of hydrogen to form 
 the compound molecule of ammonia gas ; and an 
 atom of the element carbon combines with four 
 atoms of hydrogen to form the compound mole- 
 
170 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 cule of marsh gas. No compound of chlorine is 
 known in the molecule of which one atom of 
 that element is combined with more than one 
 atom of hydrogen ; no compound of oxygen is 
 known in the molecule of which one atom of 
 that element is combined with more than two 
 atoms of hydrogen ; no compound of nitrogen is 
 known in the molecule of which one atom of 
 nitrogen is combined with more than three atoms 
 of hydrogen ; nor is any molecule known of a 
 compound of carbon wherein one atom of that 
 element is combined with more than four atoms 
 of hydrogen. There is a limit to the number of 
 atoms of hydrogen which a single atom of 
 chlorine can hold in union with itself so as to 
 produce a stable molecular building ; and there 
 is a limit to the number of atoms of hydrogen 
 which a single atom of oxygen, or a single atom 
 of nitrogen, or a single atom of carbon, is able 
 to unite with to form a molecular structure that 
 does not fall to pieces. 
 
 There is then a limit to the number of 
 hydrogen atoms which can be held by a single 
 atom of any one of the four elements, chlorine, 
 oxygen, nitrogen, and carbon. And what is 
 true of the atoms of these elements is true of 
 the atoms of all the elements. An examination 
 of the numbers of atoms in the molecules of 
 compounds of all the elements that combine 
 with hydrogen, reveals the fact that there is a 
 maximum value for the number of hydrogen 
 atoms wherewith one atom of any particular 
 element can combine to produce a firm molecular 
 structure. 
 
MOLECULAR ARCHITECTURE. 171 
 
 And as with atoms of hydrogen, so with 
 atoms of the other elements. A study of the 
 atomic compositions of molecules, on the lines 
 that have been indicated in the foregoing ex- 
 amples, leads to the notion of a limited atom- 
 holding power as belonging to each kind of 
 elementary atom with respect to any other 
 kind of elementary atom. The results of the 
 study of the atomic compositions of molecules 
 go further than this ; they show that there are 
 five elementary atoms which can be used as 
 measurers of the atom-holding power of any 
 other elementary atom. These five atoms are 
 the atoms of hydrogen, of fluorine, of chlorine, 
 of bromine, and of iodine. When we have 
 determined the maximum number of atoms of 
 hydrogen, or the maximum number of atoms 
 of fluorine, or of chlorine, or of bromine, or of 
 iodine, wherewith a single atom of any specified 
 element combines to produce a stable molecule, 
 we have also determined the maximum number 
 of atoms of any kind which that specified atom 
 is able to hold to itself in any molecular building 
 into which it enters. 
 
 The moment we grasp the fact that there is 
 a limit to the number of atoms of any kind 
 wherewith a specified atom can enter into direct 
 chemical union to produce a molecular building 
 which does not fall to pieces, we begin to have 
 definite conceptions regarding the structure of 
 molecules. It is now only necessary to deter- 
 mine the limiting value for each kind of atom ; 
 I have indicated how this can be done with the 
 help of the atoms of the five eh ments that serve 
 
172 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 as measurers of atom-holding power ; then to 
 devise a convenient language for expressing the 
 atom-holding powers of the elementary atoms, 
 and their joining together in accordance with 
 the atom-holding power of each ; and, finally, 
 to represent the reactions of compounds as con- 
 nected with the arrangement of the atoms that 
 form the molecules of these compounds, that 
 arrangement being expressed in the special 
 language which has been devised. 
 
 All the elementary atoms belong to one or 
 other of six classes : there are those which can 
 unite directly with but a single other atom ; 
 there are those which are able to bind two other 
 atoms to themselves ; there are those which 
 can hold three other atoms ; there are those 
 which can retain within their sphere of direet 
 action four other atoms ; there are those which 
 are able to act directly on, and be directly 
 acted on by, five other atoms ; and there are 
 those which can unite into one structure with 
 themselves six other atoms. 
 
 I have purposely varied the words whereby 
 I have sought to convey the conception of 
 direct chemical union between atoms. What 
 that union is we do not definitely know ; it 
 may be brought about by an attractive force 
 between the atoms, it may be due to the force 
 of gravity conditioned by the forms and the 
 motions of the atoms, but it is more probable 
 that the forces between the atoms are electrical 
 forces. Whatever future research may shew 
 the nature of what we call "direct chemical 
 union between atoms " to be, we are compelled 
 
MOLECULAR ARCHITECTURE. 173 
 
 by the facts to think of a molecule as held to- 
 gether by some kind of direct action between 
 atom and atom ; we must imagine the molecule 
 as a definite and orderly structure, with each 
 atom arranged in a particular way with respect 
 to the other atoms, although all the atoms are 
 ia motion, and with a direct action and re- 
 action between any one atom and a limited 
 number of other atoms. The molecule must 
 be pictured as a sort of miniature solar system. 
 A simple device is adopted in chemistry for 
 representing the number of atoms between 
 which and any specified atom direct action and 
 reaction can take place in any molecule. The 
 symbol of an element, as we know, is the first 
 letter, or the first and some other letter, of its 
 name ; that symbol represents one atom of the 
 element. To the symbol is attached either a 
 Roman numeral, or a number of lines ; and the 
 figure, or the number of lines, indicates the 
 atom-holding power of the atom of the element 
 in question. For instance, the symbols C IV and 
 
 _C- have the same meaning ; and the meaning 
 
 is that an atom of carbon can hold to itself 
 directly four, and not more than four, other 
 atoms, in a molecule. So the symbols O n and 
 -0- tell that an atom of oxygen is able to unite 
 directly with not more than two other atoms. 
 Then take the symbol for a compound expressed 
 with the help of the device I am endeavouring 
 to explain. Let the compound be water; the 
 symbol H-O-H is meant to set forth the 
 arrangement of the atoms in the molecule of 
 
174 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 water in so far that the symbol implies direct 
 action, and reaction, between the atom of oxygen 
 and each atom of hydrogen, but the absence of 
 direct union between the two hydrogen atoms. 
 Alcohol is a more complicated compound than 
 water; its composition and molecular weight, 
 and the number of each kind of the atoms that 
 compose the molecule of alcohol, are represented 
 by the symbol C 2 H 6 0. Now, the reactions of 
 alcohol can be expressed in the special language 
 we are considering, that is, the reactions can be 
 represented as conditioned by the arrangement 
 of the atoms in the molecule of alcohol, by 
 writing the symbol in this way ; 
 H H 
 
 H C H 
 
 This symbol is meant to express the fact that 
 if we suppose the atoms of carbon to be in 
 direct union with one another, and if we picture 
 one atom of carbon as directly holding to itself 
 three atoms of hydrogen, and the other atom of 
 carbon as binding to itself two atoms of hydrogen 
 and one atom of oxygen, and if we also think 
 of the atom of oxygen as directly holding the 
 sixth atom of hydrogen ; if we keep this arrange- 
 ment of the atoms C 2 H 6 in our mind, then 
 we have a consistent, and, so far as it goes, 
 satisfactory, way of thinking about the reactions 
 of alcohol as conditioned by the number and 
 arrangement of the atoms that form the molecule 
 of that compound. This formula must not be 
 
MOLECULAR ARCHITECTURE. 175 
 
 regarded as pretending to be a likeness of the 
 molecule of alcohol ; whatever that molecule is 
 like, it is certainly not a collocation of atoms 
 fixed relatively to one another, and all in one 
 plane. This formula, like all formulae con- 
 structed on the same conceptions, is merely a 
 device for aiding accurate thinking about the 
 fundamental question of chemistry, the question, 
 namely, of the connexions between composition 
 and properties. And the device has worked 
 admirably ; moreover, it has been found to carry 
 in itself the germs of further developments. 
 
 Let us turn back to the special case of con- 
 nexions between composition and reactions pre- 
 sented by urea and ammonium cyanate. These 
 two compounds have the same molecular weight, 
 and the molecules of the two compounds are 
 composed of the same numbers of the same 
 atoms. The reactions of one of the compounds 
 shew that it is a salt of ammonium, analogous to 
 salammoniac (ammonium chloride) and ammo- 
 nium carbonate. Now, the only way of repre- 
 senting the compositions of salts of ammonium, 
 agreeably with the reactions of these salts, in the 
 special language of the theory of molecular struc- 
 ture which I have endeavoured to outline in the 
 preceding paragraphs, is to suppose the presence 
 in the molecules of all of them of an atomic group 
 consisting of an atom of nitrogen in direct union 
 with four atoms of hydrogen. The representa- 
 tion of the molecule of ammonium cyanate on 
 this hypothesis is given by the formula 
 
176 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 The reactions of urea on the other hand are in 
 keeping with an atomic arrangement which is 
 symbolised by the formula 
 
 Ammonium cyanate reacts as if the molecule of 
 the salt were constructed by joining two groups 
 of atoms, one group being itself made of an atom 
 of nitrogen in direct union with four atoms of 
 hydrogen, and the other group being composed of 
 an atom of carbon directly united with an atom 
 of oxygen and an atom of nitrogen ; these 
 groups are indicated in the formula by dotted 
 lines. Moreover, the supposition that these two 
 groups of atoms are hold together by direct 
 action and reaction between the oxygen atom 
 and that nitrogen atom which is in connexion 
 with four atoms of hydrogen is more in keeping 
 with the reactions of the salt than any other 
 supposition that can be made concerning the 
 joining of the two atomic groups in the molecule 
 of ammonium cyanate. 
 
 If we wish to translate the reactions of urea 
 into the language of the only theory of molecular 
 structure that has been found useful in generalis- 
 ing and suggesting facts, we must represent the 
 molecule of urea as constructed by putting 
 together three groups of atoms ; one of these is 
 composed of an atom of carbon directly bound to 
 an atom of oxygen (the group C-O), and the 
 carbon atom of this group is in direct union with 
 two groups, each of which is composed of an 
 
MOLECULAR ARCHITECTURE. 177 
 
 atom of nitrogen holding to itself a couple of 
 atoms of hydrogen ; the three groups of atoms 
 are indicated in the formula given on p. 176 by 
 dotted lines. 
 
 These formulae, the formula for ammonium 
 cyanate and that for urea, include in themselves 
 the conception of the compound radicle, which 
 was illustrated without the aid of the atomic and 
 molecular theory in Chapter VIII. Now that we 
 have become acquainted with the conceptions of 
 the atom and the molecule, and have gained some 
 slight insight into the ways of representing the 
 connexions between reactions and composition 
 as dependent, among other conditions, on the 
 arrangement of the atoms in the molecules of 
 compounds, we are in a position to think more 
 exactly regarding compound radicles. We now 
 think of a compound radicle as a definite group 
 of atoms, which atoms are arranged in a definite 
 way, and are so firmly held together that the 
 whole group may be moved from one molecule to 
 another without falling to pieces ; and we think 
 of many molecules as constructed by putting to- 
 gether various compound radicles. 
 
 As we facilitate the comparison, the contrast- 
 ; ng, and the classification, of compounds by 
 thinking of the reactions of these compounds as 
 the reactions of extremely small portions of 
 them which we call molecules, so we further 
 facilitate the comparison, the contrasting, and the 
 classification, of compounds by thinking of their 
 reactions as definitely connected with the ar- 
 rangement in groups of those parts of their 
 molecules which we call atoms ; and we yet 
 M 
 
178 THE STORY OF THE CHEMICAL ELEMENTS 
 
 further facilitate the accurate classification of 
 compounds by the use of a device for represent- 
 ing to ourselves the structure of their molecules, 
 which device has arisen from the use of the 
 atomic and molecular theory, and from the con- 
 ception of a limit to the number of atoms which 
 can be held directly by any special atom in a 
 molecule, a conception that is itself an outcome 
 of the application to chemical occurrences of the 
 theory of the grained structure of matter. 
 
 If the properties of a compound are the pro- 
 perties of the molecule of that compound ; if the 
 properties of the molecule are conditioned not 
 only by the nature and number of the atoms 
 that compose it, but also by the arrangement of 
 these atoms ; and if it is possible to arrange the 
 same number of the same atoms in various ways 
 consistently with the knowledge we have of the 
 number of atoms which any special atom can 
 hold in direct chemical union ; then there ought 
 to be very many instances of the existence of 
 compounds with different properties but the 
 same molecular weights and having molecules 
 composed of the same number of the same atoms. 
 Hundreds, probably thousands, of such cases are 
 known; and almost all the compounds of this 
 kind that have been carefully studied are com- 
 pounds of carbon, and very many of them are 
 compounds of carbon with some or all of the 
 three elements hydrogen, oxygen, and nitrogen. 
 
 The study of the connexions between com- 
 position and chemical properties, especially the 
 compositions and properties of compounds of car- 
 bon, is guided, nowadays, by the conception of the 
 
MOLECULAR ARCHITECTURE. 179 
 
 molecule as a structure of atomic groups. For 
 instance, the study of the aniline colours, to 
 which I referred in Chapter VII., would be a 
 chaos were it not for this illuminating concep- 
 tion. So that the manufacturer who wishes to 
 make new colours, or to improve the processes 
 whereby well known colours are made, must be 
 thoroughly at home in handling the difficult and 
 intricate questions of molecular structure. We 
 have no other language wherein to express the 
 relations of such bodies as the aniline colours 
 except the language of atoms and molecules, of 
 atomic groups and stable and unstable atomic 
 systems ; and to use that language aright de- 
 mands constant mental alertness, a vivid ima- 
 gination, and a knack, which comes only from 
 habit, of translating the phrases of the language 
 into the actual reactions of the laboratory, and 
 the reactions observed in the laboratory into the 
 expressions of the language. And this language, 
 like all other living tongues, grows as the wants 
 that have to be expressed by it increase and be- 
 come more complicated. It is but yesterday 
 that chemists began to try to picture to them- 
 selves, in definite wavs, the possible arrangements 
 of the atoms of molecules in three dimensions in 
 space, and already many new terms have been 
 introduced to express the new conceptions. And 
 it is so easy to construct a little phrase book for 
 oneself, and to set out hoping to understand and 
 to be understood. But the traveller soon returns 
 discomfited, lost, perplexed. He thought he had 
 asked correctly, in the laboratory, for a new 
 colour, or a new drug ; but nature gave him only 
 
180 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 a nasty mess. He must begin at the beginning, 
 and he must learn the language in sentences, not 
 in detached words. 
 
 Success in many of our staple industries is im- 
 possible without a fair knowledge, a knowledge 
 that is accurate so far as it goes, of chemical 
 transformations, and of the means whereby these 
 transformations are expressed, compared, con- 
 trasted, and classified. Success in the more 
 refined branches of the chemical trade comes 
 only to the man who is himself an accomplished 
 chemist, or who has the foresight to employ, 
 and to be guided by, a thoroughly trained and 
 thoroughly efficient chemist. And to become an 
 efficient and practically successful chemist means 
 that a man shall have devoted years to the study 
 of problems that deal with the connexions be- 
 tween the structures of molecules and the pro- 
 perties of the compounds which are formed by 
 the coalescence of molecules; it means an apparent 
 wandering far from the path trodden bare by the 
 feet of those who with a gentle irony call them- 
 selves " practical men " ; but it means treading 
 the only path that leads to those practical re- 
 sults which can be translated into hard cash, I 
 would appeal once more to the history of the 
 German chemical trade in support of this asser- 
 tion. 
 
 Soap-making ; the manufacture of paraffin, and 
 the separation and purification of petroleum oils; 
 cheese-making, and most of the operations con- 
 nected with the feeding of plants and animals 
 on the farm; sugar-making; the preparation of 
 alcoholic drinks ; the manufacture of explosives 
 
MOLECULAR ARCHITECTURE. 181 
 
 and smokeless powders ; tanning ; dyeing ; calico 
 printing, and the making and applying of colours; 
 preparing paints for use, and devising suitable 
 vehicles for the application of paints, and proper 
 varnishes for locking up the painted surfaces 
 from the effects of weather ; the action and ap- 
 plication of disinfectants, and antiseptics ; the 
 manufacture of anaesthetics ; the preparation of 
 new therapeutic agents : to carry on any one of 
 these industries, or arts, successfully, even if by 
 success is meant nothing more than monetary 
 success, requires a knowledge of chemical facts 
 and chemical laws ; and not one of these arts or 
 industries can be improved, permanently and 
 fundamentally, otherwise than by the application 
 to the practice of them of that profound chemical 
 knowledge which is gained, and gained only, by 
 the study of problems which seem to the unimagi- 
 native man to be absolutely unconnected with the 
 practice of anything that is useful to mankind. 
 
 Let us turn back for a short time to the study 
 of the structure of molecules. This study has 
 led to the recognition of molecular symmetry. 
 In the light of recent researches into the con- 
 nexions between composition and properties, it 
 is necessary to think of the properties of a fairly 
 complex molecule as conditioned by an adjust- 
 ment of the properties and the positions re- 
 latively to one another, of the various compound 
 radicles which form the molecule. Perhaps the 
 substitution of a certain compound radicle for 
 some other atom, or atomic group, in a molecule 
 may carry with it such excessively pronounced 
 properties of one kind or another that the 
 
182 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 molecule becomes unstable and ready to fall 
 to pieces ; then the introduction of another 
 atomic group, with properties very different 
 from those of the group that has made the 
 molecule, as we may say, lopsided, will re- 
 store the threatened equilibrium of the structure. 
 As an illustration, look at some of the aniline 
 colours. Let us suppose that the structure of 
 the molecule of a certain compound which dyes 
 a blue-purple is so far known that a definite 
 structural formula has been assigned to the 
 compound ; a certain atomic group is introduced 
 in the place of one of the hydrogen atoms in 
 the molecule of this compound, and the result 
 is a compound which dyes an extremely pro- 
 nounced blue ; another compound radicle is 
 introduced in place of an atom of hydrogen in 
 the molecule of the original compound, and the 
 product dyes a red-purple. Now it is wished 
 to produce a compound which will dye a rich 
 purple, neither too blue nor too red; therefore 
 two atomic groups are introduced into the 
 molecule, one of which groups is known to carry 
 with it the property of dyeing blue, and the 
 other the property of dyeing red; the result 
 is that the blueing tendency of one group of 
 atoms is balanced by the reddening tendency of 
 the other group, and a compound is produced 
 which dyes the wished - for purple shade. 
 Molecular symmetry is insured by opposing one 
 group of atoms to another within the molecule. 
 
 "Symmetry,' 7 says Euskin, "is the opposition 
 of equal quantities to each other. " In molecular 
 architecture symmetry is attained by opposing 
 
MOLECULAR ARCHITECTURE. 183 
 
 various groups of atoms which are not neces- 
 sarily equal quantities, if by that is meant con- 
 taining equal quantities of matter as measured 
 by weight, but which produce equal changes, 
 although changes in opposite, or at least in 
 different, directions, in the properties of the 
 molecule. Now the effect of this or that atomic 
 group on the properties of a molecule can be 
 determined only by introducing the group in 
 question into many different molecules, and 
 examining the effects produced on the properties 
 of these molecules. It is the old difficulty. You 
 cannot take a group of atoms in your hand and 
 examine and tabulate its properties, and then 
 tell in what ways the properties of any particular 
 molecule will be modified by the substitution of 
 that group for some one or more of the atoms 
 in the molecule; just as a study of the pro- 
 perties of an element taken by itself does not 
 help you to foretell what kind of properties will 
 belong to the compounds of that element. There 
 is only one method : in the late-t developments of 
 chemistry, which may be called the study of 
 molecular symmetry, as in the earliest attempts 
 to connect the properties of elements with the 
 properties of their compounds, the only way by 
 which real knowledge is gained is the constant 
 and accurate examination of changes of pro- 
 perties, and changes of composition, and the 
 attempt, constantly made, to connect these two 
 kinds of changes and to express the connexions 
 in some clear and exact, although it may be, and 
 indeed must be, extremely symbolic, language. 
 There always have been, and there always must 
 
184 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 be, two lines along which chemistry advances : 
 the science advances by studying composition, and 
 by studying properties ; but the study of chemi 
 cal composition means the looking to changes of 
 composition, and the study of chemical properties 
 is the examination of changes of properties ; and 
 all the great advances are made by combining 
 the two lines of research, and by framing gener- 
 alisations which connect definite changes of one 
 kind with definite changes of the other kind. 
 
 But what is the good, what is the use, says 
 some one, of theories about molecules and atoms, 
 things that have only a hypothetical existence, 
 things that if they exist at all are so minute that 
 no mortal eye can ever see them ? Let us keep 
 to facts, and then we are on safe ground. Well 
 then, keep to what you call facts ; and you will 
 be left behind ; you will be left floating on your 
 little crazy raft of crumbling facts, desolate, 
 panting, distressed, when by the help of the 
 swimming belt of theory you might have reached 
 solid land. 
 
 The theory of atoms and molecules is a guide 
 to definite and accurate thinking about facts ; 
 every day it leads to the discovery of fresh facts 
 one sometimes wishes there were not quite so 
 many of them ; and every day it extends the 
 meaning and the scope of the facts that were 
 thought to be known already. And to be in the 
 habit of thinking about such things as mole- 
 cular structure, and the connexions between the 
 arrangements of the parts, and the properties of 
 the whole, of these minute portions of matter; 
 to be in the habit of connecting these considera- 
 
MOLECULAR ARCHITECTURE. 185 
 
 tions with operations that proceed on a vast scale 
 in nature, on a smaller but yet very large scale 
 in manufacturing operations, and on a yet smaller 
 scale in the laboratory ; to have such conceptions 
 of order, plan, arrangement, and symmetry in 
 one's mind, and to be constantly translating these 
 conceptions into hard solid facts in the laboratory; 
 to be doing those things humanises a man, and 
 serves as a corrective to the brutalising tendencies 
 of life which are so numerous and so violent. 
 
 The atoms are the ultimate building stones 
 wherewith chemistry constructs all her varied 
 buildings. And there is no doubt that all the 
 properties of all the compounds, and the com- 
 positions of all compounds, are connected with 
 the relative weights of the atoms of the elements 
 in a definite way, and in a way which can 
 be stated in such a form as to admit of direct 
 application. It was a profound truth that was 
 expressed by the great Russian chemist Men- 
 dele*eff, when he said that, " chemistry recognises 
 how changes take place in combinations of the 
 unchanging.' 7 The unchanging things are the 
 atoms of the elements ; the combinations are the 
 molecules both of elements and compounds ; and 
 the business of chemistry is to trace the changes 
 of properties which are brought about by com- 
 bining those unchanging atoms so as to form 
 different kinds of molecules. 
 
CHAPTER X. 
 
 SUMMARY AND CONCLUSION. 
 
 I HAVE tried in the preceding chapters of this 
 book to indicate the prominent characteristics of 
 those material changes which are the subject 
 matter of the science of chemistry ; to illustrate 
 the chemist's classification of material things into 
 elements and not-elements, and his division of 
 the latter class into compounds and mixtures ; 
 to sketch the methods, and to state the main 
 results of applying these methods, which lead to 
 the representation of the compositions of com- 
 pounds in terms of the elements that unite to 
 form, and can be obtained by breaking up, those 
 compounds ; to give examples of chemical pro- 
 parties, or reactions, as distinguished from 
 physical properties, and to indicate by the 
 examples given the way in which chemists set 
 about the study of the reactions of elements and 
 compounds, and the general character of the 
 results they obtain. I have also attempted to 
 state and illustrate the essential problem of 
 chemistry, which is to connect properties with 
 composition, and changes of properties with 
 changes of composition ; in doing this I have 
 been obliged to deal with the only theory of 
 the structure of matter which has furnished a 
 good working hypothesis for setting forth the 
 observed connexions between composition and 
 reactions ; and I have found it necessary to say 
 something about the special language, arising out 
 of that theory, which chemists use as an instru- 
 
 186 
 
SUMMARY AND CONCLUSION. 187 
 
 ment to aid them in expressing the connexions 
 that they observe between reactions and com- 
 position, and also as a means for suggesting 
 relations that may be submitted to the test of 
 experiment. 
 
 I have not attempted to follow the historical 
 order of the development of chemical science ; 
 but from time to time I have suggested contrasts 
 between older and more modern views of the 
 phenomena with which chemistry concerns itself. 
 I have been obliged, from the condition of my 
 task, to ignore vast portions of the chemical 
 domain ; I have, indeed, been able only to sketch 
 in outline a chapter taken here and there, but I 
 hope not altogether in a haphazard way, from 
 the story of chemistry. 
 
 Of the chemists themselves, the men who 
 have studied the transformations of matter, and 
 have gradually brought the elucidation of those 
 transformations to the point it has reached to- 
 day for the goal is yet far oif nothing, or prac- 
 tically nothing, has been said. These men be- 
 longed to various nations ; they were of different 
 temperaments, some were slow and plodding, 
 others brilliant, rapid, and versatile ; but they 
 were all incited by a regulated curiosity, and 
 were led on by strong, imaginative, common- 
 sense. In this country we had Black, Professor 
 at Edinburgh, the most methodical of men ; 
 Priestley, erratic, original, and full of new dis- 
 coveries ; Dalton, essentially a thinker rather 
 than an experimenter ; Davy, the most brilliant 
 and the most enthusiastic of English chemists; 
 Cavendish, the careful worker, and the founder 
 
188 THE STORY OF THE CHEMICAL ELEMENTS. 
 
 of large branches of experimental chemistry; 
 Graham, the atomist, and the forerunner of the 
 physical chemists of to-day ; and Faraday, the 
 perfect type of a scientific student of nature. 
 France boasts of Lavoisier, the founder of scien- 
 tific chemistry, and the greatest name in the 
 history of the science ; but the Republic declared 
 there was no need for men of science, and the 
 guillotine fell remorselessly. France produced 
 Dumas also, who lived and worked from the 
 time when organic chemistry began till our own 
 days ; an enthusiastic student, a brilliant orator, 
 a great chemist. Of the many great names 
 in the history of chemistry, Germany claims a 
 fair share : Liebig, who alone would add lustre 
 to the annals of any nation ; Humboldt, who 
 might almost be said to have taken all knowledge 
 as his province ; Wdhler, one of the greatest 
 workers in the domain, of mineral chemistry ; 
 Hofmann, the greatest of organic chemists ; and 
 Bunsen, who is still with us, although his 
 working days are finished. Sweden is immor- 
 talised in chemistry by the work of Scheele 
 and Berzelius ; Italy boasts of Avogadro, and 
 Cannizzaro who still lives and labours ; and 
 among chemists none has been more distin- 
 guished for philosophic insight and penetrative, 
 accurate imagination than the Russian Mende- 
 le"eff, who is still in the thick of the fight. 
 There are many other great names in the his- 
 tory of chemistry ; but those I have mentioned 
 suffice to prove that science knows no nationality. 
 Chemistry extends in two directions , it leads 
 the man who studies it to the highest and most 
 
SUMMARY AND CONCLUSION. 189 
 
 recondite theories, and the most imaginative 
 speculations, concerning the nature of matter ; 
 and it has a part in very many of our manu- 
 facturing industries, and touches almost every 
 action of our daily lives. It is the most homely, 
 and also the most detached, of the sciences. 
 Chemistry is concerned with the commonest acts 
 of the ordinary lives of men; and in studying 
 chemistry it is literally true that " there is not a 
 moment that passes in which we do not hold 
 the infinite in our hand." 
 
GENERAL INDEX. 
 
 A. 
 
 Acetic acid (the acid of wine 
 vinegar), 121, r=C 2 H 4 2 
 123, 125. 
 
 Acids, 82, 111. 
 
 Acids and alkalis, 84. 
 
 Acidic. Thdle oxides which 
 dissolve in water to form 
 acrid liquids that dissolve 
 metals (and several oxides 
 which do not dissolve in 
 water, but which combine 
 with basic oxides to form 
 salts) are called ACIDIC 
 OXIDES, 106. They are 
 mostly oxides of " oxygen- 
 like elements," more com- 
 monly called ' ' non-metals, " 
 111. The acidic, or basic 
 characters of oxides are 
 conditioned by the relative 
 quantities of oxygen they 
 contain, 112. 
 
 Alchemy, ] 4 sag., 22, 72-75, 87. 
 
 Alcohol, 13, 42, 125, 128. 
 
 (absolute), 37. 
 
 Alizarin, 115. 
 
 Alkalis, 84, 88. 
 
 Aluminium, 57, 101. 
 
 chloride, 102. 
 
 fluoride, 103. 
 
 Ammonia, 21, 31, 32, 84, 151. 
 
 Ammonium cyanate, 142. 
 
 Aniline, 13, 114, 179, 134 = 
 (C 6 H 5 )NH 2 dyes, various 
 colour of, 182. 
 
 Animals, 11, 114. 
 
 Antimony, 57, 58. 
 
 Antipyrine, 138. 
 
 Apparatus (chemical), 141. 
 
 Aqua fortis, 12, 124, (the 
 common acid called) 82. 
 
 Argon, 57. 
 
 Arsenic, 57, 58. 
 
 Atom. The hypothetical ulti- 
 mate particles of an element 
 are called atoms, 145, cf. 
 
 154, 165. 
 
 (properties, number, 
 
 and) arrangement of atoms, 
 167. 
 
 always in motion, 
 
 173. 
 
 Atomic weight. The number 
 which tells how many times 
 an atom of an element is 
 heavier than an atom of 
 hydrogen is called the 
 
 ATOMIC WEIGHT of that 
 
 element. Thus the atomic 
 weight of hydrogen is 1 
 (159), of nitrogen, 14 (150), 
 of oxygen 16 (149, 163), of 
 sulphur 16, (148), of 
 chlorine, 35 '5. 
 
 Atomic -holding power, 171, 
 172, 173. 
 
 Attar of roses, 11. 
 
 Attractive force, 172. 
 
 Avogadro, 154, 188. His Law, 
 
 155. " Equal volumes of 
 gases, equal numbers of 
 
 molecules," 156, 160. 
 
 191 
 
192 
 
 INDEX. 
 
 B, 
 
 Balance, the, 22, 26. 
 
 Barley (malting of), 41. 
 
 Baryta (an alkali), 84. 
 
 Basic. Those oxides whose 
 aqueous solutions are not 
 acrid, and do not dissolve 
 metals (and many oxides 
 which do not dissolve in 
 water, but dissolve in 
 aqueous solutions of acidic 
 oxides, and combine with 
 these acidic oxides to form 
 salts), are called BASIC 
 OXIDES, 106. They are 
 oxides of metals, 110. 
 
 oxide of sodium, 108. 
 
 Beer, 13, 41, (German) 44. 
 
 Benzene, 134. 
 
 Bergamot (oil of), 11. 
 
 Beryllium, 58. 
 
 Berzelius, 58, 188. 
 
 Bessemer, 104. 
 
 Bicarbonate of soda, how 
 produced, 108. 
 
 Binary compounds, 105. 
 
 Bismuth, 57, 58. 
 
 Black (Prof.), 187. 
 
 Blast-furnaces, 100. 
 
 Blood and bones, 114 
 
 Borax and boron, 57, 58, 59. 
 
 Brazil-wood, 115. 
 
 Breathing, 60 
 
 Bromide of ammonium, 94. 
 
 of potassium, 94. 
 
 Bromine, 57, 95, 96, 102. 
 
 Bunsen, 188 ; B. -burner, 19. 
 
 Burnt-lime, 37. 
 
 C. 
 
 Camera and lenses, 94. 
 Cannizzaro, 188. 
 Carbon 29, 30, 57, 114, 115. 
 monoxide, 99, 102. 
 
 Carbonate of soda, 12. 
 
 Carbonates, 100. 
 
 Carbonic acid gas, 13, 42, 43, 
 
 72, 99, 107, 162. 
 Caustic soda, 12. 
 Cavendish, 187. 
 Cedar-wool oil, 11 
 Chalk, 12, 23, 72, 108. 
 Change (of substance or of 
 
 form or of properties), 9, 
 
 33, 91, 95. 
 
 Charcoal, 29, 98, 101, 114. 
 Charles X. (at the Tuileries), 
 
 120. 
 Chemical changes in brewing, 
 
 Chloride of silver, 92. 
 Chlorine, 31, 57, 78, 92, 102, 
 
 120, 160. 
 
 Chromate of potash, 109. 
 Chrome -ironstone, 109. 
 Citric acid, 82. 
 Clays, 101, 102. 
 Coal, 12, 114. . 
 
 gas, 115. 
 
 tar, 12 ; " coal-tar 
 
 colours," 134. 
 Cobalt, 57, 59. 
 Cochineal, 115. 
 Coffee beans (roasting of), 36. 
 Coke, 55, 98, 101, 114. 
 Combining weights, 65, 117, 
 
 145, 149. 
 Components, 24. 
 Composition, 22, 24, 30, 76, 
 
 141. 
 Compound radicle, 132 ; cf. 
 
 138, 142, 143, 177. 
 Conservation of mass, 51, 56. 
 
 See Law. 
 
 Copper (scales), 77. 
 Cryolite, 103. 
 
 D. 
 
 Daguerre, 93. 
 
INDEX. 
 
 193 
 
 Dalton (John), 69, 187; his 
 
 theory of atoms, 143 sqq. 
 Davy (Sir Humphry), 58, 
 
 187. 
 Decay (gases formed by), 
 
 115. 
 
 Decomposition, 96. 
 Deoxidised, 83. 
 Developers, 93-5-6-7. 
 Diamond, 57, 114, 
 Diastase, 42. 
 Dichloracetic acid = 
 
 C 2 H 2 C1 2 2 , 123. 
 Disintegration, 125. 
 Dross (iron oxide unmelted), 
 
 104. 
 
 Drugs, 115, 137. 
 Double hyposulphite of silver 
 
 and soda, 92. 
 Dumas, 119-123, 188. 
 
 E. 
 
 Electric, (current) 77, 83, 
 
 103, (force) 172. 
 Elements, 48, 57, 59. 
 Emulsion, 94. 
 Ether, 125,=OC 4 H 10 128. 
 Ethereal odours, 125, 126. 
 Ethyl=C 2 H 5 , 132. 
 
 F. 
 
 Faraday, 188. 
 
 Febrifuge, 137. 
 
 Fermentation, 42. 
 
 Fixing (a photograph), 96. 
 
 Flame, free from particles of 
 
 unburnt carbon is not 
 
 luminous, 20. 
 Flesh, 114. 
 Fluorine, 57, 102. 
 Flux, 109. 
 
 Formula, 68, 126, 127. 
 Frankincense, 11. 
 N 
 
 G. 
 
 Gallium, 59. 
 
 Gas, 144. 
 
 Gases, pressure, expansion 
 
 and solubility of different, 
 
 144. 
 
 Gelatine, 94. 
 Geological classification of 
 
 rocks, 40. 
 Germanium, 59. 
 German manufacturers, 137, 
 
 180. 
 
 Glass, 12. 
 Glycerin, 12, 
 Graham, 188. 
 Graphite, 114. 
 Gravity, 172. 
 Gold, 57, (chloride of) 97. 
 Gun-cotton, 114. 
 Gum-arabic, 11. 
 Gum-tragacanth, 11. 
 Gutta-percha, 11. 
 
 H. 
 
 Helium, 28, 57. 
 
 Hides, hoofs, and horns, 12. 
 
 Hofmann, 188. 
 
 Homogeneous kinds of 
 matter, 35-42. 
 
 Hops, 42. 
 
 Humboldt, 188. 
 
 Hydrochloric acid (gas), 31, 
 32, 79, 120, 122, 160. 
 
 (solution), 83, 107, 128. 
 
 Hydrogen, 29, 83 ; inflam- 
 mable, and the lightest 
 kind of matter known, 57, 
 147, 158 ; compounded with 
 carbon and oxygen, 63, 
 116 ; some reactions of, 81- 
 90. 
 
 Hyposulphite of soda, 92, 96, 
 97. 
 
194 
 
 INDEX. 
 
 I. 
 
 India-rubber, 11. 
 
 Indigo, 11, 115. 
 
 Iodide of silver, 93. 
 
 Iodine, 57. 
 
 Iron, 33, 57, (cast) 11, (filings) 
 13, 134, (pig) 56, 103 ; oxide 
 of iron, 103, 108, or ferric 
 oxide, 108, or iron rust, 
 78, is composed of iron and 
 oxygen, 56 ; (scrap) 12, 
 (smelting, process of) 98- 
 100, (ore) 11, (wire) 19 
 
 L. 
 
 Lamp-black, 114. 
 
 Lavoisier, 188. 
 
 Law of conservation of mass, 
 
 61. 
 of combining and reacting 
 
 weights, 66. 
 
 Avogadro's, 155. 
 
 Leather, 12. 
 
 Liebig, 119, 188. 
 
 Lime, 23, 55. 56, 71, (slaked) 
 
 71, 84. 
 
 Logwood, 115. 
 Lunar caustic (nitrate of 
 
 silver), 90. 
 
 M. 
 
 Madder, 11. 
 
 Magnesium, 82. 
 
 Manganese, 12. 
 
 Manufacturing industries, 11, 
 12, 45, 61, 105, 134, 185, 
 189 ; various kinds requir- 
 ing chemistry, 180, 181. 
 
 Marble, 108. ' 
 
 MendelSeff, 188. 
 
 Mercury, 57, 58, 59. 
 
 Metalic, chloride, bromide, 
 and fluoride, 102. 
 
 Metalic oxides, 78, 100, 101, 
 102. 
 
 Metals, 57, 58. 
 
 Milk, 35. 
 
 of lime, 12. 
 
 Mixtures, 35, 41, 48. 
 
 Molecule, 154. 
 
 Molecule (structure of) ; 169 ; 
 e.g. of water, 173 ; of 
 alcohol, 174; of ammonium 
 cyanate, 175 ; and of urea. 
 176. 
 
 Molecular structure and the 
 properties of compounds 
 (connection between), 180. 
 
 Molecular symmetry, 181, 182. 
 
 Molecular weights are mea- 
 sured by that of hydrogen 
 which is taken as 2, 158, 
 161. 
 
 Monochloracetic acid, 123. 
 
 Morphine, 11. 
 
 Mortar, 71. 
 
 Moon, 61, 59. 
 
 Muriatic acid. See Hydro- 
 chloric acid. 
 
 Myrrh, 11. 
 
 N. 
 
 Names of elements (origin 
 
 of), 58, 59. 
 Neutralising acids by alkalis, 
 
 85. 
 
 Nickel, 57, 59. 
 Niobium, 59. 
 Nitrate of potassium, 94. 
 of silver (solution of), 90, 
 
 94. 
 
 Nitric acid, 129, 130. 
 Nitrog;en, 31, 57, 83, 111, 150. 
 
 dioxide, 54. 
 
 Nitro-benzene, 134. 
 Nitro-glycerin, 130. 
 Non-metals, 111. 
 
INDEX. 
 
 195 
 
 o. 
 
 Oils (fuel), 115. 
 Oil of almonds (an oil smelling 
 like), 13, 134. 
 
 bergamot, 11. 
 
 cedar- wood, 11. 
 
 cotton-seed. 12. 
 
 olive, 12. 
 
 vitriol, 12, 81, 125, 134. 
 Organic compounds, 118. 
 Oxides. Compounds of oxygen 
 
 and one other element are 
 called OXIDES, 105 (see 
 acidic and basic), 110. 
 
 Oxide of aluminium, 101. 
 
 chromium 108, (red- 
 acidic) 113, (gregn-basic) 
 109, 113. 
 
 copper, 99. 
 
 iron, 78, 99, 104. 
 
 lead, 99. 
 
 manganese (basic) 112, 
 
 (acidic) 113. 
 
 potassium 129, (basic), 
 
 109, (solid) 109. 
 
 sodium, 129. 
 
 sulphur (gaseous), 54. 
 Oxygen, 29, 57, 111, &c. 
 
 P. 
 
 Paraffin, 39. 
 Pearlash, 12. 
 Peat, 115. 
 Pewter, 45. 
 Phenacetin, 138. 
 Phenyl=C 6 H ft , 134. 
 Phosphorus, 57, 58, 111. 
 Photography, 90-98, 104. 
 Plants, 11, 114. 
 Platinum, 57. 
 
 Potash,84; (solid) or hydroxide 
 of potassium, 128. 
 
 Potassium, 67, 78. 
 acetate, 130, 131. 
 chloride, 128. 
 
 cyanide, 102. 
 
 iodide, 130, 131. 
 
 Priestley, 187. 
 
 Properties, 21 ; chemical pro- 
 perties (chap, v.), better 
 called '* chemical reactions," 
 77. 
 
 Prussian blue, 12. 
 
 Prussiate of potash, 12. 
 
 Pyrites, 12 
 
 Q. 
 
 Qualitative facts, 78. 
 
 Quantitative experiments, 24, 
 25, 70, 108. 
 
 Quantities of matter mea- 
 sured by their weights, 25. 
 
 Quartz, 12. 
 
 Quinine, 11, 137, 138. 
 
 R. 
 
 Reacting weights. See com- 
 bining weights and law. 
 Reagent, 63, 93, 95. 
 Red-lead, 77. 
 Ruskin, on symmetry, 182. 
 
 S. 
 
 Saffron, 115. 
 Salammoniac, 20, 31, 32. 
 Salt (common), 12, 55 ; 
 
 solution of, 90 ; boiling 
 
 solution of, 92. 
 Salts, 85, 106, 110. 
 Sand, 12, 71. 
 Scents, 115. 
 Scheele, 188. 
 Scientific hypotheses, 19. 
 
 method, 90. 
 
 Selenion, 59. 
 
196 
 
 INDEX. 
 
 Sensitive surface (photo- 
 graphy), how prepared, 
 94. 
 
 Silicate of lime, 72. 
 
 Silicon, 58, 72, 101. 
 
 Silver, 95. 
 
 bromide, 94, 95. 
 
 chloride, 96. 
 
 Slag, 56. 
 
 Smokeless powders, 114. 
 
 Soap, 12. 
 
 Soda, 84 (hydroxide of 
 sodium), 128, 189. 
 
 calcined, 79. 
 
 lye, 20, 107. 
 
 washing, 55, 79, 108. 
 
 Sodium, 67, 78, 102. 
 
 chloride, 102, 130, 131. 
 
 fluoride, 103. 
 
 nitrate, 129, 130. 
 
 Spectroscope, 26. 
 
 Spirits of hartshorn, 21, 31, 84. 
 
 of salt, 31, 82, 83. 
 
 Steam, 78, 82; (dry), 156- 
 158. 
 
 Steel,' 12, 104. 
 
 Sublimate, 25. 
 
 Substitution (in organic 
 chemistry), 120-140; (128, 
 133, 135). 
 
 Sugar, 11, 20, 28, 35, 43, 56, 
 79. 
 
 cane, 11. 
 
 Sulphide, 100. 
 
 Sulphur, 12, 54, 57, 58, 111. 
 
 dioxide, 100. 
 
 trioxide, 108. 
 
 Sulphuric acid, 54, 55, 68, 77. 
 
 Sunshine, 83, 91. 
 
 Synthesis, 142. 
 
 T. 
 
 Tables of combining weignts 
 (68), (atomic weights), 145, 
 mentioned. 
 
 Talbot, 90 sqq. 
 
 Tantalum, 59. 
 
 Tellurium, 59. 
 
 Three dimensions, 179. 
 
 Tin, 57, 
 
 Tissues, 114. 
 
 Toning (a photograph), 97. 
 
 Trace, 92, 95, 104. 
 
 Trichloracetic acid, 123. 
 
 U. 
 Urea, 119, 142, 166 
 
 V. 
 
 Vanadium, 59. 
 Vinegar, 13, 82. 
 "Vital force," 119. 
 
 W. 
 
 Water, (pure) 39, 68, 78, (some 
 reactions of) 177-80. 
 
 gas, 158. 
 
 Wax, 121. 
 
 Wax-candles, 120. 
 
 Weights (atomic), how found 
 161, (combining) 149, 
 (molecular) 161, (combin- 
 ing and reacting) 166. 
 
 Whist, 60. 
 
 Wine, 13, 115. 
 
 Wohler, 119, 166, 188. 
 
 Wood, 115 ; (chips of), 20. 
 
 Wort, 42. 
 
 Y. 
 
 Yeast, 42. 
 Ytterbium, 58. 
 
 Zinc, 57, 102. 
 
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