D. 0. CAULDWELL 206 NEALE AVENUS KETTEBlHGi A MANUAL PHOTOGRAPHIC CHEMISTEY. A MANUAL OF PHOTOGRAPHIC CHEMISTRY, INCLtTDING THE PRACTICE OF THE COLLODION PROCESS. BY T. FREDEEICK HARDWICH, LA.TE DEMOKSIBAIOB OF CHBHISIBT IN Klira'S COLIiEQX, hOSDOS. EfjirlJ (ElJttton. LONDON: JOHN CHURCHILL, NEW BURLINGTON STREET. MDCCOIVI. ne Author reterves to himtelf the right of translating this Edition. mm&wm PRINTED BY JOHJf BDWAED TATtOE, IITTIE QUEEN STBEET, LINCOLN'S INN FIELDS. PREFACE TO THE THIRD EDITION. It is a source of much gratification to the Author to find himself csLlled. upon to prepare a Third Edition of his Ma- nual in less than fourteen months from the date of its first publication. No greater proof could have been afforded of the rapid advance which the Photographic Art is now making in this country. On once more entering upon the task of revision, the Writer has been led to reflect in what way the utUity of the Work may be promoted ; and from numerous inquiries he believes that this result will best be attained by care- fully omitting everything which does not possess practical as well as scientific interest. The majority of Photogra- phers look to the Art to furnish them with amusement as well as instruction, and they are deterred from entering upon a study which seems to involve a great amount of technical detail : these remarks however are not intended to discourage a habit of perseverance and careful observa- tion, but simply to distinguish between the essential and the non-essential in the theory of the subject. The present Edition differs in many important particu- lars from those which have preceded it. It has undergone a fresh arrangement throughout. In some parts it is con- densed, in others enlarged. The Chapters on Photogra- phic printing are entirely re-written, and include the whole PEEFACE. of the Author's investigations, as published in the Society's J ournal. The minute directions given in this part of the Work will show how much success in Photography is thought to depend upon a careful attention to minor par- ticulars. Another point which has been kept in view, is to re- commend, as far as possible, the employment of chemical agents which are used in medicine, and vended by all druggists throughout the united kingdom. It is often an advantage to the Amateur to be able to purchase his ma- terials near at hand ; and, if the common impurities of the commercial articles are pointed out, and directions given for their removal,— the 'London Pharmacopoeia' will be found to include almost all the chemicals necessary for the practice of the Art. Great additions have been made to the Index of the pre- sent Edition ; which is now so complete that a reference to it will at once point out the most important facts rela- ting to each subject, and the different parts of the Work at which they are described. In conclusion, a hope is expressed that this ' Manual of Photographic Chemistry' may be found to be a complete and trustworthy guide on every point connected with the theory and practice of the Collodion process. London, June 2nd, 1856. CONTENTS. PART I. THE SCIENCE OE PHOTOGRAPHY. Page Introduction 1 CHAPTER I. Historical Sketch of Photography . . . ... . 6 CHAPTER II. the salts of silver employed in photography. Section I. — The Chemistry of the Salts of Silver. — The pre- paration and properties of the Nitrate of Silver — of the Chloride, Bromide, and Iodide of Silver. — The Chemistry of the Oxides of Silver 12 Section II.— 2%^ Photographic Properties of the Salts of Silver. — The action of Light upon Nitrate of Silver— upon Chlo- ride, Bromide, and Iodide of Silver.— The blackening of Chloride of Silver explained.— Simple experiments with sensitive Photographic paper 18 CONTENTS. CHAPTER III. ON THE DEVELOPMENT OF AN INVISIBLE IMAGE. Page Simple Experiments illustrating the ])roeess 25 Section I. — Chemistry of the Substatices employed as Develo- pers. — Development shown to he a process of reduction. — The chemistry of the principal reducing agents, Gallic Acid, PyrogaUic Acid, and the Protosalts of Iron 26 Section II. — The Reduction of Salts of Silver by Develop- ing Agents. — Reduction of Oxide of Silver — of Nitrate and Acetate of Silver. — Varied appearance of metallic Silver when finely divided. — The reduction of the Iodide and other salts of Silver containing no Oxygen 30 Section III. — Hypothesis on the Formation of an Invisible Image upon Iodide of Silver. — Explanation of the terms under- and over-exposure. — Diagram of Molecular change. Moser's experiments on development. — Curious perver- sions of development 34 CHAPTER IV. ON " FIXING " THE PHOTOGRAPHIC IMAGE. Chemistry of the various substances which may be employed as Fixing Agents. — Ammonia, Alkaline Chlorides, Bromides, and Iodides. — Hyposulphite of Soda. — Cyanide of Potas- sium 40 CHAPTER V. ON THE NATURE AND PROPERTIES OF LIGHT. Section I. — The Compound Nature of Light. — Its decompo- sition into elementary coloured rays. — Division of these rays into Luminous, Heat-producing, and Chemical Rays 45 Section II. — The Refraction of Light. — Phenomena of simple refraction by parallel and inclined surfaces. — Refraction from curved surfaces. — ^The various forms of Lenses. — CONTENTS. ix The Foci of Lenses. — Formation of a Luminous Image by a Lens . Section III. — The Photographic Camera. — Its simplest form. — The field of the Camera. — Chromatic aberration.— Spherical aberration.— The use of Stops.— The double, or Portrait combination of Lenses. — Variation between the Visual and Chemical Foci in Lenses Section IV.— The Photographic Action of Coloured Light.— Diagram of Chemical Spectrum.— Illustrative experiments. — Superior sensibility of Bromide of SUver to coloured light.— Mode in which dark-coloured objects are Photo- graphed CHAPTER VI. THE PHOTOGKAPHIC PKOPERTIES OP IODIDE OP SILTEE UPON COLLODION. Section I. — Collodion. — Pyroxy line— its varieties— its chemical composition.— Means of obtaining Nitro-sulphuric Acid of the proper strength.— Solvents for Pyroxyline.— Variation of physical properties in different samples of Collodion. — The changes which Iodized Collodion undergoes by keep- ing 67 Section II.— 27*^ Chemistry of the Nitrate Bath.— Its solvent action on Iodide of Silver.— Acidity and Alkalinity of the Nitrate Bath.— Formation of Acetate of Silver in the Bath. — The substances which decompose the Bath ... 79 Section III.— The Formation of the Sensitive Film.— The physi- cal characters of the film modified by the amount of Io- dide of SUver — also by the time allowed to elapse before, and during, immersion in the Bath. — Characteristics of a perfect film 84 Section IV. — The Conditions which influence the Formation and Development of the Latent Image. — The causes which increase or diminish the sensitiveness of the film to Light. — The conditions which hasten or retard development . . 87 Section v.— 0« cer^in irregularities in the Developing Pro- X CONTENTS. cess. — Effect of certain states of the Bath in producing mistiness of image, and of acids in obviating it ... . R V represents the focus of the violet ray, Y of the yellow, and E of the red. Hence, as the chemical action corresponds more to the violet, the most marked actinic effect would be produced at V. The luminous portion of the spectrum however is the yellow, consequently the visual focus is at Y. Photographers have long recognized this point; and therefore, with ordinary Lenses, not corrected for colour, rules are laid down as to the exact distance which the sen- sitive plate should be shifted away from the visual focus, in order to obtain the greatest amount of distinctness of outline in the image impressed by chemical action. These rules do not apply to the Achromatic Lenses re- cently described. The coloured rays being m that case bent together again and reunited, the two foci also nearly correspond. By a little further correction to a point higher in the Spectrum, they are made to do so perfectly. SECTION IV. On the FhotografUc Action of Coloured Light. It has already been mentioned in the first section of this chapter that certain of the elementary colours of white light, viz. the violet and indigo, are peculiarly active in decomposing the Photographic Salts of Silver ; but there are some points of importance relating to the same subject which require a further notice. 60 JTATUEE AND PEOPEETIES OF LIGHT. The term "actinism" (Gr. uktIs, a ray or flash) has been proposed as convenient to designate the property pos- sessed by light of producing chemical change ; the rays to which the effect is especially due being known as actinic rays. If the pure Solar Spectrum formed by prismatic analysis in the manner represented at page 46 be allowed to impinge upon a prepared sensitive surface of Iodide of Silver, the latent image being subsequently developed by a reducing agent, the effect produced will be something similar to that represented in the following diagram : — J- Fig. 2. Fig. 1 shows the visible spectrum as it appears to the eye ; the brightest part being in the yellow space, and the light gradually shading off until it ceases to be seen. Pig. 2 represents the chemical effect produced by throwing the Spectrum upon Iodide of Silver. Observe that the dark- ening characteristic of chemical action is most evident in the upper spaces, where the light is feeble, and is altogether absent at the point corresponding to the bright yellow spot of the visible spectrum. The actinic and luminous spectra are therefore totally distinct from each other, and the word " Photography," which signifies the process of taking pic- tures by liffht, is in reality inaccurate. To those who have not the opportunity of working with NATURE AND PEOPEETIES OF LIGHT. 61 the Solar Spectrum, the following experiments will be useful in illustrating the Photographic value of coloured light. Experiment I. — Take a sheet of sensitive paper prepared with Chloride of SUver, and lay upon it strips of blue, yeUow, and red glass. On exposure to the sun's rays for a few minutes, the part beneath the blue glass darkens rapidly, whilst that covered by the red and yellow glass is perfectly protected. This residt is the more striking from the extreme transparency of the yeUow glass, giving the idea that the Chloride would certainly be blackened first at that point. On the other hand, the blue glass appears very dark, and effectually conceals the tissue of the paper from view. Experiment II. — Select a vase of flowers of different shades of scarlet, blue, and yellow, and make a Photo- graphic copy of them, by development, upon Iodide of Silver. The blue tints vrill be found to act most violently upon the sensitive compound, whilst the reds and yellows are scarcely visible ; were it not that it is difficult to pro- cure in nature pure and homogeneous tints, free from ad- mixtvire with other colours, they would make no impres- sion whatever upon the plate. In exemplifying further the importance of distinguishing between visual and actiuic rays of light, we may observe that if the two were in aU respects the same. Photography must cease to exist as an Art. It would be impossible to make use of the more sensitive chemical preparations from the difficulties which would attend the previous prepara- tion and subsequent development of the plates. These operations are now conducted in what is termed a darle room ; but it is dark only in a Photographic sense, being illuminated by means of yellow light, which, whilst it en- ables the operator easily to watch the progress of the work, produces no injurious effect upon the sensitive surfaces. If the windows of the room were glazed with blue in place of yeUow glass, then it would be strictly a " dark 62 NATUEE AND PEOPEETIES OF LIGHT. room," but one altogether unfitted for tlie purpose in- tended. Another point connected with the same subject and worthy of note is — the extent to which the sensibility of the Photographic compounds is influenced by atmosphe- ric conditions not visibly interfering with the brightness of the light. It is natural to suppose that those days on which the sun's rays are the most powerful would be the best for rapid impression, but such is not by any means the case. If the light is at all of a yellow cast, however bright it may be, its actinic powers will be small. It wiU also be often observed in working towards the evening, that a sudden diminution of sensibility in the plates begins to be perceptible at a time when but little difference can be detected in the brilliancy of the light ; the setting sun has sunk behind a golden cloud, and aU chemical action is soon at an end. In the same manner is explained the difficulty of obtain- ing Photographs in the glowing light of tropical climates ; the superiority of the early months of spring over those of the midsummer ; of the morning sun to that of the afternoon, etc. April and May are usually considered the best months for rapid impression in this country ; but the light continues good until the end of July. In August and September a longer exposure of the plates will be re- quired. THE STTPEEIOE SENSIBILITY OP BEOMIDE OF SILVEE TO COLOUEED LIGHT. In copying the Solar Spectrum alternately upon a sur- face of Iodide and Bromide of Silver, we notice a difference in the Photographic properties of these two salts. The latter is affected more extensively, to a point lower in the spectrum, than the former. In the case of the Iodide of Silver, the action ceases in the Blue space ; but with the Bromide it reaches to the Green. This is shown in the following diagrams, which are drawn from the obser- NATUEE AND PEOPEETIES OF LIGHT. 63 vations of Mr. Crookes (' Pliotographic Journal,' vol. i. p. 100):— Fig. 1. Fig. 3. Fig. 3. Violet. Indigo. Blue. Green. YeUow. Orange. Pig. 1 represents tlie chemical spectrum on Bromide of Silver ; fig. 2 tke same upon Iodide of Silver ; and fig. 3 tlie visible spectrum. It might periiaps be supposed tbat tbe superior sensi- bility of the Bromide of Silver to green rays of light would render that salt useful to the Photographer in copying land- scape scenery ; and indeed it is the opinion of many that, in the Calotype paper process, the dark colour of foliage is better rendered by a mixture of Bromide and Iodide of Silver than by the latter salt alone. This however cannot depend upon the greater sensibility of the Bromide to coloured light, as may easily be proved. — The diagrams given above are shaded to represent as nearly as possible the relative intensity of the chemical 64 NATtTEE AND PEOPEETIES OF LIGHT. action, exerted by tlie rays at different points of the spec- trum ; and on referring to tliem it wiU be seen that the maximum poiut of blackness is in the indigo and violet space, the action being more feeble in the blue space lower down ; there are also highly refrangible rays ex- tending upwards far beyond the visible colours, and these invisible rays are actively concerned in the formation of the image. It is evident therefore that the amount of effect pro- duced by a pure green, or even a light blue tint, upon a surface of Bromide of Silver is very small as compared with that of an indigo or violet ; and hence, as in copying natural objects radiations of all kinds are present at the same time, the green tints have not time to act before the image is impressed by the more refrangible rays. Sir John Herschel has proposed to render coloured light more available in Photography by separating the actinic rays of high refrangibUity, and working only with those which correspond to the blue and green spaces in the spectrum. This may be done by placing in front of the Camera a vertical glass trough containing a solution of Sulphate of Quinine. Professor Stokes has lately shown that this liquid possesses curious properties. In transmit- ting rays of light it modifies them so that they emerge of lower refrangibUity, and incapable of producing the same actinic effect. Sulphate of Quinine is, if we may use the term, opaque to all actinic rays higher than the blue- coloured space. The proposition of Sir John Herschel above referred to was therefore to employ a bath of Sul- phate of Quinine, and having eliminated the actinic rays of high refrangibihty, to work upon Bromide of Silver with those corresponding to the lower coloured spaces. In this way he conceived that a more natural effect might be obtained. If Photographic compounds should be discovered of greater sensibility than any we at present possess, the use of the Quinine bath wiU perhaps be adopted ; but at NATTJEE AND PEOPEETIES OF LIOHT. 65 present we trust to the superior intensity of the invisible rays for the formation of the image, and hence the em- ployment of Bromide of Silver is less strongly indicated. These remarks apply to Photographs taken by sunlight. Mr. Crookes has lately shown that in working with arti- ficial light, such as gas or camphine, the case is different. Actinic rays of high refrangibility are comparatively want- ing in gas-light, the great bulk of the Photographic rays being found to lie within the limits of the visible spectrum, and consequently acting more energetically upoh Bromide than on Iodide of Silver. Explanation of the mode in xvJiicTi Coloured Objects im- press the Sensitive Film. — The fact of which we have been speaking, viz. that the natural colours are not always cor- rectly represented in photography, is often urged in depre- ciation of the art, — " when lights are represented by sha- dows, how, it is said, can a truthful picture be expected ?" The insensitiveness of Iodide of Silver to the colours oc- cupying the lower portion of the spectrum would indeed present an insuperable difficulty if the tints of Nature were pure and homogeneous : such however is not the case. Even the most sombre colours are accompanied by scattered rays of white hght in quantity amply sufficient to affect the sen- sitive film. This is especially seen when the coloured body possesses a good reflecting surface, and hence some varieties of foli- age, as for instance the Ivy with its smooth and polished leaf, are more easily photographed than others. So again with regard to drapery in the department of portraiture — it is necessary to attend not only to the colour, but also to the material of which it is composed. Silks and satins are favourable, as reflecting much light, whilst velvets and coarse stuffs of aU kinds, if at aU dark, produce very little effect upon the sensitive film. 66 CHAPTER VI. THE PHOTOGEAPHIC PEOPEETIES OE IODIDE OE SILTEE UPON COLLODION, In the preceding part of tliis work tlie physical and che- mical properties of Chloride and Iodide of Silver have been described, with the changes which they experience by the action of Light. Nothing however has been said of the surface used to support the Iodide of Silver, and to expose it in a finely divided state to the iafluence of the actinic radiations. This omission will now be supplied, and the use of Collodion will engage our attention. The sensibility of Iodide of Silver upon CoUodion is greatly superior to that of the same salt employed in con- jimction with any other vehicle at present known. Hence the CoUodio-Iodide film will supersede the paper and Albu- men processes in all cases where objects liable to move are to be copied. The causes of this superior sensitiveness, as far as ascertained, may be principally referred to the state of loose coagulation of a Collodion film and other particulars presently to be noticed. It must however be allowed that there are many points affecting the sensitive- ness of Iodide of Silver, both mechanical and chemical, of the exact nature of which we are ignorant. The present chapter may be divided into five Sections — the chemistry of Collodion — of the Nitrate Bath — of the sensitive film prepared ready for the Camera — the causes affecting the impression and development of the Image COLLODIO-IODIDE OF SILVEE. 67 upon Collodion — the various irregularities wiiicli interfere with the development of the Image. SECTIOIS" I. Collodion. Collodion, so named from the Greek word KoWaa, to sticJc, is a glutinous, transparent fluid, procured — as gene- rally said — by dissolving Gun Cotton in Ether. It was originally used for surgical purposes only, being smeared over wounds and raw surfaces, to preserve them from contact with the air by the tough film which it leaves on evaporation. Photographers employ it to support a deli- cate film of lodijde of Silver upon the surface of a smooth glass plate. Two elements enter into the composition of Collodion : first, the Gun Cotton ; second, the fluids used to dissolve it. Each of these will be treated in succession. A. CHEMISTRY OE PTEOXYLINE. Gun Cotton or Fyroxyline is Cotton or Paper which has been altered in composition and properties by treatment with strong acids. Both Cotton and Paper are, chemically, the same. The microscope reveals fibres which are found on analysis to have a constant composition. They contain three ele- mentary bodies, Carbon, Hydrogen, and Oxygen, united together in fixed proportions ; and to this combination the term Lignine has been applied. Lignine is a definite chemical compound, in the same sense as Starch or Sugar, and consequently, when treated with various reagents, it exhibits properties peculiar to itself. It is insoluble in most liquids, such as Water, Alcohol, Ether, etc., and also in dilute acids ; but when acted upon by Nitric Acid of a certain strength it liquefies and dissolves. 68 ON THE CHEMISTET OF THE It has been already shown (p. 12) tliat wlien a body dissolves in Nitric Acid the solution is not usually of the same nature as an aqueous solution ; and so in the case of Lignine — the Nitric Acid does not take it up as Lignine, but it imparts Oxygen first, and afterwards dissolves it. Preparation of Pyroxyline. — If, instead of treating Lignine with Nitric Acid, a mixture of Nitric and Sul- phuric Acids in certain proportions be used, the effect is peculiar. The fibres contract slightly, but undergo no other visible alteration. Hence we are at first disposed to think the mixed Acids ineffectual. This idea however is not correct, since on making the experiment the properties of the cotton are found to be changed. Its weight has increased by more than one-half ; it has become soluble in various liquids, such as Acetic Ether, Ether and Alcohol, etc., and, what is more remarkable, it no longer burns in the air quietly, but explodes on the application of flame with greater or less violence. This change of properties clearly shows, that although the fibrous structure of the Cotton is unaffected, it is no longer the same substance, and consequently chemists have assigned to it a different name — that of Pyroxyline. To produce the peculiar change by which Lignine is converted into Pyroxyline, both Nitric and Sulphuric Acids are, as a rule, required ; but of the two the former is the most important. On analyzing Pyroxyline, Nitric Acid* or a body analogous to it, is detected in considerable quan- tity, but not Sulphuric Acid. The latter Aqjd, in fact, serves but a temporary purpose, viz. to prevent the Nitric Acid from dissolving the Pyroxyline, which it would be liable to do if employed alone. The Sulphuric Acid pre- vents the solution by removing water from the Nitric Acid, and so producing a higher degree of concentration ; Pyroxyline, although soluble in a dilute, is not so in the strong Acid, and hence it is preserved. The property possessed by OD. of Yitriol of removing COLLODIO-IODIDE OF SILVEE. 69 water from otlier bodies, is one with which it is well to be acquainted. A simple experiment will serve to illus- trate it. Let a small vessel of any kind be filled to about two-thirds with Oil of Vitriol, and set aside for a few days ; at the end of that time, and especially if the at- mosphere be damp, it will have absorbed sufficient mois- ture to cause it to flow over the edge. Now even the strongest reagents employed in chemistry contain, almost invariably, water in greater or lesser quan- tity. Pure Anhydrous Nitric Acid is a white, solid sub- stance ; Hydrochloric Acid is a gas ; and the liquids sold xmder those names are merely solutions. The effect then of mixing strong Oil of Vitriol with aqueous Nitric Acid is to remove water in proportion to the amount used, and to produce a liquid containing Nitric Acid in a high state of concentration, and Sulphuric Acid more or less diluted. This liquid is the Nitro- Sulphuric Acid employed in the preparation of Pyroxyline. Various Forms of Pyroxyline. — Very soon after the first announcement of the discovery of Pyroxyline, most animated discussions arose amongst chemists with regard to its solubility and general properties. Some spoke of a " solution of Gun Cotton in Ether ;" whilst others denied its solubility in that menstruum ; a third class, by follow- ing the process described, obtained a substance which was not explosive, and therefore could scarcely be termed Gun Cotton. On further investigations some of these anomalies were cleared up, and it was found that there were varieties of Pyroxyline, depending mainly upon the degree of strength of the Nitro- Sulphuric Acid employed in the preparation. Still the subject was obscure until the publication of re- searches by Mr. E. A. Hadow, of Bristol. These investi- gations, conducted in the Laboratory of King's College, London, were published in the J ournal of the Chemical Society. Constant reference will be made to them in the following remarks. 70 ON THE CHEMISTRY OE THE We notice — first, tlie cliemical constitution of Py- roxyline ; secondly, its varieties ;^ and third, the means adopted to procure a Nitro-Sulphuric Acid of the proper strength. a. Constitution of Pyroxyline. — Pyroxyline has been sometimes spoken of as a Salt of Nitric Acid, a Nitrate of Lignine. This view however is erroneous, since it can be shown that the substance present is not Nitric Acid, al- though analogous to it. It is the Peroxide of Nitrogen, which is intermediate in composition between Nitrous Acid (NO3) and Nitric Acid (NO5). Peroxide of Nitrogen (NO4) is a gaseous body of a dark red colour ; it possesses no acid properties, and is incapable of forming a class of salts. In order to understand in what state this body is combined with Lignine to form Pyroxyline, it will be necessary to digress for a short time. Law of Substitution. — By the careful study of the action of Chlorine, and of Nitric Acid, upon various organic sub- stances, a remarkable series of compounds has been disco- vered, containing a portion of Chlorine or of Peroxide of Nitrogen in the place of Hydrogen. The peculiarity of these substances is, that they strongly resemble the origi- nals ia their physical, and often in their chemical proper- ties. It might have been supposed that agents of such active chemical afl&nities as Chlorine and Oxide of Nitro- gen would, by their mere presence in a body, produce a marked effect ; yet it is not so in the case before us. The primitive type or constitution of the substance modified remains the same, even the crystalline form being often unaffected. It seems as if the body by which the Hy- drogen had been displaced had stepped in quietly and taken up its position in the framework of the whole with- out disturbance. Many compounds of this kind are known ; they are termed by chemists "substitution compounds." The law invariably observed is, that the substitution takes place in equal atoms : a single atom of Chlorine, for in- stance, displaces one of Hydrogen ; two of Chlorine dis- COILODIO-IODIDE OF SILVEE. 71 place two of Hydrogen, and so on, until, in some cases, tlie whole of the latter element is separated. In illustration of these remarks, take the following in- stances : — Acetic Acid contains Carbon, Hydrogen, and Oxygen ; by the action of Chlorine the Hydrogen may be removed in the form of Hydrochloric Acid, and an equal number of atoms of Chlorine be substituted. In this way a new compound is formed, termed Chloracetic Acid, re- sembling in many important particulars the Acetic Acid itself IS'otice particularly that the peculiar properties characteristic of Chlorine are completely masked in the substitution body, and no indication of its presence is obtained by the usual tests ! A soluble Chloride always gives with Nitrate of Silver a white precipitate of Chlo- ride of Silver unaflPected by Acids, but the Chloracetic Acid does nothing of the kind ; hence it is plain that the Chlorine exists in it in a peculiar and intimate state of combination different from what is usual. The substance we have been previously considering, viz. Pyroxyline, affords another illustration of the Law of Substitution. Omitting, for the sake of simplicity, the number of atoms concerned in the change, the action of concentrated Nitric Acid upon Lignine may be thus ex- plained : — r Carbon Lignine.. Hydrogen ^ ^tric Acid (, Oxygen equals S Carbon Si&itrogen + W^t- Oxygen Or in symbols : — CH^0+N05=C (H„_iN04) 0+HO. By a reference to the formula, it is seen that the fifth atom of Oxygen contained ia the Nitric Acid takes one of 72 ON THE CHEMISTEY OF THE Hydrogen, and forms an atom of Water ; the NO^ then steps in, to fill the gap which the atom of Hydrogen has left. All this is done with so little disturbance that even the fibrous structure of the cotton remains as before. b. Chemical Composition of the Varieties of Pyroxyline. — Mr. Hadow has succeeded in establishing four different substitution compounds, which, as no distinctive nomen- clature has been at present proposed, may be termed com- pounds A, B, C, and D. Compound A is the most explosive Gun Cotton, and contains the largest amount of Peroxide of IsTitrogen. It dissolves only in Acetic lather, and is left on evaporation as a white powder. It is produced by the strongest Nitro- Sulphuric Acid which can be made. Compounds B and C, either separate or in a state of mixture, form the soluble paper or cotton of the Photo- grapher. They both dissolve in Acetic Ether, and also in a mixture of Ether and Alcohol. The latter, viz. C, also dissolves in glacial Acetic Acid. They are produced by a Nitro-Sulphuric Acid shghtly weaker than the last, and contain a smaller amount of Peroxide of JSTitrogen. Compound D resembles what has been termed Xyloidine, that is, the substance produced by acting with Nitric Acid upon Starch. It contains less Peroxide of Nitrogen than the others, and dissolves ia Ether and Alcohol, and also in Acetic Acid. The ethereal solution leaves, on evapora- tion, an opaque film, which is highly combustible, but not explosive. By bearing in mind the properties of these compounds, many of the anomalies complained, of in the manufacture of G-un Cotton disappear. If the Nitro-Sulphuric Acid employed is too strong, the product will be insoluble in Ether ; whilst if it is too weak, the fibres are gelatinized, and partly dissolved by the acid. c. Means adopted to procure a Nitro- Sulphuric Acid of the requisite strength for preparing Pyroxyline. — This is a point of more difficulty than would at first appear. It COLLODIO-IODIDE OF SILTEE. 73 is easy to determine an exact formula for the mixture, but not so easy to hit upon the proper proportions of the acids required to produce that formula ; and a very slight de- parture from them will altogether modify the result. The maia difficulty lies in t?ie uncertain strength of commercial Nitric Acid. Oil of Vitriol is more to be depended upon, and has a tolerably uniform Sp. Gr. of 1-836 ; but Nitric Acid is constantly liable to variation ; hence it becomes necessary to make a preliminary determination of its real strength, which is done either by taking the specific gravity and referring to tables, or, better still, by a direct analysis. As each atom of Sulphuric Acid removes only a given quantity of water, it foUows that the weaker the Nitric Acid, the larger the amount of Sulphuric which will be required to bring it up to the proper degree of concentra- tion. To avoid the trouble necessarily attendant upon these preliminary operations, many prefer to use, in place of Nitric Acid itself, one of the salts formed by the combina- tion of Nitric Acid with an alkaliae base. The composi- tion of these salts, provided they are pure and nicely crys- tallized, can be depended on. Nitrate of Potash, or Saltpetre, contains a single atom of Nitric Acid united with one of Potash. It is an anhi/- drous salt, that is, it has no water of crystaHization. Now when strong Sulphuric Acid is poured upon this Nitrate of Potash in a state of fine powder, in virtue of its su- perior chemical afiinities it appropriates to itself the Alkali and liberates the Nitric Acid. If care be taken to add a sufficient excess of the Sulphuric Acid, a solution is ob- tained containmg Sulphate of Potash dissolved in Sul- phuric Acid, and free Nitric Acid. The presence of the Sulphate of Potash (or, more strictly speaking, of the Bi- Sxilphate) does not in any way interfere with the result, and therefore the efiect is the same as if the mixed acids themselves had been used. The reaction may be thus represented : — OTS THE CHEMISTET OF THE Nitrate of Potasli^^ws Sulphuric Acid in excess = Bisulpliate Potash ^^ms Nitro-Sulphuric Acid. Hecapitulation. — The chemistry of Pyroxyhne, and of the materials employed to form it, having been suiEciently explained, we proceed to speak of its solution in the proper solvents. Before doing so, however, it may be well to in- sist once more on the fact, already stated, that Pyroxyline is, strictly speaking, a neutral substance. Although it con- tains an element of such activity as the Peroxide of JSTitro- gen, yet nevertheless, being a substitution compound, the properties of that oxide are masked and concealed. It is true that when heated a violent explosion ensues, the com- pound being broken up, and the elements reuniting to form simpler bodies ; but at common temperatures Pyroxyline is very stable. As far as relates to its application to Photography, in the present state of our knowledge, we attribute the advantage gained rather to the physical pro- perties of its solution than to the chemical composition of Pyroxyline itself. CHEMISTRY OF THE SOLUTION OF PYEOXYLINE IN" ETHEE AND ALCOHOL, OE " COLLODION." The substitution compounds B and C, already alluded to as forming the Soluble Cotton of Photographers, are both abundantly soluble in Acetic Ether. This liquid however is not adapted for the purpose required, inasmuch as on evaporation it leaves the Pyroxyline in the form of a white powder, and not as a transparent layer. The rectified Ether of commerce has been found to answer better than any other liquid as a solvent for Py- roxyline. If the sp. gravity be about -750, it contains invariably a small proportion of Alcohol, which appears to be necessary ; the solution scarcely taking place with absolutely pure Ether. The Pyroxyline, if properly prepared, begins al- most immediately to gelatinize by the action of the Ether, COLLODIO-IODIDE OF SILVER. 75 and is soon completely dissolved. In this state it forms a slimy solution, wliicli, when poured out on a glass plate, speedily dries up into a liorny transparent layer. In preparing Collodion for Photographic purposes, we find that its physical properties are liable to considerable variation. Sometimes it appears to be very thru and fluid, flowing on the glass almost like water, whilst at others it is thick and glutinous. The causes of these differences wiU now engage our attention. They may be divided into two classes : first, those relating to the Pyroxyhne ; second, to the solvents employed. a. Variation of Properties in different Samples of so- luble Tyroxyline. — The substitution compounds A, B, C, and D differ, as already shown, in the percentage amount of Peroxide of ^Nitrogen present ; it is not therefore sur- prising that the former should be more explosive and inso- luble than the latter. But it often happens in preparing Pyroxyline that two portions of Nitro-Sxilphuric Acid taken from the same bottle yield products which vary in properties, although necessarily the same in composition. Taking extremes in illustration, we notice two principal modifications of soluble Pyroxyline. The first, when treated with the mixture of Ether and Alcohol, sinks down to a gummy or gelatinous mass, which gradually dissolves on agitation. The solution is very fluid in proportion to the number of grains used, and when poured out spreads into a beautifully smooth and glassy surface, which is quite structureless, even when highly magnified. The film adheres tightly to the glass, and is not removed by washing with water. The second variety often dissolves instantly, without pre- viously gelatinizing, and appears at first to be more soluble than the other ; but this is not the case, the liquid formed being thick and glutinous, flowing over the glass in a slimy manner, and soon setting into numerous small waves and cellular spaces. The film lies loose upon the glass, and is apt to separate during the washing which it has subse- quently to undergo. 76 ON THE CHEMISTEY OF THE This subject is not thoroughly understood, but the fol- lowing hints will be found useful. First, the temperature of the JSTitro- Sulphuric Acid at the time of immersing the Cotton influences the result. The soluble variety is pro- duced by warm acids ; the second, or glutinous, by the same acids employed cold. The best temperature appears to be about 120° or 130° Fahrenheit ; if it rises much beyond that point, the acids act upon and dissolve the Cotton. It is also found by experience that the texture of the film is more uniformly good when " Swedish filtering paper " is used in place of cotton-wool. The reason of this is not obvious, but it may possibly be due to a difference in the structure or arrangement of the fibres, since it is stated that other varieties of unsized paper may be substi- tuted for the Swedish paper. Mr. Shadbolt has proposed the use of Chloroform (10 minims to the ounce) as a remedy for the glutinous con- dition of some samples of Collodion. The first effect of its addition is to precipitate a portion of the Pyroxyline in a gelatinous form ; which however redissolves on agitation, and the solution after a few hours usually becomes more fluid. b. The physical properties of Collodion affected hy the proportions and purity of the Solvents. — Pyroxyhne of the varieties termed B and C dissolves freely in a mixture of Ether and Alcohol; but the characters of the resulting solution vary with the relative proportions of the two sol- vents. When the Ether is in large excess, the film is strong and tough, so that it can often be raised by one corner and lifted completely off* the plate without tearing. It is also very contractile, so that a portion of the Collodion poured on the hand draws together and puckers the skiu as it dries. If spread upon a glass plate in the usual way, the same pro- perty of contractility causes it to retract and separate from the sides of the glass. These properties, produced by Ether in large proportion, disappear entirely on the addition of more Alcohol. The COLIiODIO-IODIDE OP SILVEE. 77 transparent layer is now soft and easily torn, possessing but little coherency. It adheres to the surface of the glass naore firmly, and exhibits no tendency to contract and sepa- rate from the sides. In hot weather advantage will be gained by somewhat increasing the quantity of Alcohol in Collodion ; the eva- poration of the solvents being retarded, and the film ren- dered less liable to become dry before development. An- Jiydrous Alcohol of Sp. Gr. '796, may be mixed with pure Ether of "715, even to the extent of equal parts ; but this is the extreme limit, and with the strongest spirit ordi- narily obtainable, the Collodion will become somewhat glutinous if the proportions (by measure) of 5 parts of Ether to 3 of Alcohol be exceeded. The physical properties of Collodion are afiected by an- other cause, viz. by the strength and purity of the solvents, or, in other words, their freedom from dilution with water. If a few drops of water be purposely added to a sample of Collodion, the efiect is seen to be to precipitate the Py- roxyline in flakes to the bottom of the bottle. There are many substances known in chemistry which are soluble in spirituous liquids, but behave in the same manner as Pyroxyhne in this respect. The manner in which water gains entrance into the Pho- tographic Collodion is usually by the employment of Alco- hol or Spirit of Wine which has not been highly rectified. In that case the Collodion is somewhat thicker, and flows less readdy than if the Alcohol were stronger. Sometimes the texture of the film left upon evaporation is injured ; it is no longer homogeneous and transparent, but semi-opaque and reticulated — composed of a network of small fibres enclosing spaces, and so rotten that a stream of water pro- jected upon the plate washes it away. These effects are to be attributed not to the Alcohol, but to the Water introduced with it ; and the remedy is to procure a stronger spirit, or, if that cannot be done, to in- crease the proportion of Ether in the Collodion. 78 ON THE CHEMISTEY OF THE CHEMISTEY OF " IODIZED " COLLODION. The Iodides of Potassium, Ammonium, and Cadmium are those commonly enployed in iodizing Collodion. Of these the second is considered by many operators to be the best, not only on account of its ready solubility when pure, but also from the fact that certain complicated changes — which wiU presently be spoken of as occurring spontane- ously in Collodion, and which at an early stage are favour- able to sensitiveness — take place more readily with the Ammonium Salt than with the others. Iodide of Potassium is sparingly soluble in Alcohol and Ether free from water, and hence in some cases it cannot be used alone. T/ie Coloration of Iodized Collodion explained. — Iodized Collodion, when first prepared, is colourless if the mate- rials employed are pure. It soon however begins to change to a lemon-yellow colour, afterwards to an orange-yellow, and finally to a dark red. These effects, due to the liber- ation of free Iodine, are caused by the Ether, or a pro- duct of its oxidation, gradually reacting upon the alkaline Iodide. It is common amongst Photographers to speak of a slight acidity of the Ether as a cause of coloration ; the simple fact of acidity however does not explain the phenomenon. The tendency of ordinary acid placed in contact with an Iodide is to set free, not Iodine, but Hydriodic Acid, that is. Iodine in combination with Hydrogen (HI) ; and although Hydriodic Acid is an unstable substance and soon becomes decomposed, it does not do so sufficiently quickly to strike an immediate colour. Ether may, with proper precaution, be preserved for a long time in a pure state, but on exposure to the joint ac- tion of air and light it undergoes a slow process of oxida- tion, attended with formation of Acetic Acid and a pecu- liar principle resembling in properties ozone, or Oxygen in an allotropic and active condition. An alkaline Iodide, COLLODIO-IODIDE OF SILVEE. 79 sucli as the Iodide of Potassium or Ammonium, is decom- posed by Ether in this state, Acetate of Potash and Hy- driodic Acid (HI) being first produced. The ozonized sub- stance then removes Hydrogen from the latter compound, and liberates Iodine, which dissolves and tinges the liquid yellow. By distillation from quicklime or caustic Potash, oxidized Ether may be purified and again rendered fit for the manu- facture of Collodion. . All acids which contain loosely combined Oxygen elimi- nate Iodine from Iodide of Potassium ; Nitric Acid does so, especially the yellow acid containing Peroxide of Nitrogen. Peroxide of Nitrogen and Hydriodic Acid instantly react upon each other with formation of Water and free Iodine, thus : — HI + NO4 = HO + I + NO3. If we consider therefore the nature of Pyroxyline, it will be readily understood that a partial decomposition of its ethereal solution might at once, by setting free Peroxide of Nitrogen, produce the efiect which we are now considering. In keeping Collodion this always occurs to a greater or less extent, and hence, after the lapse of some months, and especially in an elevated atmospheric temperature, the fluid acquires this property of liberating Iodine to so great an extent as to render it almost useless. SECTION II. Tlie Chemistry of the Nitrate Bath. The solution of Nitrate of Silver in which the plate coated with iodized Collodion is dipped, in order to form the layer of Iodide of Silver, is known technically as the Nitrate Bath. At present we notice the following points connected with the Chemistry of the Bath : — its property of dissolving a certain portion of Iodide of Silver ; — con- tamination with free Nitric Acid ; — occasional alkaline 80 ON THE CHEMISTRY OF THE reaction ; — the substances wliicli decompose and render it useless. Soluhility of Iodide of Silver in the Nitrate Bath. — Aqueous solution of Nitrate of Silver may be mentioned in the list of solvents of Iodide of Silver. The proportion dissolved is in all cases small, but it increases with the strength of the solution. If no attention were paid to this point, and the precaution of previously saturating the Ni- trate Bath with Iodide of Silver neglected, the film would be produced equally well at first, but would afterwards be attacked and dissolved away. This solvent power of Nitrate of Silver on the Iodide is well shown by taking the excited CoUodion film out of the Bath, and allowing it to dry spontaneously. The layer of Nitrate on the surface, becoming concentrated by evapora- tion, eats away the film, so as to produce a transparent, spotted appearance. In the solution of Iodide of Silver by Nitrate of Silver a double salt is formed, which corresponds in properties to the double Iodide of Potassium and Silver in being de- composed by the addition of water. Consequently, in order to saturate a Bath with Iodide of Silver it is necessary only to dissolve the total weight of Nitrate of Silver in a small bulk of water, and to add to it a few grains of Iodide of Silver ; perfect solution takes place, and on subsequent dilution with the full amount of water, the excess is pre- cipitated in the form of a milky deposit. In operating by the Calotype and waxed paper processes this preliminary saturation of the Bath with Iodide of Silver is not required, the sensitive film being thicker and less liable to suffer injury. Acidity of the Nitrate Bath. — A solution of pure Nitrate of Silver is neutral to test-paper, but that prepared from the commercial Nitrate has usually an acid reaction ; the crystals having been imperfectly drained from the acid mother-liquor in which they were formed. Hence, in making a new Bath it is often advisable not only to satu- COLLODIO-IODIDE OF SILVEE. 81 rate it with Iodide of Silver, but to neutralize tlie free acid it contains. The IS^itrate Bath however, although perfectly neutral when first prepared, becomes acid by continued use, if Collodion containing free Iodine be employed. In that case a portion of Nitric Acid is liberated each time the plate is immersed, thus : — Nitrate of Silver + Iodine = Iodide of Silver + Nitric Acid + Oxygen; Observe that not only Nitric Acid but also an atom of Oxygen is liberated by the Iodine. The impression at first entertained by the Author was, that this Oxygen would combine with the organic matter of the Bath ; but later experiments have proved that a true lodate of Silver is formed, in the same manner as Iodide of Potassium and lodate of Potash are produced by dissolving Iodine in caustic Potash. The Photographic effect of Iodine in Col- lodion is not precisely the same as that of Nitric Acid in the Bath, which may be accounted for by the presence of the lodate of Silver (?). Alkalinity of the Bath. — By "alkalinity" of the Bath is meant a condition in which the blue tint is restored to reddened litmus-paper. This indicates that an Oxide of some kind is present in solution, which, by combining with the acid in the reddened paper, neutralizes it and removes the red colour. If a small portion of caustic Potash or Soda be added to a strong solution of Nitrate of Silver, it speedUy produces a brown precipitate, which is Oxide of Silver. The solu- tion however, from which the precipitate has separated, is not left in a neutral state after such addition, but possesses an alkaline reaction ; a minute quantity of the Oxide of Silver dissolving in the liquid. Oxide of Silver and Carbonate of Silver are also abun- dantly soluble in water containing Nitrate of Ammonia ; which salt is continually accumulating in the Bath when compounds of Ammonium are used for iodizing. (J 82 ON THE CHEMISTRY OF THE These facts indicate tliat caution sliould be used in adding any substance to the Bath which will produce Oxide or Carbonate of Silver. A Collodion containing free Ammonia, often sold in the shops, does so, tending by degrees to render the Bath alkaline. The use of Potash, Carbonate of Soda, Chalk, or Marble to remove free Nitric Acid from the Bath has the same elFect ; and hence it will be advisable after adding these substances to test for alkalinity by reddened litmus-paper, and, if found to neutralize it, by a slight addition of Acetic Acid. The mode in which Acetate of Silver is formed in the Nitrate Sath. — In preparing a new Bath, if the crystals of Nitrate of Silver are acid, it is usual to add an alkali in small quantity. This removes the Nitric Acid, but leaves the solution faintly alkaline. Acetic Acid is then dropped in, which, by combining with the Oxide of Silver, forms Acetate of Silver. Another source of Acetate of Silver is the addition of an alkali to a Nitrate Bath containing Acetic Acid, with the view of removing free Nitric Acid, which sometimes accumulates in injurious quantity from constant use of brown Collodion. The operator, in adding the alkali, usually introduces a considerable excess ; hence he neu- tralizes not only the Nitric Acid, but also the Acetic Acid in the Bath, producing a soluble Acetate, which by double decomposition is converted into Acetate of Silver. Acetate of Silver is not formed in the Bath by the simple addition of Acetic Acid, because its production under such circumstances would imply the liberation of Nitric Acid ; but if an alkali be present to neutralize the Nitric Acid, then the double decomposition takes place, thus — Acetate of Ammonia -|- Nitrate of Silver = Acetate of SUver + Nitrate of Ammonia. Acetate of Silver is a white flaky salt, sparingly so- luble in water. It dissolves in the Bath only in small COLLODIO-IODIDE OF SILVEE. 83 proportion, but yet sufficiently to affect the Photograpliic properties of the film (see pages 99 and 112). The observance of the following simple rules will obviate its production in injurious quantity : — First, when it is re- quired to remove free Nitric Acid from Solution of Nitrate of Silver not containing Acetic Acid (as, for instance, in the preparation of a new Bath), the alkaline solution may be dropped in freely; but the liquid should be filtered be- fore adding any Acetic Acid, otherwise the brown deposit of Oxide of Silver will be taken up by the Acetic Acid, and the Bath will be charged with Acetate of Silver. Secondly, in dealing with a Bath containing both Nitric and Acetic Acids, employ an alkali much diluted (Liquor Ammonise with 10 parts of Water), and add a single drop at a time, testing the Photographic action of a sensitive film between each addition ; the Nitric Acid will neutrahze itself before the Acetic, and with care there will be no formation of Acetate of Silver in quantity. A List of the Substances which decompose the Nitrate Bath. — Most of the common metals, having superior affi- nity for Oxygen, separate the Silver from a solution of the Nitrate ; hence, contact with Iron, Copper, Mercury, etc., must be avoided, or the Bath will be discoloured, and a black deposit of metallic Silver gradually precipitated. All developing agents, such as GaUic and Pyrogallic Acids, the Protosalts of Iron, etc., blacken the Nitrate Bath, and render it useless by reducing metallic Silver. Chlorides, Iodides, and Bromides produce a deposit in the Bath; but the solution, although weakened, may again be used after passing through a filter. Hyposulphites, Cyanides, and all fixing agents decom- - pose Nitrate of Silver. Organic matters, generally, reduce Nitrate of Silver, either with or without the aid of light. Oil of Cloves, Grape Sugar, Albumen, Serum of Milk containing Ca- seine, etc., blacken the Bath, even in the dark. Alcohol 84 ON THE CHEMISTET OF THE andEtlier act more slowly, and produce no injurious elFect unless tlie liquid is exposed to light. These facts indicate that the Nitrate Bath, containing volatile organic matters, must be preserved in a dark place ; also that it should be kept exclusively for sensitizing the Collodion plate, and not used in floating papers ia- tended for the printing process. SECTION III. The Formation of the Sensitive Film. In order to produce the sensitive layer of Iodide of Silver upon Collodion, a soluble Iodide, such as that of Potas- sium or Ammonium, is added to plain Collodion. This mixture is then poured out on a glass, and when it begins to solidify from the spontaneous evaporation of the ethe- real solvent, it is immersed in a solution of Nitrate of Silver, until the alkaline Iodide is converted by double decom- position into Iodide of Silver. The colour and general appearance of the CoUodion film vary much with the strength of the iodiziag solution ; as these differences affect its Photographic properties, it will be necessary to describe them. a. Varieties in the Colour of Collodion Films. — A proper relation must always be maintaiaed between the strength of the Collodion iodizer and that of the Nitrate Bath. If the Silver solution be too weak, the formation of Iodide of Silver will be incomplete, and a pale film wiU result, containing undecomposed Iodide of Potassium. Thus, for instance, a Collodion iodized with four grains of alkaline Iodide to the ounce, wiU usually require a bath of at least 25 grains of Nitrate of Silver to the ounce of water, and will produce a film altogether lighter in colour if dipped in a 20-grain solution. Much however depends upon the glutinosity of the Collodion ; a thin CoUodion leaving less of the Iodide upon the glass may be worked in a weaker Bath. COLLODIO-IODIDE OF SILVEE. 85 Wlien tlie quantity of Iodide and Pyroxyline in tlie Collodion are both, small, the film, on immersion in the Bath, assumes by degrees a pale blue tint, which on in- spection is found to be transparent to such an extent that the letters of a newspaper can be read through it at a dis- tance of some inches with, facility. By slightly increasing the quantities, the blue film is changed to a silver-grey, still transparent, but less so than before. The next stage is a tendency to yellow, with compara- tive opacity ; and higher still, a decided creamy, yellow, and opaque film. When this point is reached, no further alteration in appearance is produced by adding Iodide ; but eventually it is found that flakes of Iodide of Silver burst out upon the surface of the film and fall away into the Bath. When this is the case, the Collodion is decidedly over-iodized, and the result will be inferior in every respect. As a rule, the proportion of alkaline Iodide should not exceed 4 grains to the ounce of Collodion. If this pro- duces a blue transparent film on dipping in the Bath, it is probable that the proportion of Nitrate of Silver has fallen too low, or, which is much more common, that too little Pyroxyline was used in the Collodion. In that case, by rubbing the finger across the plate, you see at once that the film is too slight, — there is not enough of the glu- tinous layer to support the sensitive salt, and hence it wUl be useless to attempt to increase the density by the simple addition of Iodide to the Collodion. Four grains of soluble Pyroxyline to each ounce of Col- lodion is often sufficient ; but with Pyroxyline prepared from Swedish paper, and at an unusually high tempera- ture (150° Fahr.), as much as 6 to 8 grains may be required. b. The JProper Time for immersing the Film in the Bath. — After exposing a layer of Collodion to the air for a short time, the greater part of the Ether evaporates, and leaves the Pyroxyline in a state in which it is neither wet 86 ON THE CHEHISTET OF THE nor dry, but receives the impression of the finger without adhering to it. Photographers term this setting, and when it takes place it is a sign that the proper time has come for submitting it to the action of the Bath. If the film be lowered into the Nitrate before it has set, the effect is somewhat the same as that produced by add- ing Water to Collodion. The Pyroxyline is precipitated in part, and consequently the after-structure of the film is not homogeneous. On the other hand, if it be allowed to become too dry, the Iodide of Silver does not form per- fectly, and the film, on being washed and brought out to the light, exhibits a peculiar iridescent appearance, and is paler in some parts than in others. N'o rule can be given as to the exact time which ought to elapse ; it varies with the temperature of the atmo- sphere, and with the proportions of Ether and of Pyroxy- line. A thin Collodion containing but little Alcohol must be immersed more speedily. The plate is to remain in the Bath until the conversion of the Alkaline Iodide into Iodide of Silver is complete. The principal impediment here lies in the difliculty with which Ether and Water mix together, which causes the Collodion surface on its first immersion to appear oily and covered with streaks. By gentle motion the Ether is w"ashed away, and a smooth and homogeneous layer obtained. Wlien the plate is removed from the Bath it should, if properly prepared, present the following appearance : — It is smooth and uniform, both by reflected and trans- mitted light, being of equal thickness in every part ; there are no wavy lines or markings such as would be caused by a glutinous Pyroxyline ; no opaque dots from small particles of dust or Iodide of Silver in suspension in the Collodion. The evidences of a too ra/pid immersion in the Bath are sought for on the side of the plate from which the Collo- dion was poured off. This part remains wet longer than COLLODIO-IODIDE OF SILVER. 87 the other, and always suffers the most ; horizontal cracks or marks resembling vegetation are seen, each of which would cause an irregular action of the developing fluid. On the other hand, the upper part of the plate must be examined for the pale blue colour characteristic of a film which had become too dry before immersion, since the Col- lodion is thinner at that point than at any other, SECTION IV. On the Conditions which influence the Formation and Development of the Latent Image upon Collodion. The observations contained in this section, whilst re- ferring principally to the Iodide of Silver upon Collodion, will be found in many cases to apply also to the paper and Albumen processes ; the conditions being essentially the same in each, although the relative sensibihty varies. Two terms will be used so frequently, that it is neces- sary to define them. These are " Sensitiveness" and " In- tensity." By Sensitiveness is meant a facility of receiving impres- sion from very feeble rays of light, or of receiving it quicTcly from more energetic rays. Intensity, on the other hand, relates to the appearance of the finished Photograph, independently of the time taken to produce it ; to the degree of opacity of the reduced Silver, and the extent to which it obstructs transmitted light. THE CAUSES WHICH INFLTJENCE THE SENSITIVENESS OF IODIDE OF SILVER ON COLLODION. Some of these are as follows :— The proportions of Ether and Alcohol in the Collodion employed— the pre- sence of free acid— of excess of JSIitrate of Silver in contact with the Iodide of Silver— the density of the Iodide film— 88 PHOTOGEAPHIC ACTION OF THE addition of bodies in a state of change and tending to ab- sorb Oxygen— temperature and other causes imperfectly- studied. The Sensitiveness affected hy the Relative Proportions of Ether and Alcohol in the Collodion. — This point has already been mentioned, as far as relates to the physical properties of the solution. It was shown (p. 77) that an increase in the amount of Alcohol lessened the contractility of the film, caused it to adhere more firmly to the glass, and facilitated the production of the sensitive layer of Iodide in the Bath. The Photographic properties of the film are also altered by addition of Alcohol. It is rendered more sensitive to the influence of light, the image being impressed in a shorter time than before. As much Alcohol therefore should be added to Collodion as it will bear, the exact quantity depending upon the strength of the spirit, that is, upon its freedom from dilution with water. The good effects produced by Alcohol are referred mainly not to a chemical, but to a mechanical cause. The struc- ture of the film is opened out, and being less dense and compact, the Iodide is better acted on by the light. The effect of Iodine in the Collodion, or of Acid in the Nitrate Bath. — All acids diminish the sensibility of the film to light, especially so the oxidizing acids, such as the Nitric ; free Iodine in CoUodion produces Nitric acid in the film (p. 81), and hence brown Collodion is usually slower in action than colourless Collodion recently mixed. Two drops of strong Nitric acid added to a half-pint Ni- trate Bath will produce an evident difierence in the sen- sitiveness of the film ; and when the proportion is increased to one drop per ounce it will not be easy to obtain rapid impressions. Acetic acid, being feeble in its reactions, has far less effect upon the sensitiveness than Nitric acid, and as it is found useful in the development is commonly employed. The Structure of the Film is also a point to which atten- tion must be paid. The more shght and transparent the COLLODIO-IODIDE OF SILVEE. 89 film, and tlie weaker tlie Bath in whicli it is formed, tlae greater the injury caused by free acid. The Sensitiveness influenced hy an excess of Nitrate of Silver. — When the sensitive plate is lifted from the Bath, the Iodide of Silver is necessarily left in contact with excess of Nitrate of Silver. The presence of this Nitrate is not essential during the exposure to light, since, if it be care- fully removed by washing in distilled water, the image may still be impressed. In that case however the action is slower than before, twice or perhaps three times the usual exposure being required. This accelerating influence of Nitrate of Silver is not in proportion to the quantity present. A blue transparent Collodion film is as sensitive when formed in a Nitrate Bath of 20 grains to the ounce as in one of 40 grains ; and even with a thicker film, dense and creamy, nothing is gained by increasing the strength of the Bath beyond 30 or 35 grains to the ounce. But with a Collodion film which has been washed with water to remove the whole of the Nitrate, the sensitiveness will be greater when it is after- wards dipped in a Bath of 6 grains to the ounce, than when immersed in one of 2 grains ; and hence, in partially re- moving the excess of Nitrate of Silver from plates intended for keeping, the lower edge where the liquid has drained down and accumulated, is, from containing more free Nitrate, often found to be superior in sensitiveness to the upper. The case therefore may be stated thus : — An excess of Nitrate of Silver increases the sensitiveness in proportion to its quantity to a certain point, but not beyond it; and this maximum point is exceeded when a 35-grain solu- tion of Nitrate is employed in exciting. It is no advan- tage therefore to increase the strength of the Bath to 60 or even 100 grains to the ounce, as some have proposed. Many suppose that d^pwre Iodide of Silver is unafiected by light, and that the remaining sensitiveness of a washed film is dxie to a minute quantity of Nitrate of SUver left 90 PHOTOGEAPHIC ACTION OF THE behind. To prove tliis, they remark tliat tlie Iodide of Silver precipitated on diluting a solution of double Iodide of Potassium and Silver is absolutely insensitive. This form of Iodide however may be chemically different from the other. It appears that some of the soluble Salts of Silver accele- rate Iodide of Silver more strongly than others, and this might have been anticipated upon theoretical grounds. In the Nitrate of Silver, it is the base and not the acid of the salt which is required. Supposing the acid to exert an influence, it would be in the opposite direction ; hence an Oxide of Silver in the film accelerates more powerfully than a Nitrate of Oxide or JS'itrate of Silver, so termed. In this way the excessive sensitiveness obtained by using strongly fused Nitrate of Silver may be explained ; it then yields an alkaline Bath, apparently from the presence of a basic Nitrite of Silver. The neutral Nitrite of Silver which occurs crystallized in the form of long needles, is an accelerating salt to Iodide of Silver, as wiU immediately be shown ; but it is less active than the basic Nitrite, which contains Oxide of Silver in excess. According to the Author's experiments, the use of a Bath of fused Nitrate of Silver gives the utmost degree of sensi- tiveness attainable in the Collodion process, provided the fusion has been carried to the proper point. It is not how- ever well adapted for common use, the pictures being misty and too intense in the high lights. The addition of a single drop of Nitric Acid completely alters the character of the solution, by decomposing the basic salt, and entering into combination with the excess of Oxide of SUver. The Sensibility affected by the Amount of Iodide in the Film. — The varied appearance of Collodion films, depend- ing upon the quantity of Iodide of Silver they contain, has been fully described at p. 84. The sensibility to light appears to be influenced by this to some extent. In the first place, it may be observed that the addition of too large a proportion of soluble Iodide to COLLODIO-IODIDE OF SILVEE. 91 the Collodion, so as to produce Iodide of Silver in loose flakes upon the surface, is decidedly injurious. Also avoid- ing this extreme, it will be found that, provided the Nitrate Bath be free from acid, the blue transparent films receive the impression of a feeble ray of light somewhat more readily than creamy opaque films. That such would be the case might have been antici- pated. The weaker the solution from which Iodide of Silver is precipitated, the more gradual the precipitation, and the more finely divided the particles of the precipitate. Hence it is easy to understand why they are more sensi- tive to the influence of light. The difference in such cases however, being but sHght, is alluded to principally to re- move an impression which might otherwise occur to the mind of the reader, that the most rapid effects would be obtained by increasing the amount of Iodide. Practically speaking, this idea would lead to wrong con- clusions. A dense film of Iodide gives more intensity with a powerful ray of light acting for a long time ; but under ordinary circumstances the luminous image of the Camera may be copied with equal vigour, and often with better definition, upon a comparatively transparent film, if the presence of large excess of Iodine, of Nitric Acid, etc. is avoided (seep. 88). These remarks apply only to the Col- lodion process. In the Calotype and paper processes gene- rally the sensibility is often increased by an addition of Iodide, but in this case there are retarding causes present which alter the conditions and render it impossible to pro- duce a picture with the minimum quantity of Iodide em- ployed in Collodion. Accelerating Agents to Iodide of Silver. — The effect of adding to Collodion substances which possess a very strong attraction for Oxygen, such as Gallic and Pyrogallic Acids, is to augment the influence of the actinic rays in a marked degree. The action however is too violent under such cir- cumstances, and cannot be restrained within proper limits ; so that the hopes which were at one time entertained of 92 PHOTOGKAPHIC ACTION OP THE combining tlie development and tlie impression of the image in one operation, and so producing a picture visible on removal from the Camera, do not appear at present likely to be fulfilled.* There are however certain chemical agents which may be employed as " accelerators," the characteristic property of which is, that they possess an attraction for Oxygen, although not sufficient to entitle them to rank amongst the list of developers. The principal of these are as fol- lows : — Iodide of Iron, Nitrous Acid and its salts. Grape Sugar, and the various essential ods, as Oil of Cloves, etc. Iodide of Iron added to Collodion is represented in the film by Protonitrate of Iron, thus : — Iodide of Iron + Nitrate of Silver = Iodide of Silver + Nitrate of Iron. The quantity used must be small ; or the same universal decomposition as that following the addition of G-allic and Pyrogallic Acids would result. The accelerating powers of the Nitrites were pointed out by the Abbe Laborde and by Mr. Hadow. Alkaline Nitrites tend slightly to absorb Oxygen and become Ni- trates ; but it is probable that their Photographic action is due to a liberation of Nitrous Acid, an unstable sub- stance decomposed by water and producing the Deutoxide of Nitrogen, which is a powerful reducing agent. Any substance of complex structure, the component molecules of which tend to rearrange themselves in simpler forms, may fairly be expected to act as an accelerator to Iodide of Silver ; and this independently of absorption of Oxygen. But if the decomposition results in the produc- tion of an acid, or other oxidized substance opposed to the reducing agency of Light, the ultimate effect will be * Gallic Acid is associated with the Iodide of SOver employed in the Calo- type process in order to increase the sensibility, a portion of Acetic Acid being added at the same time, to preserve the clearness of the whites under the influence of the developer. COLLODIO-IODIDE OF SILVEE. 93 to injure the sensitiveness of the film rather than to pro- mote it. It is important to remark, as far as the use of accelera- tors is concerned, that their efiects are most evident when a brown Collodion containing much Iodine is employed. The Author found that the addition of Iodide of Iron in moderate proportion to a pure colourless Collodion, the Nitrate Bath being also chemically neutral, increased the sensitiveness by no more than one-sixth of the total expo- sure ; a comparatively insignificant effect to what is com- monly noticed. In the case of the Essential Oils no difier- ence at all could be observed ; and yet it has been shown that they accelerate strongly under some circumstances. These facts may perhaps be explained by supposing that the accelerating agent acts partly by absorbing the atom of free Oxygen, shown at p. 81 to be liberated in conjunc- tion with Nitric Acid by the action of Iodine upon Nitrate of Sdver. Mr. Mayall has added four more organic substances to the hst of accelerators to brown Collodion. These are Benzine, Naphthahne, HeUenine, and Terpenole ; they pos- sess complicated formulae, and act probably in the same manner as the essential oils before noticed. I7ie effect of Depression of Temperature and other causes not yet thoroughly explained. — The influence of tempera- ture upon Photographic processes is seen most evidently during the development of the latent image : but indepen- dent of this, there is reason to think that the sensitive- ness of the plate is injured more or less by a reduction of atmospheric temperature. This point would probably be better understood were it not that during the winter months, when the cold is intense, the hght is also more feeble, and hence a longer exposure in the Camera is ne- cessitated. Photographers complain much of the difference in sensi- bility between various samples of Collodion. This depends no doubt in part upon the presence or absence of the oxidized 94 PHOTOGEAPHIC ACTION OF THE bleacliing principle described at p. 78. Sometimes Iioav- ever the Collodion proves to be insensitive, witbout any unusual liberation of Iodine having been observed. In that case possibly Ether which had been redistilled from residues of old Collodion might have been used, since the Author has found that the fluid under those circumstances is contaminated with a substance producing a retarding effect, but not possessing the property of liberating Iodine from Iodide of Potassium.* On the other hand, the fact of a sample of Collodion be- coming brown immediately on iodizing does not prove of necessity that it wiU be insensitive. There are some acce- lerators which favour that effect, as, e.g., the Nitrites and the Proto-iodide of Iron. Collodion often improves in sensitiveness for a few hours or days after being mixed, although the general tendency is in the opposite direction, since free Iodine, the cause of the brown colour, produces, in the Bath, both Nitric Acid and Oxygen. The insensitiveness of Old Collodion how- ever is supposed by many to depend upon a product of the action of lodiae upon Ether, and the experience of the writer, above given, is ui favour of this. The quality of the Pyroxyline may be said occasionally to affect the sensitiveness of CoUodion, since there are some varieties of that substance which decompose spontaneously, giving off Peroxide of Nitrogen. This however is rare. THE CONDITIONS WHICH AFFECT THE DEVELOPMENT OF THE LATENT IMAGE. The general theory of the development of a latent image by means of a reducing agent having been already given, the subject may now be more fully examined. The presence of free Nitrate of Silver essential to the development.— This subject has been alluded to in the third • The employment of Alcohol containing " fusel oil" and other impurities has been mentioned to the Author as injuring the sensitiveness of Col- lodion (?). COLLODIO-IODIDE OF SILVEE. 95 Chapter, but needs a further notice. As far as the action of light upon the film is concerned, free Nitrate of Silver, although useful as an accelerator, is not essentially re- quired, since the plate receives the radiant impression in the Camera, even if previously washed with care. It is not however immediately seen that such is the case, since on removing it to the dark room and covering it with a re- ducing agent, no image is brought out. If however it be dipped for an instant in the Bath, in order to restore the Nitrate of Silver which had been washed away, then the picture developes well. The same fact is shown more completely by further ex- periments. The ultimate intensity of the image is regu- lated entirely by the supply of Nitrate of Silver :— if too little of that salt be added, it is feeble, or altogether im- perfect, in parts ; whilst, on the other hand, by using too much, the action is hastened, and leads at length to perfect opacity and destruction of half-tones. These remarks refer to the presence of Nitrate of Silver upon the film during the development, independently of the manner in which it was added, whether by the use of a strong Bath, or by subsequent addition to the developing solution. TJie Development affected hy the Strength of the Re- ducing Agent. — No increase of power in the developer will suffice to bring out an image either when insufficiently impressed by Light, or when the proportion of Nitrate of Silver is too small. The advantage gained by the use of a strong developing agent is principally in point of time ; a weak developer takes longer to act, but produces the same effect in the end. Galhc Acid is not usually employed to develope CoUodion pictures, because it reduces so slowly that there is a danger of pouring it off before the action is completed, and in that case the half-tones of the picture would be deficient. Pyrogallic Acid, Protosulphate and Protonitrate of Iron are more active, but there is a marked difference between the three. As far as relates to the 96 PnOTOGEAPHIC ACTION OF THE number of grains required to produce a given effect, Pyro- gallic Acid is at least four times as strong as the crys- tallized Protosulphate of Iron, and twenty times more so than the Protonitrate of Iron. The comparative feebleness of the Protonitrate of Iron is probably attributable to the oxidizing nature of the acid it contains. It is the protoxide of Iron, or base of the salt, which acts as the reducing agent, and the association of an acid like the Nitric with this base impedes the effect. PyrogaUic Acid, so called, being a strictly neutral sub- stance, is in that respect favourably constituted for a re- ducing agent ; indeed in practice it is found to be too violent in action, and to produce universal decomposition, unless a portion of weak acid like Acetic be added. The effect of adding Free Acid. — All acids tend to re- tard the reduction of the image, as well as to increase the length of the exposure in the Camera. Nitric Acid especially does so, from the powerful oxidizing properties it possesses. This acid is sometimes used in developing, from the property it possesses of hrightening the reduced Silver ; but it has disadvantages, which are mostly seen when the film is very transparent, and the excess of Nitrate of Silver, as measured by the strength of the Bath, small. Under such circumstances the proper development of the image is often suspended, and spangles of metaUic Silver separate. This indicates that the quantity of acid should be diminished, or the strength of the Nitrate Bath and of the reducing agent be increased, as a counterpoise to the retarding action of acid upon the development. Acetic Acid also moderates the rapidity of development, but it has not that tendency altogether to suspend it pos- sessed by Nitric Acid. It is therefore usefully employed, to enable the operator to cover the plate evenly with liquid before the development commences, and to preserve the white parts of the impression from any accidental deposit of metallic Silver due to irregular action of the Gallic or PyrogaUic Acid. Sulphate of Iron developes well COLLODIO-IODIDE OF SILVER. 97 without Acetic Acid ; but even in that case the chance of curved lines of over-development appearing with the image is lessened by its employment. On comparing the retarding effects of free acid upon the Hght's action, and upon the development, we see that the former is the most marked, — that a smaU quan- tity of Nitric Acid produces a more decided influence upon the impression of the image in the Camera than upon the bringing out of that image by means of a redu- cing agent. The Development affected hy the Presence of Nitrite or of Oxide of Silver in the Film. — Nitrate of Silver which has been strongly fused contains sub-Nitrite of Silver, as shown in the Second Chapter. This Nitrite is an accele- rator to the luminous agency ; but independently of this, it produces a curious change in the process of develop- ment. The image comes out almost instantaneously and with great force, thus inducing the operator to suppose that he has mistaken the proportions, and that his deve- loper is too strong. It is stated by the Abbe Laborde, that when the Bath contains Nitrites, even Gallic Acid, a comparatively feeble agent, will suffice to bring out the image rapidly. The cause of these peculiarities is easily explained. As strong acids are opposed to reduction, so their removal, or their substitution by others of a weaker nature, facilitates reduction. Thus acid Nitrate of Silver is more difficult to reduce than neutral Nitrate, — Nitrate of Silver more so than Nitrite, — and Nitrite more than the basic Nitrite, containing Oxide of Silver in excess. Effect of Temperature on Development. — deduction of the oxides of noble metals proceeds more rapidly in pro- portion as the temperature rises. In cold weather it wiU be found that the development of the image is slower than usual, and that greater strength of the reducing agents and more free Nitrate of Silver is required to produce the effect. 98 PHOTOGRAPHIC ACTION OF THE On tlie other liand, if the heat of the atmosphere be ex- cessive, the tendency to rapid reduction will be greatly increased, the solutions decomposing each other ahnost immediately on mixing. In this case the remedy will be to use Acetic Acid freely both in the Bath and in the developer, at the same time lessening the quantity of PyrogalUc Acid, and omitting the Nitrate of Silver which is commonly added towards the end of the de- velopment. In the Calotype paper process a little Gallic Acid is usually employed in rendering the sheets sensitive, but in hot weather it is often found necessary to omit it in order to prevent universal blackening under the action of the developer. Also in the case of films which are to be kept for a long time in a sensitive condition, the modifying influence of temperature must be observed, and the quan- tity of free Nitrate of Silver left upon the film be re- duced to a minimum if the thermometer stands higher than usual, SECTION V. On certain irregularities in the Developing Process. The characteristics of the proper development of a la- tent image are these : — that the action of the reducing agent should cause a blackening of the parts exposed to light, but produce no effect upon others which have been shielded from it. In operating both on Collodion and paper however we are often liable to a failure in this respect; the film beginning after the application of the developer to change in colour to a greater or less extent over the whole sur- face. This state of things has been termed " fogging," from the grey tint of the Silver by reflected light, giving an ap- pearance as if the picture were veiled behind a dense fog. There are two main causes which produce it : — the first COLLODIO-IODIDE OF SILVEE. 99 being due to an irregularity in the action of tlie light ; the second to impurity in the chemicals employed. It is common to find, from some defect in the con-< struction of the Camera, or from other causes which will be pointed out more particularly in the Second Part of this work, that diffused white light gains entrance and pro- duces indistinctness of image by affecting the Iodide more or less universally. Even supposing errors of this kind to be avoided, yet mere over-exposure of the sensitive plate has the same effect. In such a case, when the developer is poured on, a faint image first appears, and is followed by a general de- composition. Impurity of Chemicals as a cause of irregular reduction. — The Nitrate of Silver Bath, in which the sensitive layer of Iodide of Silver on Collodion is formed, is usually kept faintly acid to test-paper ; if this precaution be not ob- served, the pictures are liable to be indistinct from a fine deposit of metallic Silver produced by the developer and adhering to every part of the plate, independent of any action of the light. Fure crystallized Nitrate of Silver can be employed accurately neutral, but fused Nitrate of Silver, or a Bath to which Nitrite of Silver, Acetate of Silver, or any alkali has been added, requires the presence of a trace of acid to preserve the clearness of the plates under the influence of the developer. There are also many organic matters, amongst which (although not at the head of the list) may be mentioned Ether and Alcohol, which tend to reduce Nitrate of Silver, and hence it will be found that a new Bath accurately neutral to test-paper must be acidified after being some' time in use. If the CoUodion employed however is tinted with free Iodine, the necessary acid reaction will probably be spontaneously acquired (see p. 81). The kind of film which usually produces irregular action of the developer, is that which is the most favourable to the reduction of metallic Silver. Take, for instance, the 100 PHOTOGRAPHIC ACTION OF THE alkaline Bath, wMcli is especially troublesome, and com- pare it with a solution acidified with Nitric Acid :— In the first case, when the developer is poured on, the image starts out instantaneously, and immediately afterwards the plate begins to blacken. With the second, on the other hand, the action is slow and gradual and the white parts are preserved. Hence also the sensitive films will, as a rule, be developed clean and brilliant in cold weather, but unless proper pre- cautions are taken (p. 98) will become misty during the heat of summer. The great remedy for fogging is the addition of acid ; either Acetic acid, which will often be found sufficient, or a minute quantity of Nitric acid. It must however be borne in mind that although the image can often be developed with great clearness when the Bath contains a smaU quan- tity of Nitric acid, yet that such a condition is not favour- able to intensity of the image ; on the other hand, those films, which are prone to irregular reduction, such as films prepared in a bath containing Acetate or Nitrite of Silver, are the most sensitive to Hght, and give the greatest depth of impression. Hence, when these qualities are sought, the use of Nitric Acid will be adopted cautiously and as a last resource. Acetic Acid is superior to Nitric Acid for a reason which has already been mentioned, p. 96, viz. that too much Ni- tric acid is apt to interfere with the proper development of the half-tones of the impression, and often originates a state in which the image is feeble, and a deposit of Silver occurs more or less upon the shadows as weU as on the lights of the picture. The use of impure water in the preparation of the Ni- trate Bath — and all those conditions which have been men- tioned at p. 83 as favouring the decomposition of Nitrate of Silver, are the causes most likely to produce fogging. If these be avoided, and the remarks made in the Third Section of Chapter III. on the effect of surface conditions COLLODIO-IOPIDE OF SILVEE. 101 in modifying tlie deposition of vapour and of metallic SUver be carefully studied, tlie operator will experience no difficulty in dealing with those numerous irregularities in the action of the developing fluid, which often prove the greatest hindrance to the successful practice of the Collodion process. 102 CHAPTER VII. OK POSITIVE AND FEGATITE COLLODION PHOTO- GEAPHS. The terms "Positive" and "Negative" occur so frequently in all works and conversations upon the subject of Photo- grapliy, that it will be impossible for the student to make any progress without thoroughly understanding their meaning. In attempting to give a clear and simple expla- nation, the words will be taken in their usual sense, with- out reference to peculiar varieties of Photographs : such as transparencies for the Stereoscope or Magic Lantern, etc. ' A Positive may be defined to be a Photograph which gives a natural representation of an object, as it appears to the eye. A ISTegative Photograph, on the other hand, has the lights and shadows reversed, so that the appearance of the object is changed or negatived. In Photographs taken upon a layer of CMoride of Sil- ver, either in the Camera or -by superposition, the effect must necessarily be I^egative ; the Chloride being dcu^k- ened by luminous rays, the lights are represented by shadows. The following simple diagrams will make this obvious. Fig. 1 is an opaque image drawn upon a transparent ground; fig. 2 represents the effect produced by placing it POSITIVE AND NEGATIVE PHOTOGEAPHS. Fig. 1. Kg. 3. in contact witli a layer of sensitive Chloride and exposing to liglit ; and fig. 3 is the result of copying this negative again on Chloride of Silver. Fig. 3 therefore is a Positive copy of Fig. 1, obtained by means of a Negative. By the first operation the tints are reversed ; by the second, being reversed again, they are made to correspond to the original. The possession of a Negative therefore enables us to obtain Positive copies of the object, indefinite in number, and all precisely similar in appearance. This capability of multiplying impressions is of the utmost importance, and it has rendered the pro- duction of good Negative Photographs of greater conse- quence than any other branch of the Art. The same Photograph may often be made to show either as a Positive or as a Negative. For instance, supposing a piece of silver leaf to be cut into the shape of a cross and pasted on a square of glass, the appearance presented by it would vary under different circumstances. Fig. 1 represents it placed on a layer of black velvet ; Fig. 1. I'ig- 3- 104 ON POSITIVE AND NEGATIVE fig. 2 as held up to the light. If we term it Positive in the first case ^ e. by reflected light, then it is I^egative m the second, that is, by transmitted light. The exoll nation is obvious. P^^' Therefore, to carry our original definition of Positives and Negatives a little further, we may say, that the forme! Z^:^'' w 1. ^^l,^^f.*^^^P^s liowever cannot be made to represent both Positives and Negatives. In order to possess this c^- pabdity, It IS necessary that a part of the image shoiJld be transparent, and the other opaque but ^ith a hrinU surface. These conditions are fulfiUed when the Iodide of dXT/ag^ - -^i--^- -ith a Every Collodion picture is to a certain extent both Nega- tive and Positive, and hence the processes for obtainfng both varieties of Photographs are substantially the same Although however the general characters of a Positive Td a Negative are similar, there are some points of difference A surface which appears perfectly opaque when looked down upon, becomes somewhat translucent on being held up to he hght ; hence to give the same effect, the deport of metal in a Negative must be proportionably thicker than m a Positive; otherwise the minor details of the 7Z,Xj o^^^ir^^i^^ the light suf! With these preliminary remarks, we are prepared to investigate more closely the rationale of the processes for obtammg CoUodion Positives and Negatives. refers to paper Positives upon Chloride of Silver wiU be treated m a subsequent Chapter. SECTION I. On Collodion Positives. CoUodion Positives are sometimes termed direct, becau;se COLLODION PHOTOGEAPHS. 105 obtained by a single operation. The Chloride of Silver, acted upon by light alone, is not adapted to yield direct Positives, the reduced surface being dark and incapable ot representing the hghts of a picture. Hence a developing agent is necessarily employed, and the Iodide of Silver substituted for the Chloride, as being a more sensitive pre- paration. Collodion Positives are closely alUed m their nature to Daguerreotypes. The difference between the two consists principally in the surface used to sustam the sensitive layer, and the nature of the substance by which the invisible image is developed. , In a Collodion Positive the Hghts are formed by a bright surface of reduced Silver, and the shadows by a black background showing through the transparent portions of the plate. _ Two main points are to be attended to m the production of these Photographs. ^ t, ^ ^ First, to obtain an image distinct in every part, but 0/ comparatively small intensity.— I? the deposit of reduced metal be too thick, the dark background is not seen to a sufficient extent, and the picture in consequence is deficient in shadow. i i j. i Secondly, to whiten the surface of the reduced metal as much as possible, in order to produce a sufficient con- trast of light and shade. Iodide of Silver developed m the usual way presents a dull yellow appearance which is sombr e and unpleasing. The Exposure to Light required for Collodion Positives. —The rule to be followed is— to expose the plate so long that aifter developing, the darkest shadoius are distinctly seen by reflected light. If the film is in good condition (and the plate has not been over-developed) the highest lightis will be scarcely, or not at aU, too intense ; but if, on thie' other hand, the film is not adapted for taking Posi- tives then the operator must be content to sacrifice the shad(^^ws to a certain extent, and to regulate the exposure by tke appearance of the lights. In that way a tolerably POSITIVE AND NEGATIVE good picture will be obtained, altliough somewhat ©mbre m the dark parts. The exposure required for a Collodion Positive is never more than half as much as that necessary for obtaiune a JN egative with the same fihn. _ The Collodion and Nitrate Bath for Positives. -Good. Positives may often be obtained by diluting a sanple of I^egatiye CoUodion with Ether and Alcohol until it gives a bluish fahn in the Bath. The proportion of locide of bilver bemg m that case small, the action of the high lights IS less violent, and the shadows are allowed more time to impress themselves. The dilution lesseis the ^"i^"^! T (•^''^^^^^ ^^^^'^ Collodion at the same time with the Iodide, which is an advantage, the slight and transparent films always giving more sharpness ard de- finition m the picture, although it becomes necessary to clean the glasses with extra care hi order to avoid sttins The employment of a very thin film for Positives is not admissible in aU cases ; if free Iodine or other substances which have a retarding effect on the action of light are pre sent to a considerable extent, the CoUodion will not work wen with a smaU proportion of Iodide. The Author found in experunentrng on this subject that with perfect pure Col- lodion and a neutral Bath most vigorous impressions were produced when the density of the film had been so far re- duced that scarcely anythmg could be seen upon the /rlass • but with Collodion strongly tinted with Iodine, or with a Bath contaming Nitric Acid, it was necessary to stop the dilution at a certain point or the film became absolutely insensitive to feeble radiations of light, and the shadows could not be brought out by any amount of exposure (see the last Ime m page 88). In this case, by adding more Iodide a better effect was obtained. Iodide of Iron is sometimes added to Collodion as an accelerator, but when Positives are to be taken it is iiriiu nous ; the intensity of the high lights is too much in- creased, which causes a flat and indistinct appearance COLLODION PHOTOGEAPHS. 107 If tie materials are pure, the Negative Nitrate Bath may f,dvantageously be diluted down at the same time with tie Collodion, wlien Positives are to be taken ; but tbe eir.ployment of a very weak Nitrate Bath (such as one of ^3 grains to the ounce), although highly useful in ob- viatin" excess of development, has some disadvantages ; it becomes necessary to exclude free Nitric Acid, and to avoid the employment of a Collodion too highly tmted with Iodine. On the other hand, with a strong Nitrate Bath, and a tolerably dense film of Iodide of Silver, a better re- sult is often secured by the use of Iodine or Nitric Acid. The sensitiveness of the plates is unpaired, but at the same time the intensity is diminished, and the picture shows weU upon the surface of the glass. _ A new Bath is better for taking Positives than one which has been a long time in use. The latter often causes hazi- ness and irregular markings on the fihn during the action of the developer. In this case the plan would be to acidify verv faintly with Nitric Acid, increasing at the same time, if necessary, the thickness of the film and the proportion of Nitrate of Silver in the Bath. The presence oi Acetate of Silver is objectionable m a Positive Nitrate Bath as giving too much mtensity of image, and therefore those precautions which obviate its formation must be adopted (p. 83). CrystaUized Nitrate of Sdver should be used m prefer- ence to the fused Nitrate, which often contains a basic Ni- trite of Silver, and yields an image intense and often misty. Also when Nitrite of Silver is present in the Bath, a pecu- liar greenish tint is seen on the high hghts of the image, which would be objectionable in a direct Positive. The Developers for Collodion Posi7i«e5.— PyrogaUic Acid when used with Acetic Acid, as is usual for negative pic tures, produces a surface which is dull and yellow. Mr. Home proposed to obviate this by substituting Nitric Acid in small quantity for the Acetic. The surface produced by PyrogaUic with Nitric Acid is lustreless, but very white, POSITIVE AND NEGATIVE if the solution be used of the proper strength. On at- tempting to increase the amount of Nitric Acid the deposit becomes slightly metallic, and the half-tones of the pict^e are injured ; PyrogaUic Acid, although an active developer does not a low of the addition of mineral acid to the same extent as the Salts of Iron. It requires also, when com! bmed with IN^tne. Acid, a fair proportion of Nitrate of ^^-^-^ - Sulphate oflron.-The Protosalts of Iron were first employed m Photography by Mr. Hunt. The Sulphate is a most energetic developer and often brings out a picture when others would fail. To produce by means of it a dead white tint with absence of metallic lustre, it may be used m conjunction with Acetic Acid. The addition of Mtric Acid to Sulphate of Iron modi- fies the development and produces a bright sparkling sur- face of metalhc Silver. Too much of this acid however must not be used, or the reduction wiU be irregular. The Nitrate Bath also must be tolerably concentrated, iu order to compensate for the retarding effect of Nitric Acid upon the development The blue and transparent films of Iodide of Silver, formed in a very dUute Nitrate Bath, are not weU adapted for Positives to be developed in this way They are apt to be injured by the acid, and the develop, ment of the image becomes imperfect Protonitrate of Iron.-Thi, salt was suggested as a de- veloping agent for Collodion Positives by Dr. Diamond. It IS remarkable as giving a surface of briUiant metallic lustre without any addition of free acid. Theoretically it may be eonsidered as closely corresponding to the Sulphate of Iron with Nitric Acid added. There are however sliight practical differences between them, which are perhap.s in favour of the Protonitrate. _ The reducing powers of Protoxide of Iron appear tj 5 „ Observe, that two only out of the five possess acid pn perties, the others being simple oxides. Nitric Acid i- strictly speaking, the " Peroxide," but as it belongs to tli AND THEIE COMBINATIONS. 319 class of acids, that term naturally falls to the compound below. The binary compounds of Sulphur with Oxygen all pos- sess acid properties ; they may be represented (in part) as foUows : — Sulphur. Oxygen. Hyposulphurous Acid ... 2 atoms. 2 atoms. Sulphurous Acid 1 „ 2 „ Hyposulphuric Acid . , . 2 „ 5 „ Sulphuric Acid 1 „ 3 In this case the Stdphuric and Sulphurous Acids had become familiarly known before the others, intermediate in composition, were discovered. Hence, to avoid the con- fusion which would result from changing the nomencla- ture, the new bodies are termed 5^^osulphuric and Hypo- sulphurous (from v-no, under). Nomenclature of Salts. — Salts are named according to the acid they contain ; the termination " ic " being changed into "ate," and "ous" into "ite;" thus, Sulphuric Acid forms Sulphcfifes ; Nitric Acid, 'Nitrates ; but Sulphurowif Acid forms Sulph^Yes, and Nitrous Acid, Nitrites. In naming a salt, the base is always placed after the acid, the term oxide being omitted ; thus. Nitrate of Oxide of Silver is more shortly known as " Nitrate of Silver," the presence of oxygen being understood. When there are two oxides of the same base, both of which are salifiable, — in naming the salts, the term " proto" is prefixed to the acid of the salt formed by the lowest, and "per" to that of the higher oxide ; as, the Protosvl- phate of Iron, or Sulphate of the Protoxide ; the Per- sulphate of Iron, or Sulphate of the Peroxide. Many salts contain more than one atom of acid to each atom of base. In that case, the usual prefixes expressive of quantity are adopted : thus, the ^^sulphate of Potash contains twice as much Sulphuric Acid as the neutral Sulphate, etc. On the other hand, there are salts in which the base is 320 THE CHEMICAL ELEMENTS in excess witli regard to tlie acid, and wliicli are usually known as " basic salts ;" thus, tlie red powder wliicli de- posits from solution of Sulphate of Iron, is a hasic Per- sulpliate of Iron, or a Sulphate of the Peroxide of Iron with more than the normal proportion of oxide. Nomenclature of the Sydracid Salts. — The composition of these salts being different from those formed by Oxygen Acids, the nomenclature varies also. Thus, in neutralizing Hydrochloric Acid with Soda, the product formed is not known as Hydrochlorate of Soda, but as Chloride of So- dium ; this salt, and others of a similar constitution, being binary, and not ternary, compounds. The salt produced by Hydrochloric Acid and Ammonia, however, is often called " Muriate or Hydrochlorate of Ammonia," although more strictly it should be the Chloride of Ammonium. ON SYMBOLIC NOTATION. The list of symbols employed to represent the various elementary bodies is given at page 308. — Commonly the initial letter of the Latin name is used, a second or smaller letter being added when two elements correspond in their initials : thus C stands for Carbon, CI for Chlo- rine, Cd for Cadmium, and Cu for Copper. The chemical symbol hovi'ever does not simply repre- sent a particular element ; it denotes also a definite weight, or equivalent proportion, of that element. This will be explained more fully in the next page, when speaking of the Laws of Combination. Formulce of Compounds. — In the nomenclature of com- pounds it is usual to place the Oxygen or analogous ele- ment first in the case of binary compounds, and the acid before the base in the ternary compounds, or salts ; but in representing them symbolically this order is reversed ; thus. Oxide of Silver is written AgO, and never as OAg ; Nitrate of Silver as AgO NO5, not NOgAgO. The juxtaposition of symbols expresses combination ; thus, FeO is a compound of one proportion of Iron with AND THEIE COMBINATIONS. 321 one of Oxygen, or the "Protoxide of Iron." If more than one equivalent be present, smaU figures are placed below the symbols; thus, Fe^O^ represents two equivalents of Iron united with three of Oxygen, or the " Peroxide of Iron ;" SO3, one equivalent of Sulphur with three of Oxygen, or Siilphuric Acid. Larger figures placed before and in the same line with the symbols, afiect the whole compound which the symbols express : thus, 2 SO3 means two equivalents of Sulphuric Acid ; 3 NOg, three equivalents of Nitric Acid. The in- terposition of a comma prevents the influence of the large figure from extending further. Thus, the double Hypo- sulphite of Soda and Silver is represented as follows : — 2 NaO S2O2, AgO S2O2, or ttco equivalents of Hyposulphite of Soda with one of Hyposulphite of Silver ; the large figure referring only to the first half of the formula. Sometimes brackets, etc. are employed, in order to render a complicated formula more plain. For example, the formula for the double Hypo- sulphite of Gold and Soda, or " Sel d'Or," may be written thus : — 3 (NaO S2O2) AuO S2O2+4 HO. In this formula, the plus sign (+) denotes that the four atoms of water which foUow, are less intimately united with the framework of the salt than the other constituents. The use of a plus sign is commonly adopted in repre- senting salts which contain water of crystallization. Thus, the formula for the crystallized Protosulphate of Iron is written as follows : — FeO SO3+7 HO. These atoms of water are driven off by the application of heat, leaving a white substance, which is the anhydrous salt, and would be written simply as FeO SO3. ^ The plus sign however is often employed in token of simple addition, no combination of any kind being in. Y 322 THE CHEMICAL ELEMENTS tended. Thus the decomposition which follows on mixing Chloride of Sodium with Nitrate of Silver may be written as follows ; — NaCl+AgO N05= AgCl+NaO NO^; that is, — Chloride of Sodium added to Nitrate of Silver = Chloride of Silver and Nitrate of Soda. ON EQUIVALENT PEOPOETIONS. When elementary or compound bodies enter into che- mical union with each other, they do not combine in in- definite proportions, as in the case of a mixture of two liquids, or the solution of a saline body in water. On the other hand, a certain definite weight of the one unites with an equally definite weight of the other ; and if an excess of either be present, it remains free and uncombined. Thus, if we take a single grain of the element Hydrogen — to convert that grain into Water there will be required exactly 8 grains of Oxygen ; and if a larger quantity than this were added, as for instance ten grains, then two grains would be over and above. So, to form Hydrochloric Acid, 1 grain of Hydrogen takes 36 grains of Chlorine : — for i the Kydriodic Acid, 1 grain of Hydrogen unites with 126 grains of Iodine. ; Again, if separate portions of metallic Silver, of 108 grains each, are weighed out, — in order to convert them into Oxide, Chloride, and Iodide of Silver respectively, there would be required Oxygen 8 grains. Chlorine 36 „ Iodine 126 „ Therefore it appears that 8 grains of Oxygen are equi- valent to 36 grains of Chlorine and to 126 grains of Iodine, seeing that these quantities all play the same part in com- bining ; and so it is with regard to the other elements, — to AND THEIE COMBINATIONS. 323 every one of tliem a figure can be assigned which repre- sents the number of parts by weight in which that ele- ment unites with others. These figures are the " equiva- lents" or " combining proportions," and they are denoted by the symbol of the element. A symbol does not stand as a simple representative of an element, but as a repre- sentative of one equivalent of an element. Thus"0" indicates 8 parts by weight of Oxygen ; " CI" one equiva- lent, or 36 parts by weight, of Chlorine ; and so with the rest. Observe however that these figures, termed "equiva- lents," do not refer to the actual number of parts by weight, but only the ratio which exists between them : if Oxygen is 8, then Chlorine is 36 ; but if we term Oxygen 100, as some have proposed, then Chlorine would be 442'65. In the scale of equivalents now usually adopted, Hydro- gen, as being the lowest of all, is taken as unity, and the others are related to it. Equivalents of Compounds. — The law of equivalent pro- portions applies to compounds as well as to simple bodies, the combining proportion of a compound being always the sum of the equivalents of its constituents. Thus Sulphur is 16, and Oxygen 8, therefore Sulphuric Acid, or SO3, equals 40. The equivalent of Nitrogen is 14, that of Ni- tric Acid, or NO5, is 54. The same rule applies with regard to salts. Take for instance the Nitrate of Silver : it contains Total of equivalents, or equiva-") -j^^q lent of the Nitrate of Silver ) Practical application of the Laws of Combination. — The utility of being acquainted with the law of combining pro- portions is obvious when their nature is understood. As Equivalent. Nitrogen 6 Oxygen Silver . , 14 48 108 324 THE CHEMICAL ELEMENTS bodies both, unite with, and replace each other in equiva- lents, a simple calculation shows at once how much of each element or compound will be required in a given reaction. Thus, supposing it be desired to convert 100 grains of Ni- trate of Silver into CJiloride of Silver, the weight of Chlo- ride of Sodium which will be necessary is deduced thus : — one equivalent, or 170 parts, of JN'itrate of Silver, is de- composed by an equivalent, or 60 parts, of Chloride oi Sodium. Therefore as 170 : 60 : : 100 : 35-2; that is, 35"2 grains of salt will precipitate, in the state ol Chloride, the whole of the Silver contained in 100 grainf of Nitrate. So again, in order to form the Iodide of Silver, wha are the proportions in which the two salts should be mixed! The equivalent of Iodide of Potassium is 166, and that o Nitrate of Silver is 170. These numbers so nearly corre spond, that it is common to direct that equal loeigJits of th two salts should be taken. : One more illustration wiU. suffice. Supposing it be re quired to form 20 grains of Iodide of Silver — how muol Iodide of Potassium and Nitrate of Silver must be used? One equivalent, or 166 parts, of Iodide of Potassium, will yield an equivalent, or 234 parts, of Iodide of Silver; therefore as 234 : 166 : : 20 : 14-2. Hence, if 14"2 grains of the Iodide of Potassium be dis-' solved in water, and an equivalent quantity, viz. 14'5 grains, of the Nitrate of Silver added, the yeUow preci])i- tate, when washed and dried, wiU weigh precisely 20 grains. OK THE ATOMIC THEOEY. The atomic theory, originally proposed by Dalton, sc much facilitates the comprehension of chemical reactions AND THEIE COMBINATIONS. 325 generally, that it may be useful to give a sliort sketch, of it. It is supposed that all matter is made up of an infinite number of minute atoms, which are elementary, and do not admit of further division. Each of these atoms pos- sesses an actual weight, although inappreciable by our present methods of investigation. Simple atoms, by imi- ting with each other, form compound atoms ; and when these compounds are broken up, the elementary consti- tuent atoms are not destroyed, but separate from each other, in possession of all their original properties. In representing the simple atomic structure of bodies, circles may be used, as in the following diagram. Fig. 1 is a compound atom of Sulphuric Acid, consist- ing of an atom of Sulphur united intimately with three of Oxygen ; fig. 2 is an atom of Peroxide of Nitrogen, ; and fig. 3, an atom of Nitric Acid, composed of Nitrogen 1 atom. Oxygen 5 atoms, or in symbols NOg. The term "atomic weight" substituted for equivalent proportion. — If we suppose that the simple atoms of dif- ferent kinds of matter differ in toeight, and that this difference is expressed by their equivalent numbers, the whole laws of combination follow by the simplest reason- ing. It is easy to understand that an atom of one element, or compound, would displace, or be substituted for, a single atom of another ; therefore, taking as the illustration the decomposition of Iodide of Potassium by Chlorine, — the weight of the latter element required to liberate 126 grains of Iodine is 36 grains, because the weights of the atoms of those two elementary bodies are as 36 to 126. So again. 326 THE CHEMICAL ELEMENTS in the reaction between Cliloride of Sodium and Nitrate of Silver, a compound atom of the former, represented by the weight 60, reacts upon a compound atom of the latter, which equals 170. Therefore in place of the term " equivalent" or " com- bining proportion," it is more usual to employ that of " atomic weight." Thus the atomic weight of Oxygen is 8, represented by the symbol O, that of Sulphur is 16 ; hence the atomic weight of the compound atom of Sul- phuric Acid, or SO3, is necessarily equal to the combined weights of the four simple atoms ; id est, 16+24 = 40. ON THE CHEMISTEY OF OKGANIC SUBSTANCES. By " organic" substances are meant those which have possessed life, witli definite organs and tissues, in contra- distinction to the various forms of dead inorganic matter, in which no structural organization of that kind is found. The term organic, however, is also applied to substances which are obtained by chemical processes from the vege- table and animal kingdoms, although they cannot them- selves be said to be living bodies; thus Acetic Acid, procured by the distillation of woody fibre, and Alcohol, by fermentation from sugar, are strictly organic sub- stances. The class of organic bodies embraces a great variety of products ; which, like inorganic Oxides, may be divided into neutral, acid, and basic. The organic acids are numerous, including Acetic Acid, Tartaric, Citric, and a variety of others. The neutral substances cannot easily be assimilated to any class of inorganic compounds ; as examples, take Starch, Sugar, Lignine, etc. The bases are also a large class. They are mostly rare substances, not familiarly known : Morphia, obtained from Opium; Quinia, from Quinine; Nicotine, from Tobacco,, are illustrations. Composition of organic and imrganic bodies contrasted. AND THEIE COMBINATIONS. 327 — There are more than fifty elementary substances found in the inorganic kingdom, but onljfour, commonly speak- ing, in the organic : these four are Carbon, Hydrogen, Ni- trogen, and Oxygen. Some organic bodies, — Oil of Turpentine, Naphtha, etc., contain only Carbon and Hydrogen ; many others, such as sugar, gum, alcohol, fats, vegetable acids — Carbon, Hydro- gen, and Oxygen. The Nitrogenous bodies, so called, con- taining Nitrogen in addition to the other elements, are principally substances derived from animal and vegetable tissues, such as Albumen, Caseine, Gelatine, etc. ; Sulphur and Phosphorus are also present in many of the Nitro- genous bodies, but only to a small extent. Organic substances, although simple as regards the number of elements involved in their formation, are often highly complex in the arrangement of the atoms ; this may be illustrated by the following formulae : — Inorganic bodies, as already shown, unite in pairs, — two elements join to form a binary compound ; two binary com- pounds produce a salt ; two salts associated together form a double salt. With organic bodies, however, the arrange- ment is difierent, — the elementary atoms are all grouped equally in one compound atom, which is highly complex in structure and cannot be split up into binary products. Observe also, as characteristic of Organic Chemistry, the apparent similarity in composition between bodies which differ widely in properties. As examples take Lig- nine or cotton fibre, and Starch, — each of which contains the three elements imited as C24H20O20. Mode of distinguishing between Organic and Inorganic matter.— A. simple means of doing this is as follows: — place the suspected substance upon a piece of Platinum- Starch . . . Lignine . . . Cane Sugar . Grape Sugar . 328 THE CHEMICAL ELEMENTS. foil, and heat it to redness with a sprrit-lamp : if it first blackens, and then burns completely away, it is probably of organic origin. This test depends upon the fact, that the constituent elements of organic bodies are all either themselves volatile, or capable of forming volatile combi- nations with Oxygen. Inorganic substances, on the other hand, are often unaffected by heat, or, if volatile, are dissi- pated without previous charring. The action of heat upon organic matter may further be illustrated by the combustion of coal or wood in an ordi- nary furnace ; — first, an escape of Carbon and Hydrogen, united in the form of volatile gaseous matter, takes place, leaving behind a black cinder, which consists of Carbon and inorganic matter combined; afterwards this Carbon burns away into Carbonic Acid, and a grey ash is left, which is composed of inorganic salts, and is indestructible by heat. 329 CHAPTER 11. VOCABULARY OV PHOTOGEAPHIC CHEMICALS. ACETIC ACID. Symbol, C4H3O3+HO. Atomic weight, 60. Acetic Acid is a product of the oxidation of Alcohol. Spirituous liquids, when perfectly pure, are not affected by exposure to air ; but if a portion of yeast, or Nitro- genous organic matter of any kind, be added, it soon acts as a ferment, and causes the spirit to unite with oxygen derived from the atmosphere, and so to become sour from formation of Acetic Acid or "Vinegar." Acetic Acid is also produced on a large scale by heating wood in close vessels ; a substance distils over which is Acetic Acid contaminated with empyreumatic and tarry matter ; it is termed Pyroligneous Acid, and is much used in commerce. The most concentrated Acetic Acid may be obtained by neutralizing common vinegar with Carbonate of Soda and crystallizing out the Acetate of Soda so formed; this Acetate of Soda is then distilled with Sulphuric Acid, which removes the Soda and liberates Acetic Acid : the Acetic Acid being volatile, distils over, and may be con- densed. Properties of Acetic Acid. — The strongest acid contains only a single atom of water ; it is sold under the name 330 VOCABULARY OF of " Glacial Acetic Acid," so called from its property of solidifying at a moderately low temperature. At about 50° the crystals melt, and form a limpid liquid of pun- gent odour and a density nearly corresponding to that of water ; the specific gravity of Acetic Acid, however, is no test of its real strength, which can only be estimated by analysis. The Commercial Glacial Acetic Acid is often diluted with water, which may be suspected if it does not solidify during the cold winter months. Sulphurous and Hydro- chloric Acids are also common impurities. They are in- jurious in Photographic processes from their property of precipitating Nitrate of Silver. To detect them proceed as foUows : — dissolve a small crystal of Nitrate of Silver in a few drops of water, and add to it about half a drachm of the Glacial Acid ; the mixture should remain quite clear even when exposed to the hght. Hydrochloric and Sul- phurous Acid produce a white deposit of Chloride or Sul- phite of Silver ; and if Aldehyde or volatile tarry matter be present in the Acetic Acid, the mixture with Nitrate of Silver, although clear at fijst, becomes discoloured by the action of light. Many Photographers employ a cheaper form of Acetic Acid, sold by druggists as " Beaufoy's " acid ; it should be of the strength of the Acetic Acid fortiss. of the London Pharmacopoeia, containing 30 per cent, real acid, and must be tested for Sulphuric Acid (see Sulphuric Acid), and also by mixing with Nitrate of Silver. ACETATE OF SILVEE. See Silvee, Acetate of. ALBUMEN. Albumen is an organic principle, found both in the animal and vegetable kingdom. Its properties are best studied in the white of egg, which is a very pure form of Albumen. Albumen is capable of existing in two states ; in one of PHOTOGRAPHIC CHEMICALS. 331 ■n hicli it is soluble, in the other insoluble, in water. The aqueous solution of the soluble variety gives a slightly alkaline reaction to test-paper ; it is somewhat thick and glutinous, but becomes more fluid on the addition of a small quantity of an alkali, such as Potash or Ammonia. Soluble Albumen may be converted into the insoluble form in the following ways : — 1. By the application of heat.—K moderately strong solution of Albumen becomes opalescent and coagulates on being heated to about 150°, but a temperature of 212° is required if the liquid is very dilute. A layer of dried Albumen cannot easily be coagulated by the mere appli- (jation of heat. 2. By addition of strong acids— Witnc Acid coagulates Albumen perfectly without the aid of heat. Acetic Acid however acts differently, appearing to enter into combina- tion with the Albumen, and forming a compound soluble in warm water acidified by Acetic Acid. 3. By the action of metallic salts. — Many of the salts of the metals coagulate Albumen very completely. Nitrate of Silver does so ; also the Bichloride of Mercury. Am- ■ moniacal Oxide of Silver however does not coagulate Al- bumen. The white precipitate formed on mixing Albumen with Nitrate of Silver is a chemica compound of the animal matter with Protoxide of Silver. This substance, which has been termed Albuminate of Silver, is soluble in Am- monia and Hyposulphite of Soda ; but after exposure to light, or heating in a current of Hydrogen gas, it assumes a brick-red colour, being probably reduced to the condi- tion of a salt of the Suboxide of Silver, It is then almost insoluble in Ammonia, but enough dissolves to tiage the liquid wine-red. The Author is of opinion that the red coloration of solution of Nitrate of Silver employed in sen- sitizing the Albuminized photographic paper is produced by the same compound, although often referred to the pre- sence of Sulphuret of Silver. 332 VOCABtTLAEY OF Albumen also combines with Lime and Baryta; and Chloride of Barium has been recommended in Positive printing upon Albuminized paper, probably from this cause. Chemical composition of Albumen. — Albumen belongs to the Nitrogenous class of organic substances (seepage 327). It also contains smaU quantities of Sulphur and Phos- phorus. ALCOHOL. Symbol, C^HgOj- Atomic weight, 46. Alcohol is obtained by the careful distillation of any spirituous or fermented liquor. If wine or beer be placed in a retort, and heat applied, the Alcohol, being more volatile than water, rises first, and is condensed in an appropriate receiver ; a portion of the vapour of water however passes over with the Alcohol, and dilutes it to ai certain extent, forming what is termed " Spirits of Wine." Much of this water may be removed by redistillation from Carbonate of Potash, in the manner described at page 205 of this work ; but in order to render the Alcohol thoroughly anhydrous, it is necessary to employ quicJc Lime, which possesses a still greater attraction for water. An equal weight of this powdered lime is mixed with strong Alcohol of -823, and the two are distilled together. Properties of Alcohol— Puie anhydrous Alcohol is a limpid liquid, of an agreeable odour and pungent taste ; sp. gr. at 60°, '794. It absorbs vapour of water, and bo- comes diluted by exposure to damp air; boils at 178° Pahr. It has never been frozen. Alcohol distilled from Carbonate of Potash has a specific gravity of -815 to '823, and contains 90 to 93 per cent, of real spirit. The specific gravity of ordinary rectified Spirits of Wine is usually about '840, and it contains 80 to 83 per cent, of absolute Alcohol. PHOTOGEAPHIC CHEMICALS. 333 AMMONIA. Symbol, NB.^ or NH4O. Atomic weight, 17. The liquid known by this name is an aqueous solution of the volatile gas Ammonia. Ammoniacal gas contains 1 atom of Nitrogen combined with three of Hydrogen : these two elementary bodies exhibit no affinity for each other, but they can be made to unite under certain circumstances, and the result is Ammonia. Properties of Ammonia. — Ammoniacal gas is soluble in water to a large extent ; the solution possessing those pro- perties which are termed alkaline (see page 310) . Ammo- nia however differs from the other alkalies in one mpor- tant particular — it is volatile : hence the original colour of turmeric paper affected by Ammonia is restored on the application of heat. Solution of Ammonia absorbs Carbonic Acid rapidly from the air, and is converted into Carbonate of Ammonia : it should therefore be preserved in stoppered bottles. Besides Carbonate, commercial Ammonia often contains Chloride of Ammonium, recognized by the white precipitate given by Nitrate of Silver after acidifying with pure Nitric Acid. The strength of commercial Ammonia varies greatly ; that sold for pharmaceutical purposes, under the name of Liquor Ammonise, contains about 10 per cent, of real Am- monia. The sp. gr. of aqueous Ammonia diminishes with the proportion of Ammonia present, the Liquor Ammonise being usually about '9S6. Chemical Properties. — Ammonia, although forming a large class of salts, appears at first sight to contrast strongly in composition with the alkalies proper, such as Potash and Soda. Mineral bases generally are protoxides of me- tals, as already shown at page 310, but Ammonia consists simply of Nitrogen and Hydrogen united without Oxygen. The following remarks may perhaps tend somewhat to elucidate the difficulty: — 334 VOCABITIAEY OF Tlieory of Ammonium. — This theory supposes that a sub' stance exists possessing the properties of a metal, but dif ferent from metallic bodies generally in being compounc in structure : the formula assigned to it is NH4, 1 aton of Nitrogen xmited with 4 of Hydrogen. This hypothec tical metal is termed " Ammonium," and Ammonia, asso ciated with an atom of water, may be viewed as its Oxide . for NH3 + HO plainly equals NH4O. Thus, as Potash ii the Oxide of Potassium, so Ammonia is the Oxide of Am m,onium. The composition of the salts of Ammonia is on this vie-v assimilated to those of the alkalies proper. Thus, Sul phate of Ammonia is a Sulphate of the Oxide of Ammo ninm ; Muriate or Hydrochlorate of Ammonia is a Chk ride of Ammonium, etc. AMMONIO-NITEATE OF SILVEE. See SiLVEE, Ammonio-Nitbate or. AQUA EEGrlA. See Niteo-hydeochloeic Acid. BAEYTA, NITEATE OF. See Niteate or Baeyta. BICHLOEIDE OF MEECUEY. See Meecuey, Bichloeide or. BEOMINE. Symbol, Br. Atomic weight, 78. This elementary substance is obtained from the uncrys- tallizable residuum of sea-water, termed bittern. It exists!' in the water in very minute proportion, combined with Magnesium in the form of a soluble Bromide of Magne- sium. Properties. — Bromine is a deep reddish-brown liquid of a disagreeable odour, and fuming strongly at common tem- peratures ; sparingly soluble in water (1 part in 23, Lowig), PHOTOGEAPHIC CHEMICALS. 335 but more abundantly so in Alcohol, and especially in Ether. It is very heavy, having a specific gravity of 3'0. Bromine is closely analogous to Chlorine and Iodine in its chemical properties. It stands on the list intermedi- ately between the two ; its affinities being stronger than those of Iodine, but weaker than Chlorine. (See Chlorine.) It forms a large class of salts, of which the Bromides of Potassium, Cadmium, and Silver are the most familiar to Photographers. BEOMIDE OF POTASSIUM. Symbol, KBr. Atomic weight, 118. Bromide of Potassium is prepared by adding Bromine to Caustic Potash, and heating the product, which is a mix- ture of Bromide of Potassium and Bromate of Potash, to redness, in order to drive off the Oxygen from the latter salt. It crystallizes in anhydrous cubes, like the Chloride, and Iodide, of Potassium ; it is easily soluble in water, but more sparingly so in Alcohol ; it yields red fumes of Bromine when acted upon by Sulphuric Acid. BROMIDE OF SILVEE. See Silvee, Beomide of. CAEBONATE OF SODA. Symbol, NaO CO^ + IO Aq. This salt was formerly obtained from the ashes of sea- \\ eeds, but is now more economically manufactured on a large scale from common salt. The Chloride of Sodium is first converted into Sulphate of Soda, and afterwards the Sulphate into Carbonate of Soda. Properties. — The perfect crystals contain ten atoms of water, which are driven ofi" by the application of heat, leav- ing a white powder — the anhydrous Carbonate. Common Washing Soda is a neutral Carbonate, contaminated to a certain extent with Chloride of Sodium and Sulphate of 336 VOCABUIAEY OF Soda. The Carbonate used for effervescing draughts is either a Bicarbonate with 1 atom of water, or a Sesquicar- bonate, containing about 40 per cent, of real alkaU ; it is therefore nearly double as strong as the washing Carbo- nate, which contains about 22 per cent, of Soda. Carbo- nate of Soda is soluble in twice its weight of water at 60° the solution being strongly alkahne. CAEBONATE OF POTASH. See Potash, Carbonate of. CASEINE. See Milk. CHAECOAL, ANIMAL. ^ Animal Charcoal is obtained by heating animal sub- stances, such as bones, dried blood, horns, etc. to redness, in close vessels, until all volatile empyreumatic matters have been driven off, and a residue of Carbon remains. When prepared from bones it contains a large quantity of inorganic matter in the shape of Carbonate and Phos- phate of Lime, the former of which produces alkalinity in reacting upon Nitrate of Silver (see p. 82). Animal! Charcoal is freed from these earthy salts by repeated di- gestion in Hydrochloric Acid ; but unless very carefully washed it is apt to retain an acid reaction, and so to libe- rate free Nitric Acid when added to solution of Nitrate of Silver. Properties. — Animal charcoal, when pure, consists solt> !y of Carbon, and burns away in the air without leaving any residue : it is remarkable for its property of decolorizing solutions ; the organic colouring substance being separated, but not actually destroyed, as it is by Chlorine employed as a bleaching agent. This power of absorbing colouring matter is not possessed in an equal degree by all varieties of Charcoal, but is in great measure peculiar to tin derived from the animal kingdom. PHOTOGEAPHIC CHEMICALS. 337 CHINA CLAY, OR KAOLIN. This is prepared, by careful levigation, from mouldering granite and other disintegrated felspathic rocks. It con- sists of the Silicate of Alumina, — that is, of Silicic Acid or Flint, which is an Oxide of Silicon, united with the base Alumina (Oxide of Aluminum). Kaolin is perfectly inso- luble in water and acids, and produces no decomposition in solution of Nitrate of Silver. It is employed by Photo- graphers to decolorize solutions of Nitrate of Silver which have become brown from the action of Albumen or other organic matters. CHLORINE. Symbol, CI. Atomic weight, 36. Chlorine is a chemical element found abundantly in na- ture, combined with metallic Sodium in the form of Chlo- ride of Sodium, or Sea-salt. Preparation. — By distilling common Salt with Sulphuric Acid, Sulphate of Soda and Hydrochloric Acid are formed. Hydrochloric Acid contains Chlorine combined with Hy- drogen ; by the action of nascent Oxygen (see Oxygen), the Hydrogen may be removed in the form of water, and the Chlorine left alone. Properties. — Chlorine is a greenish-yellow gas, of a pun- gent and suffocating odour ; soluble to a considerable 3xtent in water, the solution possessing the odour and colour of the gas. It is nearly 2\ times as heavy as a cor- d^esponding bulk of atmospheric air. Chemical properties. — Chlorine belongs to a small na- ;ural group of elements which contains also Bromine, [odine, and Fluorine. They are characterized by having ir strong affinity for Hydrogen, and also for the metals, )ut are comparatively indifferent to Oxygen. Many me- iallic substances actually undergo combustion when jjro- jected into an atmosphere of Chlorine, the xxnion between z 338 VOCABTJLAEY OF the two taking place witli extreme violence. The cha- racteristic bleaching properties of Chlorine gas are ex- plained in the same manner : — Hydrogen is removed from the organic substance, and in that way the structure is broken up and the colour destroyed. Chlorine is more powerful in its affinities than either Bromine or Iodine. The salts formed by these three ele- ments are closely analogous in composition and often in properties. Those of the Alkalies, Alkaline Earths, and many of the Metals are soluble in water, but the Silver salts are insoluble ; the Lead salts sparingly so. The combinations of Chlorine, Bromine, Iodine, and Fluorine, with Hydrogen, are acids, and neutralize Alka- lies in the usual manner, with formation of Alkaline Chlo- ride and water (see page 313). The test by which the presence of Chlorine is detected either free or in combination with bases, is 'Nitrate of Sil ver ; it gives a white curdy precipitate of Chloride of Silver insoluble in Nitric Acid, but soluble in Ammonia. Th( solution of Nitrate of Silver employed as the test must not contain Iodide of Silver, as this compound is precipitated' by dilution. CHLOEIDE OF AMMONIUM. Symbol, NH4 CI. Atomic weight, 54. This salt, also known as Muriate or Hydrochlorate of Ammonia, occurs in commerce in the form of colourless and translucent masses, which are procured by sublimation, the dry salt being volatile when strongly heated. It dis- solves in an equal weight of boiling, or in three parts of cold, water. It contains more Chlorine in proportion to the weight used than Chloride of Sodium, the atomic weights of the two being as 54 to 60. PHOTOGRAPHIC CHEMICALS. 339 CHLOEIDE OF BAEIUM. Symbol, BaCI+2 HO. Atomic weight, 123. Barium is a metallic element, very closely allied to Cal- cium, tlie elementary basis of Lime. The Chloride of Ba- rium is commonly employed as a test for Sulphuric Acid, mth which it forms an insoluble precipitate of Sulphate of Baryta. It is also said to affect the colour of the Pho- tographic image when used in preparing Positive paper ; which may possibly be due to a chemical combination of Baryta with Albumen: but it must be remembered that this Chloride, from its high atomic weight, contains less Chlorine than the alkaline Chlorides (see page 122). Properties of Chloride of Barium. — Chloride of Barium occurs in the form of white crystals, soluble in about two parts of water, at common temperature. These crystals contain two atoms of water of crystallization, which are expelled at 212°, leavmg the anhydrous Chloride. CHLOEIDE OF GOLD. See G-old, Chloeide op. CHLOEIDE OF SODIUM. Symbol, JSTaCl. Atomic weight, 60. Common Salt exists abundantly in nature, both in the form of solid rock-salt and dissolved in the waters of the ocean. Properties of the pure Salt. — Fusible without decompo- sition at low redness, but sublimes at higher temperatures ; the melted salt concretes into a hard white mass on cool- ing. Nearly insoluble in absolute alcohol, but dissolves in ^minute quantity in rectified spirit. Sokible in three parts of water, botli hot and cold. Crystallizes in cubes, which are anhydrous. Impurities of Common Salt. — Table Salt often contains yOCABTJLAET OF large quantities of the Chlorides of Magnesium and Cal- cium, which, being deliquescent, produce a dampness by absorption of atmospheric moisture : Sulphate of Soda is also commonly present. The salt may be purified by re- peated recrystallization, but it is more simple to prepare the pure compound directly, by neutralizing Hydrochloric Acid with Carbonate of Soda. CHLOEIDE OF SILVEE. See Silver, Chloride of. CITEIC ACID. This acid is found abundantly in lemon-juice and in lime- juice. It occurs in commerce in the form of large crystals, which are soluble in less than their own weight of water at 60°. Commercial Citric Acid is sometimes mixed with Tar- taric Acid. The adulteration may be discovered by making a concentrated solution of the acid and adding Acetate of Potash ; crystals of Bitartrate of Potash will separate if Tartaric Acid be present. Citric Acid is tribasic. It forms with Silver a white in- soluble salt, containing 3 atoms of Oxide of Silver to 1 atom of Citric Acid. If the Citrate of Silver be heated in a current of Hydrogen gas, a part of the acid is liberated and the salt is reduced to a Citrate of Suboxide of Silver ; which is of a red colour. The action of white light in red- dening Citrate of Silver is shown by the Author to be of a similar nature. CYANIDE OF POTASSIUM. Symbol, K, CjN, or KCy. Atomic weight, 66. This salt is a compound of Cyanogen gas with the me- tal Potassium. Cyanogen is not an elementary body, like Chlorme or Iodine, but consists of Carbon and Nitrogen united in a peculiar manner. Although a compound sub- stance, it reacts in the manner of an element, ^nd is there- PHOTOGEAPHIC CHEMICALS. 341 fore (like Ammonium, previously described) an exception to the usual laws of chemistry. Many other bodies of a similar character are known. Properties of Cyanide of Potassium. — These have been sufficiently described at page 43, to which the reader is re- ferred. ETHEE. Symbol, C4HgO. Atomic weight, 37. Ether is obtained by distilling a mixture of Sulphuric Acid and Alcohol. If the formula of Alcohol (C4Hg02) be compared with that of Ether, it will be seen to differ from it in the possession of an additional atom of Hydro- gen and of Oxygen: in the reaction the Sulphuric Acid removes these elements in the form of ioater, and by so doing converts one atom of Alcohol into an atom of Ether. The term Sulphuric applied to the commercial Ether has reference only to the manner of its formation. Properties of Ether. — The properties of Ether have been described to some extent at pages 78 and 203. The fol- lowing particulars however may be added. It is neither acid nor alkaline to test-paper. Specific gravity, at 60°, about -720. Boils at 98° Fahrenheit. The vapour is ex- ceedingly dense, and may be seen passing off from the liquid and falhng to the ground : hence the danger of pour- ing Ether from one bottle to another if a flame be near at hand. Ether does not mix with water in all proportions ; if the two arc shaken together, after a short time the former rises and floats upon the surface. In this way a mixture of Ether and Alcohol may be purified to some extent, as in the common process of washing Ether. The water em- ployed however always retains a certain portion of Ether (about a tenth part of its bulk), and acquires a strong ethereal odour ; washed Ether also contains water in small quantity. 342 YOCABULAEY OF Bromine and Iodine are both soluble in Ether, and gra- dually react upon and decompose it. The strong alkalies, such as Potash and Soda, also de- compose Ether slightly after a time, but not immediately. Exposed to air and light. Ether is oxidized and acquires a peculiar odour (page 78). Ether dissolves fatty and resinous substances readily, but inorganic salts are mostly insoluble in this fluid. Hence it is that Iodide of Potassium and other substances dissolved in Alcohol are precipitated to a certain extent by the addition of Ether. FLUOEIDE OE POTASSIUM. Symbol, KE. Atomic vreight, 59. Preparation. — Fluoride of Potassium is formed by satu- rating Hydrofluoric Acid with Potash, and evaporating to dryness in a platinum vessel. Hydrofluoric Acid contains Eluorine combined with Hydrogen ; it is a powerfully acid and corrosive liquid, formed by decomposing Eluor Spar, which is a Fluoride of Calcium, with strong Sulphuric Acid ; the action which takes place being precisely ana- logous to that involved in the preparation of Hydrochloric Acid. Properties. — A deliquescent salt, occurring in smaU and imperfect crystals. Very soluble in water : the solution acting upon glass in the same manner as Hydrofluoric Acid. EOEMIC ACID. Symbol, CgHOg. Atomic weight, 37. This substance was originally discovered in the red ant {Formica rufd), but it is prepared on a large scale by distil- ling /S^arc/^ with Binoxide of Manganese and Sulphuric Acid. Properties. — The strength of commercial Formic Acid is Tmcertain, but it is always more or less dilute. The strongest acid, as obtained by distilling Eormiate of Soda PHOTOGEAPHIC CHEMICALS. 343 witli Sulphuric Acid, is a fuming liquid with a pungent odour, and containing only one atom of water : it inflames the skin in the same manner as the sting of the ant. Formic Acid reduces the Oxides of Gold, Silver, and Mercury, to the metallic state, and is itself oxidized into Carbonic Acid. The alkaline formiates also possess the same properties. GALLIC ACID. Symbol, CyHjOj + HO. Atomic weight, 94, The chemistry of Gallic Acid is sufficiently described at page 27, to which the reader is referred. GELATINE. Symbol, CjaH^OgNg. Atomic weight, 156. This is an organic substance somewhat analogous to Albumen, but differing from it in properties. It is ob- tained by subjecting bones, hoofs, horns, calves' feet, etc. to the action of boiling water. The jelly formed on cool- ing is termed size, or when dried or cut into slices, glue. Gelatine, as it is sold in the shops, is a pure form of Glue. Isinglass is Gelatine prepared, chiefly in Russia, from the air-bladders of certain species of sturgeon. Properties of Gelatine. — Gelatine softens and swells up in cold water, but does not dissolve until heated : the hot solution, on cooling, forms a tremulous jelly. One ounce of cold water will retain about three grains of Isinglass without gelatinizing ; but much depends upon the tempe- rature, a few degrees greatly affecting the result. Gelatine forms no compound with Oxide of Silver ana- logous to the Albuminate of Silver : which fact explains the difference in the Photographic properties of Albumen and Gelatine. GLYCEEINE. Patty bodies are resolved by treatment witli an alkali 344 VOCABIJLAEY OP into an Acid — wliicli combines with tlie alkali, forming a soap, — and Glycerine, remaining in solution. Pure Glycerine, as obtained by Price's patent process of distillation, is a viscid liquid of sp. gr. about 1'23 ; miscible in all proportions with water and Alcohol. It is peculiarly a neutral substance, exhibiting no tendency to combine with acids or bases. It has little or no action upon Ni- trate of Silver in the dark, and reduces it very slowly even when exposed to light. GOLD, CHLOEIDE OF. Symbol, AuClj. Atomic weight, 303. This salt is formed by dissolving pure metallic Gold in Nitro-hydrochloric Acid, and evaporating at a gentle heat. The solution affords deliquescent crystals of a deep orange colour. Chloride of Gold, in a state fit for Photographic use, may easily be obtained by the following process : — Place a half-sovereign in any convenient vessel, and pour on it half a drachm of Nitric Acid mixed with two and a half drachms of Hydrochloric Acid and three drachms of water; digest by a gentle heat, but do not hoil the acid, or much of the Chlorine will be driven off in the form of gas. At the expiration of a few hours add fresh Aqua Kegia in quantity the same as at first, which wUl probably complete the solution, but if not, repeat the process a third time. Lastly, neutralize the liquid by adding Carbonate of Soda until all effervescence ceases, and a green precipitate forms ; this is Carbonate of Coffer, which must be allowed several hours to separate thoroughly. The solution then contains Chloride of Gold in a neutral state, and free from Copper and Silver, with which the metallic Gold is alloyed in the standard coin of the realm. The weight of a half-sovereign is about 61 grains, of which 56 grains are pure Gold. This is equivalent to 86 PHOTOGRAPHIC CHEMICALS. 345 grains of Chloride of Gold, which will therefore be the quantity contained in the solution. The following process for preparing Chloride of Grold is more perfect than the last : — Dissolve the Gold coin in Aqua Eegia as before ; then boil with excess of Hydro- chloric Acid to destroy the Nitric Acid, dilute largely with distiQed water, and add a filtered aqueous solution of com- mon Sulphate of Iron (6 parts to 1 of Gold) ; collect the precipitated Gold, which is now free from copper ; redis- solve in Aqua Eegia, and evaporate to dryness on a water bath. Avoid using Ammonia to neutralize Chloride of Gold, as it would be liable to occasion a deposit of " Fulminating Gold," the properties of which are described below. Properties of Chloride of Gold. — As sold in commerce it usually contains excess of Hydrochloric Acid, and is then of a bright yellow colour ; but when neutral and some- what concentrated it is dark red (Leo ruber of the alche- mists). It gives no precipitate with Carbonate of Soda, unless heat be applied ; the free Hydrochloric Acid pre- sent forms, with the alkali, Chloride of Sodium, which unites with the Chloride of Gold, and produces a double salt, Chloride of Gold and Sodium, soluble in water. Chloride of Gold is decomposed with precipitation of metallic Gold by Charcoal, Sulphurous Acid, and many of the vegetable acids ; also by Protosulphate and Protoni- trate of Iron. It tinges the cuticle of an indelible purple tint. It is soluble in Alcohol and in Ether. GOLD, FULMINATING. This is a yeUowish-brown substance, precipitated on adding Ammonia to a strong solution of Chloride of Gold. It may be dried carefully at 212°, but explodes violently on being heated suddenly to about 290°. Friction also causes it to explode when dry ; but the moist powder may be rubbed or handled without danger. It is decomposed by Sulphuretted Hydrogen. 346 VOCABULAET OF rulminating Gold is probably an Aurate of Ammonia, containing 2 atoms of Ammonia to 1 atom of Peroxide of Gold. GOLD, HYPOSULPHITE OF. Symbol, AuO S^O^. Atomic weigbt, 253. Hyposulphite of Gold is produced by tbe reaction of Chloride of Gold upon Hyposulphite of Soda (see page 131). The salt sold in commerce as Sel d'Or is a double Hy- posulphite of Gold and Soda, containing one atom of the former salt to three of the latter, with four atoms of water of crystallization. It is formed by adding one part of Chloride of Gold, in solution, to three parts of Hyposul- phite of Soda, and precipitating the resialting salt by Al- cohol ; the Chloride of Gold must be added to the Hypo- sulphite of Soda, and not the Soda salt to the Gold (see page 250). Properties. — Hyposulphite of Gold is unstable and can- not exist in an isolated state, quickly passing into Sulphur, Sulphuric Acid, and metallic Gold. When combined with excess of Hyposulphite of Soda in the form of Sel d'Or, it is more permanent. Sel d'or occurs crystallized in fine needles, which are very soluble in water. The commercial article is often impure, containing little else than Hyposulphite of Soda, with a trace of Gold. It may be analyzed by adding a few drops of strong Nitric Acid (free from Chlorine) diluting with water, and afterwards collecting and igniting the yel- low powder, which is metallic Gold. GEAPE SUGAE. Symbol, C24H2302g. Atomic weight, 396. This modification of Sugar, often termed Granular Sugar, or Glucose, exists abundantly in the juice of grapes, and in many other varieties of fruit. It forms PHOTOGEAPHIC CHEMICALS. 347 the saccharine concretion found in honey, raisins, dried figs, etc. It may be produced artificially by the action of fermenting principles, and of dilute mineral acids, upon Starch. Properties. — Grape Sugar crystallizes slowly and with difficulty from a concentrated aqueous solution, in small hemispherical nodules, which are hard, and feel gritty be- tween the teeth. It is much less sweet to the taste than Cane Sugar, and not so sohible in water (1 part dissolves in 1| of cold water). Grape Sugar tends to absorb oxy- gen, and hence it possesses the property of decomposing the salts of the noble metals, and reducing them by de- grees to the metallic state, even without the aid of light. The action however in the case of Nitrate of Silver is slow, unless the temperature be somewhat elevated. Cane Sugar does not possess these properties to an equal extent, and hence it is readily distinguished from the other variety. HONEY. This substance contains two distinct kinds of Sugar, Grape Sugar, and an uncrystallizable substance analogous to, or identical with, the Treacle found associated with common Sugar in the cane-juice. The agreeable taste of Honey pr#bably depends upon the latter, but its reducing power on metallic oxides is due to the former. Pure Grape Sugar can readily be obtained from inspissated Honey, by treating it with Alcohol, which dissolves out the syrup, but leaves the crystalline portion. HYDHOCHLOEIC ACID. Symbol, HCl. Atomic weight, 37. Hydrochloric Acid is a volatile gas, which may be libe- rated from the salts termed Chlorides by the action of Sul- phuric Acid. The acid, by its superior affinities, removes the base ; thus, — NaCl + HO SO3 NaO SOg-f-HCL 348 VOCABULARY OF Properties. — Abundantly soluble in water, forming the liquid Hydrochloric or Muriatic Acid of commerce. The most concentrated solution of Hydrochloric Acid has a sp. gr. 1'2, and contains about 40 per cent, of gas ; that commonly sold is somewhat weaker, sp. gr. 1'14 = 28 per cent, real acid. Pure Hydrochloric Acid is colourless, and fumes in the air. The yellow colour of the commercial acid depends upon the presence of traces of Perchloride of Iron or or- ganic matter ; commercial Muriatic Acid also often con- tains a portion of free Chlorine and of Sulphuric Acid. HYDEIODIC ACID. Symbol, HI. Atomic weight, 127. This is a gaseous compound of Hydrogen and Iodine, corresponding in composition to the Hydrochloric Acid. It cannot however, from its instability, be obtained in the same manner, since, on distilling an Iodide with Sul- phuric Acid, the Hydriodic Acid first formed is subse- quently decomposed into Iodine and Hydrogen. An aqueous solution of Hydriodic Acid is easily prepared by adding Iodine to water containing Sulphuretted Hydrogen gas ; a decomposition takes place, and Sulphur is set free : thus. HS + I = HI+S. Properties. — Hydriodic Acid is very soluble in water, yielding a strongly acid liquid. The solution, colourless at first, soon becomes brown from decomposition, and libe- ration of free Iodine. It may be restored to its original condition by adding solution of Sulphuretted Hydrogen. HYDEOSULPHUEIC ACID. Symbol, HS. Atomic weight, 17. This substance, also known as Sulphuretted Hydrogen, is a gaseous compound of Sulphur and Hydrogen, ana- PHOTOGEAPHIC CHEMICALS. 349 logous in composition to Hydrochloric and Hydriodic Acids. It is usually prepared by tlie action of dilute Sul- phuric Acid upon Sulphuret of Iron, as described at page 373 ; the decomposition being similar to that involved in the preparation of the Hydrogen acids generally : — FeS + HO SO3 = FeO SO3 + HS. Properties. — Cold water absorbs three times its bulk of Hydrosulphuric Acid, and acquires the pecuhar putrid odour and poisonous qualities of the gas. The solution is faintly acid to test-paper, and becomes opalescent on keeping, from gradual separation of Sulphur. It is de- composed by Nitric Acid, and also by Chlorine and Iodine. It precipitates Silver from its solutions, in the form of black Sulphuret of Silver ; also Copper, Mercury, Lead, etc. ; but Iron and other metals of that class are not affected, if the liquid contains free acid. Hydrosulphuric Acid is constantly employed in the chemical laboratory for these and other purposes. HYDEOSULPHATE OF AMMONIA. Symbol, NH^S HS. Atomic weight, 51. The liquid known by this name, and formed by passing Sulphuretted Hydrogen gas into Ammonia, is a double Sulphuret of Hydrogen and Ammonium. In the prepa- ration, the passage of the gas is to be continued untd the solution gives no precipitate with Sulphate of Magnesia and smells strongly of Hydrosulphuric Acid. Properties. — Colourless at first, but afterwards changes to yellow, from liberation and subsequent solution of Sul- phur. Becomes milky on the addition of any acid. Pre- cipitates, in the form of Sulphuret, all the metals which are affected by Sulphuretted Hydrogen, and, in addition, those of the class to which Iron, Zinc, and Manganese belong. 350 VOCABtJLAET OF Hydros ulpliate of Ammonia is employed in Photography to darken the Negative image, and also in the preparation of Iodide of Ammoninm ; the separation of Silver from Hyposulphite solutions, etc. HYPOSULPHITE OF SODA. Symbol, NaO S^Oj + 5 HO. Atomic weight, 125. The chemistry of Hyposulphurous Acid and the Hypo- sulphite of Soda has been sufficiently described at pages 42, 128, and 136 of the present "Work. The crystallized salt includes five atoms of water of crystallization. HYPOSULPHITE OF GOLD. See Gold, Hyposul- phite OF. HYPOSULPHITE OF SILVEE. See Silvee, Htpo- StTLPHITE OF. ICELAND MOSS. Cetraria Islandica. — A species of Lichen found in Ice- land and the mountainous parts of Europe ; when boiled in water, it first swells up, and then yields a substance which gelatinizes on cooling. It contains Lichen Starch ; a bitter principle soluble in Alcohol, termed Cetrarine ;" and common Starch ; traces of Gallic Acid and Bitartrate of Potash are also present. IODINE. Symbol, I. Atomic weight, 126. Iodine is chiefly prepared at Glasgow, from Jcelp, which is the fused ash obtained by burning seaweeds. The waters of the ocean contain minute quantities of the Iodides of Sodium and Magnesium, which are separated and stored up by the growing tissues of the marine plant. In the preparation, the mother-liquor of kelp is eva- PHOTOGEAPHIC CHEMICALS. 351 porated to dryness and distilled with Sulplmric Acid ; the Hydriodic Acid first liberated is decomposed by the high temperature, and fumes of Iodine condense in the form of opaque crystals. Properties. — Iodine has a bluish-black colour and me- tallic lustre ; it stains the skin yellow, and has a pungent smell, Hke diluted Chlorine. It is extremely volatile when moist, boils at 350°, and produces dense violet-coloured fumes, which condense in brilliant plates. Specific gravity 4-946. Iodine is very sparingly soluble in water, 1 part requiring 7000 parts for perfect solution : even this minute quantity however tinges the liquid of a brown colour. Al- cohol and Ether dissolve it more abundantly, forming dark-brown solutions. Iodine also dissolves freely in solu- tions of the alkaline Iodides, such as the Iodide of Potas- sium, of Sodium, and of Ammonium. Chemical Properties. — Iodine belongs to the Chlorine group of elements, characterized by forming acids with Hydrogen, and combining extensively with the metals (see Chlorine). They are however comparatively indifferent to Oxygen, and also to each other. The Iodides of the alkalies and alkaline earths are soluble in water ; also those of Iron, Zinc, Cadmium, etc. The Iodides of Lead, Silver, and Mercury are nearly or quite insoluble. Iodine possesses the property of forming a compound of a deep blue colour with Starch. In using this as a test, it is necessary first to liberate the Iodine (if in combina- tion), by means of Chlorine, or Nitric Acid saturated with Peroxide of Nitrogen. The presence of Alcohol or Ether interferes to a certain extent with the result. IODIDE OF AMMONIUM. Symbol, NII4I. Atomic weight, 144. The preparation and properties of this salt are described at page 208, to which the reader is referred. 352 VOCABTTLAEY OF IODIDE OF cadmium:. Symbol, Cdl. Atomic weight, 182. See page 209, for tlie preparation and properties of tkis salt. IODIDE OF lEON. Symbol, Fel. Atomic weight, 154. Iodide of Iron is prepared by digesting an excess of Iron filings with solution of Iodine (page 208). ^It is very soluble in water and Alcohol, but the solution rapidly absorbs Oxygen and deposits Peroxide of Iron ; hence the importance of preserving it in contact with metallic Iron, with which the separated lodiae may recombine. By very careful evaporation, hydrated crystals of Proto-iodide may be obtained, but the composition of the sohd salt usually sold under that name cannot be depended on. The Periodide of Iron, corresponding to the Ferclilo- ride, has not been examiued, and it is doubtful if any such compound exists. IODIDE OF POTASSIUM. Symbol, KI. Atomic weight, 166. This salt is usually formed by dissolving Iodine in so- lution of Potash until it begins to acquire a brown colour ; a mixture of Iodide of Potassium and lodate of Potash (KO lOj) is thus formed ; but by evaporation and heat- ing to redness, the latter salt parts with its Oxygen, and is converted into Iodide of Potassium. Properties. — It forms cubic and prismatic crystals, which should be hard, and very sligJitly or not at all deli- quescent. Soluble in less than an equal weight of water at 60° ; it is also soluble in Alcohol, but not in Ether. The proportion of Iodide of Potassium contained in a saturated alcoholic solution, varies with the strength of the spirit, — PHOTOGEAPHIC CHEMICALS. 353 with common Spirits of Wine, sp. gr. "836, it would be about 8 grains to the drachm ; with Alcohol rectified from Carbonate of Potash, sp. gr. '823, 4 or 5 grains ; with ab- solute Alcohol, 1 to 2 grains. The solution of Iodide of Potassiiim is instantly coloured brown by free Chlorine; also very rapidly by Peroxide of Nitrogen (page 78) ; ordinary acids however act less quickly, Hydriodic Acid being first formed, and subsequently decomposing spon- taneously. The impurities of commercial Iodide of Potassium, with the means to be adopted for their removal, are fully given at page 206. IODIDE OF SILVEE. See Silvee, Iodide of. lEOI^, PEOTOSULPHATE OE. Symbol, EeO SO3 -4- 7 HO. Atomic weight, 139. The properties of this salt, and of the two salifiable Oxides of Iron, are described at page 29. It dissolves in rather more than an equal weight of cold water, or in less of boiling water. Aqueous solution of Sulphate of Iron absorbs the Biii- oxide of Nitrogen, acquiring a deep olive-brown colour : as this gaseous Binoxide is itself a reducing agent, the liquid so formed has been proposed as a more energetic developer than the Sulphate of Iron alone. lEON, PEOTOWITEATE OF. Symbol, FeO NO5 + 7 HO. Atomic weight, 153. This salt, by careful evaporation in vacuo over Sul- phtiric Acid, forms transparent crystals, of a light green colour, and containing 7 atoms of water, like the Protosul- phate. It is exceedingly unstable, and soon becomes red from decomposition, unless preserved from contact with 2 a 354 VOCABtTLAET OF air. The preparation of solution of Protonitrate of Iron for developing Collodion Positives, is given at page 215. lEON, PEECHLOEIDE OF. Symbol, FejClg. Atomic weight, 164. There are two Chlorides of Iron, corresponding in com- position to the Protoxide and the Sesquioxide respec- tively. The Protochloride is very soluble in water, form- ing a green solution, which precipitates a dirty white Pro- toxide on the addition of an alkali. The Perchloride, on the other hand, is dark brown, and gives a foxy -red precipitate with alkalies. Properties. — Perchloride of Iron may be obtained in the solid form by heating Iron wire in excess of Chlorine ; it condenses in the shape of brilliant and iridescent brown crystals, which are volatile, and dissolve in water, the so- lution being acid to test-paper. It is also soluble in Alcohol, forming the Tinctura Ferri Sesquichloridi of the Pharmacopoeia. Commercial Perchloride of Iron ordina- rily contains an excess of Hydrochloric Acid. LITMUS. Litmus is a vegetable substance, prepared from various lichens, which are principally collected on rocks adjoining the sea. The colouring matter is extracted by a peculiar process, and afterwards made up into a paste with chalk, plaster of Paris, etc. Litmus occurs in commerce in the form of small cubes, of a fine violet colour. In using it for the preparation of test-papers, it is digested in hot water, and sheets of po- rous paper are soaked in the blue liquid so formed. The red papers are prepared at first in the same manner, but afterwards placed in water which has been rendered faintly acid with Sulphuric or Hydrochloric Acid. PHOTOGEAPHIC CHEMICALS. 355 MEECUEY, BICHLOEIDE OF. Symbol, HgClj. Atomic weight, 274. This salt, also called. Corrosive Sublimate, and some- times Chloride of Mercury (the atomic weight of Mercury being halved), may be formed by heating Mercury in excess of Chlorine, or, more economically, by subHming a mixture of Persulphate of Mercury and Chloride of Sodium. Properties. — A very corrosive and poisonous salt, usu- ally sold in semi-transparent, crystalline masses, or in the state of powder. Soluble in 16 parts of cold, and in 3 of hot water ; more abundantly so in Alcohol, and also in Ether. The solubility in water may be increased almost to any extent by the addition of free Hydrochloric Acid. The Protochloride of Mercury is an insoluble white powder, commonly known under the name of Calomel. MILK. The MUk of herbivorous animals contains three prin- cipal constituents — Fatty matter, Caseine, and Sugar ; in addition to these, small quantities of the Chloride of Potassium, and of Phosphates of Lime and Magnesia, are present. The fatty matter is contained in small cells, and forms the greater part of the cream which rises to the surface of the milk on standing ; hence shimmedms}^ is to be pre- ferred for Photographic use. The second constituent, Caseine, is an organic principle somewhat analogous to Albumen in composition and pro- perties. Its aqueous solution however does not, like Al- bumen, coagulate on boiling, unless an acid be present, which probably removes a small portion of alkali with which the Caseine was previously combined. The sub- stance termed " rennet," which is the dried stomach of the calf, possesses the property of coagulating Caseine, 356 VOCABTTLAET OF but the exact mode of its action is unknown. Sherry- Wine is also commonly employed to curdle milk ; but brandy and other spirituous liquids, when free from acid and astringent matter, have no effect. In all these cases a proportion of the Caseine usually remains in a soluble form in the whey, but when the milk is coagulated by the addition of acids, the quantity so left is very small, and hence the use of the rennet is to be pre- ferred, since the presence of Caseine facilitates the reduc- tion of the sensitive Silver salts. Caseine combines with Oxide of Silver in the same man- ner as Albumen, forming a white coagulum, which becomes hricTc-red on exposure to light. Sugar of Milk, the third principal constituent, differs from both cane and grape sugar ; it may be obtained by evaporating whey until crystallization begias to take place. It is hard and gritty, and only shghtly sweet; slowly soluble, without formiag a syrup, in about two and a half parts of boUing, and six of cold water. It does not fer- ment and form Alcohol on the addition of yeast, like grape sugar, but by the action of decomposing animal matter is converted into Lactic Acid. When skimmed milk is exposed to the air for some hours it gradually becomes sour, from Lactic Acid formed in this way ; and if then heated to ebullition, the Caseine coagulates very perfectly. NITEIC ACID. Symbol, NO5. Atomic weight, 54. Nitric Acid, or Aqua-fortis, is prepared by adding Sul- phuric Acid to Nitrate of Potash, and distilling the mix- ture in a retort. Sulphate of Potash and free Nitric Acid are formed, the latter of which, being volatile, distils over in combination with one atom of water previously united with the Sulphuric Acid. Properties. — Anhydrous Nitric Acid is a soHd substance. PHOTOGEAPHIC CHEMICALS. 357 white and crystalline, but it cannot be prepared except by an expensive and complicated process. The concentrated liquid Nitric Acid contains 1 atom of water, and has a sp. gr. of about 1'5 ; if perfectly pure it is colourless, but usually it has a slight yellow tint, from partial decomposition into Peroxide of Nitrogen : it fumes strongly in the air. The strength of commercial Nitric Acid is subject to much variation. An acid of sp. gr. 1"42, containing about 4 atoms of water, is commonly met with. If the specific gravity is much lower than this (less than 1'36), it will scarcely be adapted for the preparation of Pyroxyliue. The yellow Nitrous Acid, so called, is a strong Nitric Acid partially saturated with the brown vapours of Peroxide of Nitrogen ; it has a high specific gravity, but this is some- what deceptive, being caused in part by the presence of the Peroxide. On mixing with Sulphuric Acid the colour dis- appears, a compoimd being formed which has been termed a Sulphate of Nitrous Acid. In the Appendix a Table is given which exhibits the quantity of real anhydrous Nitric Acid contained in sam- ples of difierent densities. Chemical Properties. — Nitric Acid is a powerful oxi- dizing agent (see page 13); it dissolves all the common metals, with the exception of Grold and Platinum. Animal substances, such as the cuticle, nails, etc., are tinged of a permanent yellow colour, and deeply corroded by a pro- longed application. Nitric Acid forms a numerous class of salts, all of which are soluble in water. Hence its pre- sence cannot be determined by any precipitating reagent, in the same manner as that of Hydrochloric and Sulphuric Acid. Impurities of Commercial Nitric Add. — These are principally Chlorine and Sulphuric Acid; also Peroxide of Nitrogen, which tinges the acid yellow, as already de- scribed. Chlorine is detected by diluting the acid with an equal biilk of distilled water, and adding a few drops of 358 VOCABULAEY OF Nitrate of Silver, — a milhiness, wliicli is Cliloride of Silver in suspension, indicates the presence of CMorine. In test- ing for Sulphuric Acid, dilute the Nitric Acid as before, and drop in a single drop of solution of Chloride of Ba- rium ; if Sulphuric Acid be present, an insoluble precipi- tate of Sulphate of Baryta wiU be formed. NITEOUS ACID. See Silvek, Niteite or. NITEATE OP POTASH. Symbol, KO NO5. Atomic weight, 102. This salt, also termed Nitre or Saltpetre, is an abundant natural product, found effloresced upon the soil in certain parts of the East Indies. It is also produced artificially m what are called Nitre-beds, The properties of Nitrate of Potash are described as far as necessary at page 199. NITEATE OF BAEYTA. Symbol, BaO NOg. Atomic weight, 131. Nitrate of Baryta forms octahedral crystals, which are anhydrous. It is considerably less soluble than the Chlo- ride of Barium, requiring 12 parts of cold and 4 of boiling water for solution. It may be substituted for the Nitrate of Lead in the preparation of Protonitrate of Iron. NITEATE OF LEAD. Symbol, PbO NO5. Atomic weight, 166. Nitrate of Lead is obtained by dissolving the metal, or the Oxide of Lead, in excess of Nitric Acid, diluted with 2 parts of water. It crystallizes on evaporation in white anhydrous tetrahedra and octahedra, which are hard, and decrepitate on being heated ; they are soluble in 8 parts of water at 60°. PHOTOGEAPHIC CHEMICALS. 359 Nitrate of Lead forms with Sulplraric Acid, or soluble Sulphates, a white precipitate, which is the insoluble Sul- phate of Lead. The Iodide of Lead is also very sparingly- soluble in water. NITEATE OF SILVEE. See Silvee, Niteate oe. NITEO-HYDEOCHLOEIC ACID. Symbol, NO4 + CI. This liquid is the Aqua-regia of the old alchemists. It is produced by mixing Nitric and Hydrochloric Acids : the Oxygen contained in the former combines with the Hydrogen of the latter, forming water and hberating Chlo- rine, thus : — NO5 + HC1= NO4 + HO + CI. The presence of free Chlorine confers on the mixture the power of dissolving Gold and Platinum, which neither of the two acids possesses separately. In preparing Aqua- regia it is usual to mix one part, by measure, of Nitric Acid with four of Hydrochloric Acid, and to dilute with an equal bulk of water. The apphcation of a gentle heat assists the solution of the metal ; but if the temperature rises to the boiling point, a violent effervescence and escape of Chlorine takes place. NITEO-SULPHTJEIC ACID. For the chemistry of this acid Hquid, see page 73. OXYGEN. Symbol, 0. Atomic weight, 8. Oxygen gas may be obtained by heating Nitrate of Pot- ash to redness, but in this case it is contaminated with a portion of Nitrogen. The salt termed Chlorate of Potash (the composition of which is closely analogous to that of 360 VOCABtTLAEY OF the Mtrate, Chlorine being substituted for Nitrogen) yields abundance of pure Oxygen gas on the application of heat, leaving behind Chloride of Potassium. Chemical Properties. — Oxygen combines eagerly with many of the chemical elements, forming Oxides. This che- mical affinity however is not well seen when the elemen- tary body is exposed to the action of Oxygen in the gaseous form. It is the nascent Oxygen which acts most power- fuUy as an oxidizer. By nascent Oxygen is meant Oxygen on the point of separation from other elementary atoms with which it was previously associated ; it may then be considered to be in the Hquid form, and hence it comes more perfectly into contact with the particles of the body to be oxidized. Illustrations of the superior chemical energy of nascent Oxygen are numerous, but none perhaps are more striking than the mild and gradual oxidizing influence exerted by atmosj)heric air, as compared with the violent action of Nitric Acid and bodies of that class which contain Oxygen loosely combined. OXYMEL. This syrup of Honey and Vinegar is prepared as fol- lows : — Take of Honey 1 pound. Acid, Acetic, fortiss. (Beaufoy's Acid) 11 drachms. Water 13 drachms. Stand the pot containing the Honey in boiling water until a scum rises to the surface, which is to be removed two or three times. Then add the Acetic Acid and water, and skim once more if required. Allow to cool, and it wiU be fit for use. POTASH. Symbol, KO + HO. Atomic weight, 57. Potash is obtained by separating the Carbonic Acid PHOTOGEAPHIC CHEMICALS. 361 from Carbonate of Potash by means of Caustic Lime. Lime is a more feeble base tban Potasli, but the Carbon- ate of Lime, being insoluble in water, is at once formed on adding milk of Lime to a solution of Carbonate of Potash. (See page 316.) Properties. — TJsually met with in the form of solid lumps, or in cylindrical sticks, which are formed by melt- ing the Potash and running it into a mould. It always contains one atom of water, which cannot be driven off by the application of heat. Potash is soluble almost to any extent in water, much heat being evolved. The solution is powerfully alkaline (p. 310), and acts rapidly upon the skin; it dissolves fatty and resinous bodies, converting them into soaps. Solution of Potash absorbs Carbonic Acid quickly from the air, and should therefore be preserved in stoppered bottles ; the glass stoppers must be wiped occasionally, in order to prevent them from becoming immovably fixed by the solvent action of the Potash upon the Silica of the glass. The Liquor Potassse of the London Pharmacopoeia has a sp. gr. of 1'063, and contains about 5 per cent, of real Potash. It is usually contaminated with Carlonate of Pot- ash, which causes it to effervesce on the addition of acids ; also, to a less extent, with Sulphate of Potash, Chloride of Potassium, SUica, etc. POTASH, CAEBONATE OF. Symbol, KO COj. Atomic weight, 70. The impure Carbonate of Potash, termed Pearlash, is obtained from the ashes of wood and vegetable matter, in the same manner as Carbonate of Soda is prepared from the ashes of seaweeds. Salts of Potash and of Soda ap- pear essential to vegetation, and are absorbed and approxi- mated by the living tissues of the plant. They exist in the vegetable structure combined with organic acids in the 362 VOCABULAET OP form of salts, like tlie Oxalate, Tartrate, etc., which when burned are converted into Carbonates. Properties. — The Pearlash of commerce contains large and variable quantities of Chloride of Potassium, Sulphate of Potash, etc. A purer Carbonate is sold, which is free from Sulphates, and with only a trace of Chlorides. Car- bonate of Potash is a strongly alkaline salt, deliquescent, and soluble in twice its weight of cold water ; insoluble in Alcohol, and employed to deprive it of water. (See paee 205.) ^ ^ PYEOGALLIC ACID. Symbol, CgH^O^ (Stenhouse). Atomic weight, 84. The chemistry of Pyrogallic Acid has been described at page 28. SEL D'OE. See Gold, Hyposulphite of. SILVEE. Symbol, Ag. Atomic weight, 108. This metal, the Luna or Diana of the alchemists, is found native in Peru and Mexico ; it occurs also in the form of Sulphuret of Silver. When pure it has a sp. gr. of 10-5, and is very malleable and ductile ; melts at a bright red heat. Silver does not oxidize in the air, but when exposed to an impure atmo- sphere containing traces of Sulphiiretted Hydrogen, it is slowly tarnished from formation of Sulphuret of Silver. It dissolves in Sulphuric Acid, but the best solvent is I^itric Acid. The standard coin of the realm is an alloy of Silver and Copper, containing about one-eleventh of the latter metal. It may be converted into Mtrate of Silver, sufficiently pure for Photographic purposes, by dissolving it in Nitric Acid and evaporating the solution to the crystaUizing point : or, if the quantity be small, the solution may be boiled PHOTOGEAPHIC CHEMICALS. 363 down to complete dryness, and tlie residue fused strongly; which decomposes the Nitrate of Copper, but leaves the greater portion of the Silver salt unaffected. (N.B. Nitrate of Silver which has undergone fusion contains Nitrite of Silver, and will require the addition of Acetic Acid if used for preparing the Collodion sensitive film : see page 99.) SILVER, AMMONIO-NITHATE OF. Crystallized Nitrate of Silver absorbs Ammoniacal gas rapidly, with production of heat suflBcient to fuse the re- sulting compound, which is white, and consists of 100 parts of the Nitrate + 29*5 of Ammonia. The compound how- ever which Photographers employ under the name of Ammonio-Nitrate of Silver, may be viewed more simply as a solution of the Oxide of Silver in Ammonia, without reference to the Nitrate of Ammonia necessarily produced in the reaction. Very strong Ammonia in acting upon Oxide of Silver converts it into a black powder, termed fulminating Silver, which possesses the most dangerous explosive properties. Its composition is uncertain. In preparing Ammonio- Nitrate of Silver by the common process, the Oxide first precipitated occasionally leaves a little black powder be- hind, on re-solution ; this does not appear however, ac- cording to the observations of the Author, to be fulmi- nating Silver. In sensitizing salted paper by the Ammonio-Nitrate of Silver, ^ee Ammonia is necessarily formed. Thus — Chloride of Ammonium + Oxide of Silver in Ammonia = Chloride of Silver + Ammonia + Water. SILVEE, OXIDE OF. Symbol, AgO. Atomic weight, 116. This compound has already been described in Part I. page 17. 364 VOCABULAEY OF SILVER, CHLOEIDE OP. Symbol, AgCl. Atomic weight, 144. The preparation and properties of Chloride of Silver are given in Part I. page 14. SILYEE, BEOMIDE OF. Symbol, AgBr. Atomic weight, 186. See Part I. page 17. SILVEE, CITEATE OF. See Citbic Acid. SILVEE, IODIDE OF. Symbol, Agl. Atomic weight, 234. See Part I. page 16. SILVEE, FLUOEIDE OF. Symbol, AgF. Atomic weight, 127. This compound differs from those just described in being soluble in water. The dry salt fuses on being heated, and IS reduced by a higher temperature, or by exposure to Hght. SILVEE, SULPHUEET OF. Symbol, AgS. Atomic weight, 124. This compound is formed by the action of Sulphur upon metallic Silver, or of Sulphuretted Hydrogen, or Hydro- sulphate of Ammonia, upon the Silver salts ; the decom- position of Hyposulphite of Silver also furnishes the black Sulphuret. Sulphuret of SUver is insoluble in water, and nearly so in those substances which dissolve the Chloride, Bromide, and Iodide, such as Ammonia, Hyposulphites, Cyanides, PHOTOGEAPHIC CHEMICALS. 365 etc. ; but it dissolves in Nitric Acid, being converted into soluble Sulphate and Nitrate of Silver. (For a further account of the properties of the Sulphuret of Silver, see page 150.) SILVEE, NITEATE OF. Symbol, AgO NO5. Atomic weight, 170. The preparation and properties of this salt have been explained at page 12, part I. SILVEE, NITEITE OF. Sjrmbol, AgO NO3. Atomic weight, 154. Nitrite of Silver is a compound of Nitrous Acid, or N O3, with Oxide of Silver. It is formed by heating Nitrate of Silver, so as to drive off a portion of its Oxygen, or more conveniently, by mixing Nitrate of Silver and Nitrate of Potash in equal parts, fusing strongly, and dissolving in a small quantity of boiling water; on cooling, the Nitrite crystaUizes out, and may be purified by preasiug in blotting- paper. Mr. Hadow describes an economical method of preparing Nitrite of Silver in quantity, viz. by heating 1 part of Starch in 8 of Nitric Acid of 1'25 specific gravity, and conducting the evolved gases into a solution of pure Carbonate of Soda untU efiervescence has ceased. The Nitrite of Soda thus formed is afterwards added to Nitrate of Silver in the usual way. Properties. — Nitrite of SUver is soluble in 120 parts of cold water ; easily soluble in boiling water, and cry stallizes, on cooling, in long slender needles. It has a certain degree of affinity for Oxygen, and tends to pass into the condition of Nitrate of Silver ; but it is probable that its Photogra- phic properties depend more upon a decomposition of the salt and liberation of Nitrous Acid. Properties of Niti^ous Acid. — This substance possesses 366 VOCABULAET OF very feeble acid properties, its salts being decomposed even by Acetic Acid. It is an unstable body, and splits up, in contact witb water, into Binoxide of Nitrogen and Nitric Acid. The Peroxide of Nitrogen, NO^, is also decomposed by water and yields tlie same products. SILVEE, ACETATE OF. Symbol, AgO (C4H3O3). Atomic weight, 167. This is a diflficultly soluble salt, deposited in lamellar crystals when an Acetate is added to a strong solution of Nitrate of Silver. If Acetic Acid be used in place of an Acetate, the Acetate of SUver does not fall so readily, since the Nitric Acid which would then be liberated im- pedes the decomposition. Its properties have been suffi- ciently described at pages 82 and 287. SILVEE, HYPOSULPHITE OF. Symbol, AgO SgOj. Atomic weight, 164. This salt is fuUy described in Part I. page 128. For the properties of the soluble double salt of Hyposulphite of Silver and Hyposiilphite of Soda, see page 43. SUOAE OF MILK. See Milk. SULPHUEETTED HYDEOGEN. See Hydeosul- PHijEic Acid. SULPHUEIC ACID. Symbol, SO3. Atomic weight, 40. Sulphuric Acid may be formed by oxidizing Sulphur with boiling Nitric Acid ; but this plan would be too ex- pensive to be adopted on a large scale. The commercial process for the manufacture of Sulphuric Acid is exceed- PHOTOGEAPHIC CHEMICALS. 367 ingly ingenious and beautiful, but it iavolves reactions wliich. are too complicated to admit of a superficial explana- tion. The Sulphur is first burnt into gaseous Sulphurous Acid (SO2), and then by the agency of Biuoxide of Nitro- gen gas, an additional atom of Oxygen is imparted from the atmosphere, so as to convert the SOj into SO3, or Sul- phuric Acid. Froperties. — ^Anhydrous Sulphuric Acid is a white crys- taUine sohd. The strongest liquid acid always contains one atom of water, which is closely associated with it, and cannot be driven off" by the application of heat. This mono-hydrated Sulphuric Acid, represented by the formula HO SO3, is a dense fluid, having a specific gravity of about 1'845 ; boUs at 620°, and distils without decom- position. It is not volatile at common temperatures, and therefore does not fiome in the same manner as Nitric or Hydrochloric Acid. The concentrated acid may be cooled down even to zero without solidifying ; but a weaker com- pound, containing twice the quantity of water, and termed glacial Sulphuric Acid, crystallizes at 40° Fahr. Sulphuric Acid is intensely acid and caustic, but it does not destroy the skin or dissolve metals so readily as Nitric Acid. It has an energetic attraction for water, and when the two are mixed, condensation ensues, and much heat is evolved ; four parts of acid and one of water produce a temperature equal to that of boiling water. Mixed with aqueous Nitric Acid, it forms the compound known as Nitro- Sulphuric Acid. Sulphuric Acid possesses intense chemical powers, and displaces the greater number of ordinary acids from their salts. It chars organic substances, by removing the ele- ments of water, and converts Alcohol into Ether in a similar manner. The strength of a given sample of Sul- phuric Acid may generally be calculated from its specific gravity, and a Table is given by Dr. Ure for that purpose. (See Appendix.) Impurities of Commercial Sulphuric Acid. — The hquid 368 VOCABTJLAET OF acid sold as Oil of Vitriol is tolerably constant in compo- sition, and seems to be as well adapted for Photograpliic use as tlie pure Sulphuric Acid, which is far more ex- pensive. The specific gravity should be about 1'836 at 60°. Tf a drop, evaporated upon Platinum foil, gives a fixed residue, probably Bisulphate of Potash is present. A milkiness, on dilution, indicates Sulphate of Lead. (See page 194.) Test for Sulplmric Acid. — If the presence of Sulphuric Acid, or a soluble Sulphate, be suspected in any liquid, it is tested for by adding a few drops of dilute solution of Chloride of Barium, or Nitrate of Baryta. A white pre- cipitate, insoluble in Nitric Acid, indicates Sulphuric Acid. If the liquid to be tested is very acid, from Nitric or Hydrochloric Acid, it must be largely diluted before test- ing, or a crystalline precipitate will form, caused by the sparing solubihty of the Chloride of Barium itself in acid solutions. SULPHUEOUS ACID. Symbol, SOj. Atomic weight, 32. This is a gaseous compound, formed by burning Sulphur in atmospheric air or Oxygen gas ; also by heating Oil of Vitriol iri contact with metaUic Copper, or with Charcoal. Wlien an acid of any kind is added to Hyposulphite of Soda, Sulphurous Acid is formed as a product of the de- composition of Hyposulphurous Acid, but it afterwards disappears from the liquid by a secondary reaction, result- ing in the production of Trithionate and Tetrathionate of Soda. Properties. — Sulphurous Acid possesses a peculiar and suffocating odour, familiar to all in the fumes of burning Sulphur. It is a feeble acid, and escapes with effervescence, like Carbonic Acid, when its salts are treated with Oil of Vitriol. It is soluble in water. PHOTOGEAPHIC CHEMICALS. 369 TETEATHIONIC ACID. Symbol, S4O5. Atomic weight, 104. The chemistry of the Polythionic Acids and their salte will be found described in the first part of this Work, page 162. WATEE. Symbol, HO. Atomic weight, 9. Water is an Oxide of Hydrogen, containing single atoms of each of the gases. Distilled water is water which has been vaporized and again condensed ; by this means it is freed from earthy and saline impurities, which, not being volatile, are left in the body of the retort. Pure distilled water leaves no re- sidue on evaporation, and should remain perfectly clear on the addition of Nitrate of Silver, even lohen exposed to the light ; it should also be neutral to test-paper. The condensed water of steam-boilers sold as distilled water is apt to be contaminated with oily and empyreumatic matter, which discolours Nitrate of Silver, and is therefore injurious. P,ain-water, having undergone a natural process of dis- tillation, is free from inorganic salts, but it usually contains a minute portion of Ammonia, which gives it an alkaline reaction to test-paper. It is very good for Photographic purposes if collected in clean vessels, but when taken from a common rain-water tank should always be examined, and if much organic matter be present, tinging it of a brown colour and imparting an unpleasant smell, it must be rejected. Spring or River water, commonly known as " hard water," usually contains Sulphate of Lime, and Carbonate of Lime dissolved in Carbonic Acid: also Chloride of So- dium in greater or less quantity. On boiling the water, 2b 370 PHOTOGHAPHIC CHEMICALS. the Carbonic Acid gas is evolved, and the greater part of the Carbonate of Lime (if any is present) deposits, form- ing an earthy incrustation on the boiler. In testing water for Sulphates and Chloride's, acidify a portion with a few drops of pure Nitric Acid, free from Chlorine (if this is not at hand, use pure Acetic Acid) ; then divide it into two parts, and add to the first a dilute solution of Chloride of Barium, and to the second Nitrate of Silver, — a milkiness indicates the presence of Sulphates in the first case or of Chlorides in the second. The Photo- graphic Nitrate Bath cannot be used as a test, since the Iodide of Silver it contains is precipitated on dilution, giving a milkiness which might be mistaken for Chloride of Sdver. Common hard water can often be used for making a Ni- trate Bath when nothing better is at hand. The Chlorides it contains are precipitated by the Nitrate of Silver, leav- ing soluble Nitrates in solution, which are not injurious. The Carbonate of Lime, if any is present, neutralizes free Nitric Acid, rendering the Bath alkaline in the same manner as Carbonate of Soda. (See page 82.) Sulphate of Lime, usually present in well water, is said to exercise a retarding action upon the sensitive Silver Salts, but on this point the writer is unable to give certain information. Hard water is not often sufficiently pure for the develop- ing fluids. The Chloride of Sodium it contains decomposes the Nitrate of Silver upon the film, and the image cannot be brought out perfectly. The New River water, how- ever, supplied to many parts of London, is almost free from Chlorides, and answers very well. In other cases a few drops of Nitrate of Silver solution may be added to sepa- rate the Chlorine, taking care not to use a large excess. 371 APPENDIX. QUANTITATIVE TESTING OP SOLTTTIONS OF KITEATE OF SILVEE. The amount of Nitrate of Silver contained in solutions of that salt may be estimated with sufficient delicacy for ordinary Photographic operations by the following simple process. Take thepure crystallized Chloride of Sodium, and either dry it strongly or fuse it to a moderate heat, in order to drive off any water which may be retained between the interstices of the crystals ; then dissolve in distilled water, in the proportion of 8^ grains to 6 fluid ounces. In this way, a standard solution of salt is formed, each drachm of which (containing slightly more than one-sixth of a grain of salt) will precipitate exactly half a grain of Nitrate of Silver. In order to use it, measure out exactly one drachm of the Bath in a minim measure and place it in a two-ounce stoppered phial, taking care to rinse out the measure with a drachm of distilled water, which is to be added to the former ; then pour in the salt solution, in the proportion of a drachm for every 4 grains of Nitrate known to be present in an ounce of the Bath which is to be tested ; shake the con- tents of the bottle briskly, until the white curds are perfectly sepa- rated, and the supernatant liquid is clear and colourless ; then add fresh portions of the standard solution, by 30 minims at a time, with constant shaking. When the last addition causes no milkiness, read off the total number of drachms employed (the last half-drachm being 372 APPENDIX. subtracted), and multiply that number by 4 for the weight in grains of the Nitrate of Silver present in an ounce of the Bath. In this manner the strength of the Bath is indicated within two grains to the ounce, or even to a single grain if the last additions of standard salt-solution be made in portions of 15, instead of 30 minims. Supposing the Bath to be tested is thought to contain about 35 grains of Nitrate to the ounce, it will be convenient to begin by add- ing to the measured drachm, 7 drachms of the standard solution ; afterwards, as the milkiness and precipitation become less marked, the process must be carried on more cautiously, and the bottle shaken violently for several minutes, in order to obtain a clear solution. A few drops of Nitric Acid added to the Nitrate of Silver facilitate the deposition of the Chloride ; but care must be taken that the sample of Nitric Acid employed is pure and free from Chlorine, the presence of which would cause an error. EECOVEET OF SILTEE FEOM WASTE SOLUTIONS, — PEOM THE BLACK DEPOSIT OF HYPO-BATHS, ETC. The manner of separating metallic Silver from waste solutions varies according to the presence or absence of alkaline Hyposulphites and Cyanides. a. Separation of metallic Silver from old Nitrate Baths. — The Silver contained in solutions of the Nitrate, Acetate, etc., may easily be precipitated by suspending a strip of sheet Copper in the liquid ; the action is completed in two or three days, the whole of the Nitric Acid and Oxygen passing to the Copper, and forming a blue solution of the Nitrate of Copper. The metallic Silver however separated in this manner, always contains a portion of Copper, and gives a blue solution when dissolved in Nitric Acid. A better process is to commence by precipitating the Silver en- tirely in the form of Chloride of Silver, by adding common Salt until no further milkiness can be produced. If the liquid is well stirred, the Chloride of Silver sinks to the bottom, and may be washed by repeatedly filling the vessel with common water, and pour- ing off the upper clear portion when the clots have again settled down. The Chloride of Silver thus formed may afterwards be reduced to me- tallic Silver by a process which will presently be described (p. 374). APPENDIX. 373 h. Separation of Silver from solutions containing alkaline Hijpo- sulphites, Cyanides, or Iodides. — In this case the Silver cannot be precipitated by adding Chloride of Sodium, since the Chloride of Sil- ver is soluble in such liquids. It is necessary therefore to use the Sulphuretted Hydrogen, or the Hydrosulphate of Ammonia, and to separate the Silver in the form of Sulphuret. Sulphuretted Hydrogen gas is readily prepared, by fitting a cork and flexible tubing to the neck of a pint bottle, and having introduced Sulphuret of Iron (sold by operative chemists for the purpose), about as much as will stand in the palm of the hand, pouring upon it 1^ fluid ounce of Oil of Vitriol diluted with 10 ounces of water. The gas is generated gradually without the application of heat, and must be allowed to bubble up through the liquid from which the Silver is to be separated. The smell of Sulphuretted Hydrogen being offen- sive, and highly poisonous if inhaled in a concentrated form, the operation must be carried on in the open air, or in a place where the fumes may escape without doing injury. When the liquid begins to acquire a strong and persistent odour of Sulphuretted Hydrogen, the precipitation of Sulphuret is completed. The black mass must then be collected upon a filter, and washed by pouring water over it, until the liquid which runs through gives little or no precipitate with a drop of Nitrate of Silver. The Silver may also be separated in the form of Sulphuret from old Hypo-baths, by adding OU of Vitriol in quantity sufficient to decom- pose the Hyposulphite of Soda; and burning off the free Sulphur from the brown deposit. Conversion of Sulphuret of Silver into metallic Silver. — The black Sulphuret of Silver may be reduced to the state of metal by roasting and subsequent fusion with Carbonate of Soda ; but it is more con- venient, in operating on a small scale, to proceed in the following manner: — first convert the Sulphuret into Nitrate of Silver, by boil- ing with Nitric Acid diluted with two parts of water ; when all evo- lution of red fumes has ceased, the liquid may be diluted, allowed to cool, and filtered from the insoluble portion, which consists princi- pally of Sulphur, but also contains a mixture of Cldoride and Sulphu- ret of Silver, unless the Nitric Acid employed was free from Chlo- rine ; this precipitate may be heated in order to volatilize the Sul- phur, and then digested with Hyposulphite of Soda, or added to the Hypo-Bath. 374 APPENDIX. The solution of Nitrate of Silver obtained by dissolving Sulphuret of Silver, is always strongly acid witb Nitric Acid, and also contains Sulphate of Silver. It may be crystallized by evaporation ; but un- less the quantity of material operated on is large, it will be better to precipitate the Silver in the form of Chloride, by adding common Salt, as already recommended. EEDTJCTION OF CHLOEIDE OP SILVEE TO THE METALLIC STATE. The Chloride of Silver is first to be carefully washed, by filling up the vessel which contains it, many times with water, and pouring off the liquid, or drawing it oflP close with a siphon. It may then be dried at a gentle heat, and fused with twice its weight of dry Carbo- nate of Potash, or, better still, with a mixture of the Carbonates of Potash and Soda. The process for reducing Chloride of Silver in the moist way, by metallic Zinc and Sulphuric Acid, is more economical and less troublesome than that just given ; it is conducted as follows : — The Chloride, after having been well washed as before, is placed in a large flat dish, and a bar of metallic Zinc laid in contact with it. A small quantity of Oil of Vitriol, diluted with fom- parts of water, is then added, until a slight effervescence of Hydrogen gas is seen to take place. The vessel is set aside for two or three days, and is not to be disturbed, either by stirring or by moving the bar. The reduc- tion begins with the Chloride immediately in contact with the Zinc, and radiates in all directions. When the whole mass has become of a grey colour, the bar is to be carefully removed and the adhering Silver washed off with a stream of water ; the Zinc usually presents a honeycombed appearance, with irregularities upon the surface, which however are not metallic Silver; — they consist only of Zinc or of Oxide of Zinc. In order to ensure the purity of the Silver, a fresh addition of Sul- phuric Acid must be made, after the Zinc bar has been removed, and the digestion continued for several hours, in order to dissolve any fragments of metallic Zinc which may have been inadvertently de- tached. The grey powder must be repeatedly washed, first with Sul- phuric Acid and water (this is necessary to dissolve a portion of an in- soluble Salt of Zinc, probably an oxychloride) and then with water alone, until the liquid runs away neutral, and gives no precipitate APPENDIX. 375 with Carbonate of Soda ; it may then be converted into Nitrate of Silver by boiling with Nitric Acid diluted with two parts of water. Mr. PoUock has observed that in reducing Chloride of Silver pre- cipitated from old Nitrate Baths containing Iodide of Silver, the grey metallic powder is invariably contaminated with unreduced Io- dide of Silver, which afterwards dissolves in the solution of Nitrate of Silver formed on treating the mass with Nitric Acid. To avoid this, wash the purified Silver with solution of Hyposulphite of Soda, and then again with water. MODE OE TAKING THE SPECIFIC GEAVITY OF LIQUIDS. Instruments are sold, termed " Hydrometers," which indicate spe- cific gravity by the extent to which a glass bulb containing air, and properly balanced, rises or sinks in the liquid ; but a more exact process, and one equally simple, is by the use of the specific gravity bottle. These bottles are made to contain exactly 1000 grains of distUled water, and with each is sold a brass weight, which counterbalances it when filled with pure water. In taking the specific gravity of a liquid, fill the bottle quite fuU and insert the stopper, which being pierced through by a fine capil- lary tube allows the excess to escape. Then having wiped the bottle quite dry, place it in the scale-pan, and ascertain the number of grains required to produce equilibrium ; this number added to, or subtracted from, unity (the assumed specific gravity of water), will give the den- sity of the liquid. Thus, to take examples, supposing the bottle filled with rectified Ether to require 250 grains to enable it to counterbalance the brass weight, — then 1' minus "250, or "750, is the specific gravity ; but in the case of Oil of Vitriol the bottle when full wiU be heavier than the counterpoise by, perhaps, 836 grains ; therefore 1" plus "836, id est 1'836, is the density of the sample examined. Sometimes the bottle is made to hold only 500 grains of distilled water in place of 1000 ; in this case the number of grains to be added or subtracted must be multiplied by 2. In taking specific gravities observe that the temperature be within a few degrees of 60 ' Fahrenheit (if higher or lower, immerse the bottle in warm or cold water) ; and wash out the bottle thoroughly with water each time after use. 376 APPENDIX. ON FILTEATION AND WASHING PEECIPITATES. In preparing filters, cut the paper into squares of a sufficient size, and fold each square neatly upon itself, first into a half-square, and then again, at right angles, into a quarter-square; — round off the corners with a pair of scissors, and open out the filter into a conical form, when it will he found to drop exactly into the funnel and to be uniformly supported throughout. Before pouring in the liquid, always moisten the filter with dis- tilled water, in order to expand the fibres; if this precaution be neglected, the pores are apt to become choked in filtering liquids which contain finely divided matter in suspension. The solution to be filtered may be poured gently down a glass rod, held in the left hand {a silver spoon mny be used, in case of necessity, for Nitrate Baths, and all liquids not containing Nitric or Hydrochloric Acid), and directed against the side of the funnel, near to the upper part. If it does not immediately run clear, it will usually do so on return- ing it into the filter and allowing it to pass through a second time. Mode of Washing Precipitates—Collect the precipitate upon a filter and drain off as much of the mother-liquor as possible ; then pour in distilled water by small portions at a time, allowing each to percolate through the deposit before adding a fresh quantity^ When the water passes through perfectly pure, the washing is complete ; in testing it, a single drop may be laid upon a strip of glass and al- lowed to evaporate spontaneously in a warm place, or the proper chemical reagents may be applied, and the washing continued until no impurity can be detected. Thus, for example, in washing the Sulphuret of Silver precipitated from a Hypo-Bath by means of Hydrosulphate of Ammonia, the process will be completed when the water which runs through causes no deposit with a drop of Nitrate of Silver solution. ON THE USE OF TEST-PAPEES. The nature of the colouring matter which is employed in the preparation of Litmus-paper has already been described at page 354. In testing for the alkalies and basic oxides generally, the blue litmus-paper which has been reddened by an acid may be used, or, in place of it, the turmeric paper. Turmeric is a yellow vegetable sub- stance which possesses the property of becoming brown when treated APPENDIX. 377 with an alkali ; it is however less sensitive than the reddened litmus, and is scarcely affected by the weaker bases, such as Oxide of Silver. In using test-papers observe the following precautions : — they should be kept in a dark place, and protected from the action of the air, or they soon become purple from Carbonic Acid, always present in the atmosphere in small quantity. By immersion in water con- taining about one drop of Liquor Potassse, or Ammonise, or a grain of Carbonate of Soda, to four ounces, the blue colour is restored. As the quantities which are tested for in Photography are often infinite- simally small, it is essential that the litmus-paper should be in good condition ; and test-papers prepared with porous paper will be found to show the colour better than those upon glazed or strongly-sized paper. The mode of employing the paper is as follows : — Place a small strip in the liquid to be examined : if it becomes at once bright red, a strong acid is present ; but if it changes slowly to a wine-red tint, a weak acid, such as Acetic or Carbonic, is indicated. In the case of the Photographic Nitrate Bath faintly acidified with Acetic Acid, a purple colour only may be expected, and a decided red colour would suggest the presence of Nitric Acid. In the Hypo fixing and toning Bath which has acquired acidity, the litmus-paper will perhaps redden in about three or four minutes. Blue litmus-papers may be changed to the red papers used for alkalies by soaking in water acidified with Sulphuric Acid, one drop to half a pint ; or by holding for an instant near the mouth of a bottle containing Glacial Acetic Acid. In examining a Nitrate Bath for alkalinity, by means of the reddened litmus-paper, at least five or ten minutes should be allowed for the action, since the change of colour from red to blue takes place very slowly. REMOVAL OE SILVEE STAINS EEOM THE HANDS, LINEN, ETC. The black stains upon the hands caused by Nitrate of Silver, may readily be removed by moistening them and rubbing with a lump of Cyanide of Potassium. As this salt however is highly poisonous, many may prefer the following plan : — Wet the spot with a saturated solution of Iodide of Potassium, and afterwards with Nitric Acid (the strong Nitric Acid acts upon the skin and turns it yellow, it must therefore be diluted with two parts of water before use) ; then wash with solution of Hyposulphite of Soda. Stains upon white linen may be easily removed by brushing them 378 APPENDIX. with a solution of Iodine in Iodide of Potassium, and afterwards washing with water and soaking in Hyposulphite of Soda, or Cyanide of Potassium, until the yellow Iodide of Silver is dissolved out ; the Bichloride of Mercury (neutral solution) also answers well in many cases, changing the dark spot to white (p. 156). A TABLE SHOWING THE QUANTITY OP ANHYDEOFS ACID IN DILUTE StTLPHUEIC ACID OF DIFFEEENT SPECIFIC GEAVITIES. (UEE.) Specific Gravity. Real Acid in 100 parts of the Liquid. Specific Gravity. Eeal Acid in 100 parts of the Liquid. Specific Gravity. Real Acid in 100 parts of the Liquid. 1-8485 81-54 1-8115 73-39 1-7120 65-23 1-8475 80-72 1-8043 72-57 1-6993 64-42 1-8460 79-90 1-7962 71-75 1-6870 63-60 1-8439 79-09 1-7870 70-94 1-6750 62-78 1-8410 78-28 1-7774 70-12 1-6630 61-97 1-8376 77'46 1-7673 69-31 1-6520 61-15 1-8336 76-65 1-7570 68-49 1-6415 60-34 1-8290 75-83 1-7465 67-68 1-6321 59-52 1-8233 75-02 1-7360 66-86 1-6204 58-71 1-8179 74-20 1-7245 66-05 1-6090 57-89 A TABLE SHOWING THE QUANTITY OF ANHYDEOUS ACID IN THE LIQUID NITEIC ACID OF DIFFEEENT SPECIFIC GEAVITIES. (UEE.) Specific Gravity. Real Acid in 100 parts of the Liquid. Specific Gravity. Real Acid in 100 parts of the Liquid. Specific Gravity. Real Acid I in 100 ' parts of the! Liquid. 1-5000 79-700 1-4640 69-339 1-4147 58-978 1-4980 78-903 1-4600 68-542 1-4107 58-181 1-4960 78-106 1-4570 67-745 1-4065 57-384 1-4940 77-309 1-4530 66-948 1-4023 56-587 1-4910 76-512 1-4500 66-155 1-3978 55-790 1-4880 75-715 1-4460 65-354 1-3945 54-993 1-4850 74-918 1-4424 64-557 1-3882 54-196 1-4820 74-121 1-4385 63-760 1-3833 53-399 1-4790 73-324 1-4346 62-963 1-3783 52-602 1-4760 72-527 1-4306 62-166 1-3732 51-805 1-4730 71-730 1-4269 61-369 1-3681 51-068 1-4700 70-933 1 1-4228 60-572 1-3630 50-211 1-4670 70-136 1 1-4189 59-775 1-3579 49-414 APPENDIX. 379 WEIGHTS AND MEASURES. Troy or Apothecaries' Weight. 1 Pound = 12 Ounces. 1 Ounce = 8 Drachms. 1 Drachm = 3 Scruples. 1 Scruple = 20 Grains. (1 Ounce Troy = 480 Grains, or 1 Ounce Avoirdupois flus 42" 5 grains.) Avoirdupois Weight. 1 Pound = 16 Ounces. 1 Ounce = 16 Drachms. 1 Drachm = 27'343 grains. (1 Ounce Avoirdupois = 437'5 grains.) (1 Pound Avoirdupois = 7000 Grains, or 1 Pound Troy plus %\ Troy Ounces flus 40 grains.) Imperial Measure. 1 Gallon = 8 Pints. 1 Pint = 20 Ounces. 1 Ounce = 8 Drachms. 1 Drachm = 60 Minims. (A wine Pint of water measures 16 Ounces, and weighs a Pound.) An Imperial Gallon of water weighs 10 Pounds Avoirdupois, or 7 0,000 Grains. An Imperial Piut of water weighs 1 j Pound Avoir- dupois. A fluid Ounce of water weighs 1 Ounce Avoirdupois, or 437"5 Grains. A Drachm of water weighs 54"7 Grains. French Measures of Weight. 1 Kilogramme = 1000 Grammes = something less than 2i Pounds Avoirdupois. 1 Gramme = 10 Decigrammes = 100 Centigrammes - 1000 Milligrammes = 15"433 English Grains. A Gramme of water measures 17 English Minims, nearly, 1000 Grammes of water measure 35^ English fluid Ounces. French Measures of Volume. 1 Litre = 10 Decilitres = 100 Centilitres = 1000 Millilitres = 35 i English fluid Ounces. 1 Litre = 1 Cubic Decimetre = 1000 Cubic Centimetres. 1 Cubic Centimetre = 17 English Minims. A Litre of water weighs a Kilogramme, or something less than 2i Pounds Avoirdu^jois. A Cubic Centimetre of water weighs a Gramme. 381 INDEX. Aberration, chromatic, 53 ; sphe- rical, 55. Accelerating agents, their mode of action explained, 93 ; Iodide of ■ Iron contraindicated for glass Positives, 106; Nitrite of Sil- ver an accelerator for Nega- tives, 113. Acetate of Silver, its preparation and formula, 366 ; its formation in Nitrate Bath explained, 82; ensures absence of free Nitric Acid, 112; tends slightly to fa- vour fogging and spots, 99; useful in increasing the intensity of landscapes, 287 ; useful for engravings, 239; contraindicated for glass Positives, 107 ; mode of removing it by Nitric Acid, 287. Acetic Acid, properties and mode of testing purity of, 329 ; use- ful in preventing fogging, 100 ; in rendering the development slow and even, 96 ; in the Honey preservative process, 289 ; does not coagulate Albumen, 331 ; a good commercial form of acid, 220 ; Acetic Acid essential in Calotype, waxed paper, and Al- bumen processes, 184 ; also in printing paper Positives by de- velopment, 260, 262. Aceto-Nitrate of Silver term ex- plained, 183. Achromatic Lenses, their construc- tion explained, 54 ; the visual and chemical foci often coinci- dent in, 59. Acids, nature of, 310. Actinism explained, 60 ; impor- tance of distinguishing Actinic from visual rays, 62 ; mode of finding Actinic focus, 237. Affinity, chemical, 314. Albumen, its chemistry, 330 ; forms a compound with Silver, 21 ; used in Positive printing to pro- duce a fine surface layer, 121 ; to increase sensitiveness, 124 ; affects the colour of the prints, 126 ; protects the image from oxidation, 155 ; putrifies when exposed to moisture, 160 ; dis- colours the Nitrate Bath, 245. Albuminized paper, formula for, 242 ; slowin fixing, 130 ; not well adapted for toning by Sel d'or, 268 ; good for stereoscopic sub- jects and small portraits, 249 ; cannot be sensitized with Am- monio-Nitrate of Silver, 246. Albumen Negative process, its in- vention, 10; theory of, 187. Alcohol, its chemistry, 332; some- * The preparation and properties of the Photographic Chemicals of minor importance will be found in the Alphabetical List commencing at page 329. 382 iKD timrs too weak for maldng Col- lodion, 77 ; mode of rectifying, 205 ; must not contain impuri- tieSj 94; effects of adding to Col- lodion, 76 ; to developep, 214. Alkalies, nature of, 270. Alkalinity of Nitrate Bath ex- plained, 81 ; the evils it pro- duces, 100 ; how to test for it, 376 ; how to remove it, 276. Amber varnish, 234. Ammonia, preparation and proper- ties, 333 ; theory of its use in fixing, 41 ; Mr. Shadbolt's for- mula for, 270 ; its action upon Chloride of Gold, 345 ; effect of concentrated Ammonia upon Oxide of Silver, 363. Ammonio-Nitrate of Silver, its chemistry, 363; used in Positive printing to increase sensitive- ness, 123 ; to give black tones, 126 ; cannot be used with Al- bumen, 246 ; increases perma- nency of print, 175 ; old Ni- trate Baths not easily convertible into Ammonio-Nitrate, 247 ; mode of preparing, 247 ; best applied to the paper by brush or rod, 248 ; Oxide of Silver in Nitrate of Ammonia, a useful substitute for it, 249. Ammonio-Nitrate paper, formula for, 246 ; a more simple for- mula, but less sensitive than the last, 257. Atomic theory explained, 325. Bath for fixing and toningPositives. See Fixing and toning Bath. Bichloride of Mercury, whitening action on glass Positives ex- plained, 109 ; solution for, 216 ; used to intensify Negatives, 117 ; by Dr. Diamond in copying old prints, 239 ; bleaches paper EX. prints, 156 ; should not be added to paste used in mounting prints, 171 ; removes Silver stains, 377. Blackening Negatives, 113, 116, 2m, 239. Black tones, mode of obtaining, in paper Positives, 175. Bromide of Silver, its preparation and properties, 17 ; its superior sensibility to coloured light, 63 ; less acted on by direct light than Chloride, 19 ; less sensitive to invisible image than Iodide, 25 ; proposed employment in Collo- dion, 111 ; found useful inCalo- type, 184 ; in Photographing by artificial light, 65 ; diagram of chemical spectrum on, 63. Bromo-Iodide of Silver, 179. Brushes, mode of applying Silver solutions by, 248. Calotype process, theory of, 182. Camera, its first invention, 7 ; theory of its construction, 53 ; mode of testing accuracy of, 237 ; cause of the image being in- verted, 52 ; the term "flatness of field" exj)lained, 53 ; best position of the Camera for por- traits, 236 ; for architectural subjects, 238 ; a funnel-shaped tube placed in front of the lens, 236 ; stereoscopic Camera, 299 ; microscope Camera, 302. Causes of failure in Collodion pro- cess, 275. Chemical affinity, illustrations of, 314. Chemical elements, 308. Chemical focus, directions for find- ing, 237 ; shorter than visual in non-achromatic lenses, 59 ; longer than visual in microscopic objectives, 302 ; varies slightly with the nature of the light, 303. INDEX. 383 Chemical spectrum, 60. Chemicals, Photographic, Vocabu- lary of, 329. Chloride of Silver, its preparation and properties, 14 ; more sensi- tive to direct light than Bromide or Iodide, 19 ; less sensitive to invisible image, 24 ; its black- ening by light explained, 20, 140 ; accelerated by excess of Nitrate, 19 ; by organic matter, 20, 142 ; experiments illustra- ting darkening of papers pre- pared with, 21 ; simple expla- nation of the mode of preparing sensitive papers with, 21 ; agents which dissolve it, 41 ; mode of reducing it to metallic state, 374. Chloride of Gold, its preparation and properties, 344 ; action of Ammonia upon it, 345 ; use of an alkaline solution of for to- ning, 270 ; compounds formed on adding it to Hyposulphite of Soda, 131 ; mode of preparing the fixing and toning Bath with, 249; the Sel d'or bath with, 266. Chromatic aberration, 53. Citric Acid, forms a red compound with SuboxideofSilver,141, 340 ; used in printing to give purple tones, 126 ; formula for prepa- ring paper with, 246. Cleaning glass plates, theory of, 38 ; details of, 222. Collodion, its discovery, 10 ; che- mistry of Pyroxy line, 67; physical effect of too much Ether or Alco- hol in, 76; of water in, 77; gluti- nosity of, 75 ; use of Chloroform in, 281 ; coloration of iodized, ex- plained, 78 ; details of manufac- ture of Collodion, 193 ; Positive Collodion, theory of, 106, formula for, 210; Negative Collodion, theory of. 111 ; formula for, 216 ; Collodion for copying en- gravings, 239 ; for landscapes, 287 ; for keeping processes, 292; for hot climates, 77, 218; for working by artificial light, 804 ; to remove the brown colour from Collodion, 218. Collodion film, varieties in the colour of, 84 ; the proper time for immersing it in the Bath, 85 ; a thin film often good for di- rect Positives, 106 ; a thicker film for Negatives, 111 ; cause of the film falling away from the glass, 75, 219; spots and mark- ings on, 278 ; conditions which affect its sensitiveness to light, 87 ; causes influencing its be- haviour with the developer, 94 ; mode of preserving sensitiveness of film, 287. Colours, their nature, explained, 46 ; their chemical action on sensitive film, 60 ; their photo- graphic action assisted by re- flection of white light, 65. Combination, laws of, 309. Conjugate foci, explained, 51, 271. Crookes, Mr., remarks upon che- mical spectrum, 65 ; upon waxed paper process, 185 ; preserva- tive process for Collodion films, 288. Curvature of luminous image form- ed by lens, explained, 56. Cyanide of Potassium, its fixing ac- tion explained, 43; preparation of solution of, 215 ; used to re- move stains, 377. Daguerreotype, its invention, 8 ; theory of the process, 177. Development of invisible image, explanation of, 36-39; second. 384 INDEX. or intensifying stage, explained, 113 ; details of developing glass Positives and Negatives, 229- 231 ; development of paper Po- sitives, 258 ; conditions which increase or diminish rapidity of development, 94. Developers, their preparation and properties, 27 ; comparative strength of, 95 ; theory of, for Positives, 107 ; for Negatives, 113 ; formula3 for Positive de- velopers, 213 ; for Negative, 220. Diagrams, mode of copying, 239. Diamond, Dr., his process for copy- ing old manuscripts, 239 ; uses Bromide of Silver in Collodion, 111 ; developes Positives with Nitrate of Iron, 108. Diaphragms for lenses. See Stops. Douhle decomposition, illustrated, 14 ; explained, 316. Elementary hodies, table of, 308 ; combination of, 309. Engravings, mode of copying, 239 ; often vield dark-coloured prints, 254. ' Equivalent proportions, 322. Ether, properties of, 341 ; purifi- cation offer Photography, 203 ; must be kept in a dark place, 205 ; should not be distilled from residues of old Collodion, 94. Experiments illustrating action of Light upon Chloride of Silver, 22 ; illustrating formation and development of invisible images, 25 ; illustrating photographic action of coloured light, Gl. Exposure in the Camera, rules for Positives, 105, 229, 230 ; for Negatives, 110, 229, 232 ; for landscajje views, 287 ; ibr pre- served Collodion plates, 290 ; for microscopic photographs, 802 ; effects of under and over- exposing, 35 ; exposure required in Calotype process, 184 ; in waxed paper, 186 ; in Albumen Negative process, 187. Fading of Positives, explained at length, 166; Author's researches on,"^148. Film, sensitive. See Collodion film. Filters, mode of cutting, 376. Fixing, theory of, 40 ; of paper prints explained, 127 ; solution for fixing glass Positives and Negatives, 221; manipulatory de- tails of fixing, 234 ; fixing paper Positives with Ammonia, 270. Fixing and toning Bath, its prepa- ration, 249 ; conditions which favour or retard its action, 133 ; certain states of the bath inju- rious to the proofs, 135 ; im- . portance of keeping it in an active condition, 159, 174 ; must not be employed imme- diately after mixing, 250 ; must not be allowed to become acid by constant use, 164, 169 j theory of the gradual change of properties it undergoes, 1 37. Foci, aclinic and luminous, 59 ; ac- tinic, mode of finding, 237 ; va- riation between them in micro- scopic objectives, 303. Focussing the object, 228. Fogging, theory of, 98, mode of. detecting causes of, 275. Formula! for solutions required in Collodion process, 210 ; for pa- pers used in Positive printing, 241 ; Avant of correspondence between, 257. Gallic Acid, its preparation and INDEX. 385 properties, 27 ; not adapted to develope Collodion plates, 95 ; used in paper processes, 184 ; becomes mouldy by keeping, 261; formula for developing paper Positives with, 260. Gallo-Nitrate of Silver, 183; dis- colours rapidly when developing dishes are not clean, 186. Gelatine, its properties, 343 ; used in Positive printing to form even surface layer, 124 ; as a cement to mount Photographs, 256. Glass plates, theory of cleaning, 38 ; details of cleaning, 222 ; mode of coating with Collodion, 224 ; with Albumen, 187. Glutinous Collodion, explained, 75. Glycerine, its properties, 343 ; used in preservative processes, 291. Gold, Chloride of. See Chloride of Gold. Gold salts, their use in Photogra- phic printing explained, 130 ; in the Daguerreotype process, 181. Gradation of tone, in Collodion Photographs, affected by the density of the film, 106, 111 ; by use of Iodide of Iron, 106 ; of fused Nitrate of Silver, 90 ; of impure Ether and Alcohol, 281 ; sometimes, improved by adding Chloroform to Collodion, 111.. Gradation of tone, in paper Posi- tives, conditions affecting it in prints obtained by direct expo- sure, 122 ; in Positives printed by development, 262. Hadow, Mr., researches on Collo- dion, 69 ; uses Nitrites, 113 ; observations on Iodide of Cad- mium, 209 ; formula for mak- ing Pyroxyhne, 195. Heliography, invented by M. Niepce, 7- Historical sketch of Photography, 6 . Honey -keeping process, 289. Hunt, Mr., introduces Protosalts of Iron in developing, 108. Hypo- Bath. See Fixing and To- ning Bath. Hyposulphite of Silver, its peculiar changes in colour, 128; the sweet compound which it forms with Hyposulphite of Soda, 43. Hyposulphite of Soda, preparation and properties, 42 ; theory of its fixing action, 43 ; blackens Ni- trate of Silver, 128 ; makes a milkiness with acids, 136 ; its decomposition by constant use in fixing, 137 ; the salts it forms with Chloride of Gold, 131 ; its conversion into sulphuretting Tetrathiouate by Iodine and Per- chloride of Iron, 138 ; test for presence of, 175. Iceland moss, its use in Positive printing, 124 ; formula for pre- paring paper with, 245. Imperfections in Collodion Nega- tives, 281 ; in Positives, 282 ; in paper Positives, 284. Intensity, explanation of term, 87 ; mode of increasing in Ne- gatives, 113, 116 ; effect of Ace- tate of Silver upon, 112 ; of Ni- trite of Silver upon, 118 ; mode of diminishing in glass Positives, 106, 107 ; conditions affecting intensity in paper Positives,! 22; indeveloped paper Positives, 262. Invisible images, theory of forma- tion of, 34 ; development of, 37 ; experiments illustrating, 25. lodate, how formed in Collodion film, 81; thought to produce in- sensitiveness, 207. 2 c 386 INDEX. odide of Ammonium, preparation of, 208 ; not fitted for iodizing Collodion required to be kept long, 218. Iodide of Iron, preparation of, 308 ; an accelerator to brown Collo- dion, 92 ; contraindicated for glass Positives, 106. Iodide of Potassium and Silver, properties of, 42; mode of iodizing Calotype papers by, 183. Iodide of Potassiimi, tests of purity of, 207 ; extent of solubility in Alcohol, 352 ; dissolves Iodide of Silver, 41. Iodide of Silver, its preparation and properties, 16 ; unaffected by di- rect action of light, 19 ; highly sensitive to invisible image, 24 ; hypothesis of formation of latent image on, 35 ; possibility of its reduction by PyrogaUic Acid shown, 33 ; excess of Nitrate of Silver essential to its blackening by developer, 95 ; diagrams of chemical spectrum, on, 60, 63 ; fixing agents for, 40; its solu- bility in the Nitrate Bath, 80 ; retards the action of Hypo fixing and toning Bath, 135 ; superior permanency of developed prints on, 174; details of Negative printing process, on, 362. Iodine, in Collodion, diminishes sensitiveness, 88 ; forms Nitric Acid and lodatein the Bath, 81; often useful in Positive Collo- dion, 107 ; in Negative Collo- dion, if fogging occurs, 99 ; mode of removing from Collo- dion, 218. Iodized Collodion. See Collodion. Kaolin, properties of, 337. Landscape Photography, 286 ; with preserved Collodion plates, 287. Latent image. See Invisible image. Laws of substitution explained, 70. Le Grey, M., his toning process with Chloride of Gold, 130 ; his waxed paper Negative process, 184. Lenses, various forms of, 50; foci of, 51; formation of images by, 52 ; use of stops, 57 ; portrait, 58 ; chemical foci of, 59 ; chro- matic aberration of, 53; spheri- cal aberration of, 55 ; simple di- rections for using lenses, 235 ; for finding chemical focus, 237. Light, its action upon Silver Salts, 19; experiments illustrating, 21; formation of invisible images by, 24 ; its alternating action upon Daguerreotype plate, 38 ; its compound nature, 46 ; pho- tographic action of coloured light, 60; refraction of light, 51. Llewellyn, Mr., his Oxymel pro- cess, 289. Manipulations of Collodion pro- cess, 222; of Photographic print- ing, 251 ; of toning by Sel d'or, 267. Manuscripts, mode of copying, 239. Markings on CoUodion Pictures, 280. Measui-es and weights, 379. Microscopic Photography, 301. Moser, M. Ludwig, his researches on the development of invisible images, 37. Mounting Positive Prints, sub- stances which should be avoided, 161, 171 ; detaUs of, 256. Negative processes for printing Po- sitives, 258, 263. Negatives, definition of, 102 ; Col- INDEX. 387 lodion Negatives, theory of pro- duction of, 110 ; Calotype, 182; waxed paper, 185 ; Albumen, 187 ; Collodio-Albumen (Taupe- not)188; mode of developing Col- lodion Negatives, 113, 231 ; of converting Positives into, 116 ; formula for solutions for, 216 ; the Collodion best adapted for Negatives, 111 ; Acetate of Silver sometimes useful, 287 ; spots and markings upon Negatives, 280 ; decomposition of Pyroxyline a cause of fading of, 158. Nitric Acid, its preparation and properties, 856 ; its oxidizing powers, 12 ; impairs sensitive- ness of Collodion film, 88 ; lessens rapidity of development, 96 ; tends to prevent fogging, 100 ; sometimes usefuUy em- ployed for glass Positives, 107 ; contraindicated for Negatives, 112 ; its accumulation in the Nitrate Bath, explained, 80 ; mode of removing it, 212 ; can- not exist in contact with Acetate of Silver, 112 ; produces stains on cloth, 224; mode of determin- ing the strength of Commer- cial Nitric Acid, 194 ; table of strength of Nitric Acid of dif- ferent densities, 378. Nitrate of Silver, preparation and properties of, 12 ; preparation of from standard coin of realm, 363; often contains free Nitric Acid, 80 ; when strongly fused contains Nitrite {see Nitrite of Silver), 13 ; not acted on by light, 18 ; its reduction by Py- rogallic Acid, explained, 31 ; its presence essential in develop- ing the image, 94; increases sen- sitiveness of Collodion plate, 89 ; dissolves Iodide of Silver, 80 ; discolom'ed by Albumen, 331 ; forms a compound with Honey, 293 ; very little acted on by Gly- cerine, 346; mode of recover- ing the Silver from, 372. Nitrate Bath, mode of preparing for glass Positives, 107, 212 ; for CoUodion Negatives, 112, 219 ; its power of dissolving Iodide of Silver, its occasional acidity and alkalinity explained, 80 ; the mode in which Acetate of Silver may be formed in it, 82 ; a list of the substances by which it is decomposed, 83 ; care required to prevent it from yielding foggy pictures, 276 ; a caution against the too frequent addition of alkali, 220 ; quanti- tative testing of the bath, 371. Nitrite of Silver increases sensi- tiveness, 92 ; adds to rapidity of development, 97 ; tends slightly to produce fogging, 99 ; solarizes the high lights, 107. Nitro-Sulphuric Acid, explained, . 73 ; process for making bv mix- ed acids, 197 ; by Oil of Vitriol and Nitre, 199 ; should not be used cold, 76 ; produces an in- ferior PvroxyUne when too hot, 197. Nomenclature, chemical, 317. Notation, chemical, 320. Organic bodies, chemistry of, 326. Oxide of Silver, preparation and properties, 17 ; dissolves in the Nitrate Bath, rendering it alka- line, 81 ; properties of its solu- tion in Ammonia, 363 ; prepara- tion of ditto, 247 ; its solution in Nitrate of Ammonia used in Photography, 248. Oxymel, keeping process, 289 : preparation of Oxymel, 360. 388 INDEX. Paper, Photographic, selection of, 120, 243, 245 ; peculiarity of English papers, 126, 257. Paper, sensitive, for printing. See Sensitive Paper. Perchloride of Iron, preparation of toning Bath with, 165. Permanence of Positives^ mode of testing, 175. Photographic properties of Salts of Silver, 18 ; of Iodide of SUver upon Collodion, 66. Photographic researches, by the Author, 138. Photographic image, chemical com- position of, 139 ; action of de- structive tests on, 148. Photography, historical sketch of, 6 ; the term explained, 60. Portrait lenses, theory of their con- struction, 58 ; rules for their use, 235 ; mode of finding che- mical focus, 237 ; can often be used for views by attaching a stop, 236. Portraits, drapery for, 65 ; direc- tions for taking, 228 ; the po- sition of the Camera, and other points of importance, 236 ; the time of exposure, 229. Positive printing, on Albuminized paper, formulse for, 241 ; on plain paper, formulse for, 245 ; on Ammonio-Nitrate paper, for- mulaj for, 246, 257; by de- velopment, formulse for, 258; manipulatory details of printing, fixing, toning, washing, and mounting, 251 ; process of to- ning by Sel d'or, theory of, 132 ; practice of, 266 ; reasons for the want of correspondence between different formulse, 257 ; use of Chloride of Gold in toning, 270 ; theory of the preparation of the sensitive paper for Positives, 119 ; theory of the process of fixing, 127 ; of toning by Gold, 130 ; the Author's researches, 138 ; rationale of the printing process and composition of the image, 144 ; fading of Positive prints, 166 ; destructive action of Sulphur on, 148 ; of oxidiz- ing agents on, 153; of Chlorine, Acids, boiling water, etc., on, 156; of combustion of coal gas on, 158; eiFect of damp air on, 159 ; theory of mode of washing Positives, 168; comparative per- manency of prints, 173 ; mode of testing permanency, 175. Positives, definition of, 102 ; Col- lodion Positives, theory of pro- duction of, 104 ; formulse for solutions for, 210 ; development of, 107, 229 ; Collodion and Nitrate Bath best adapted for, 106 ; mode of whitening by Bi- chloride of Mercury, 109 ; so- lution for whitening, 216 ; mode of backing up, 234; spots and markings on, 282; mode of printing Positives on CoUodion, 229. Positives, enlarged, mode of print- ing, 271. Practice of Collodion process, 193. Preservative processes for CoUo- dion plates, 289. Printing, Photographic, theory of, 118 ; practical details of, 241. Prism, refraction of light by, 49 ; diagram of formation of spec- trum by, 46 ; explained, 54. Prismatic spectrum, 46, 60. Protonitrate of Iron, preparation of, 215 ; a feeble developer when free from excess of Sulphate of Iron, 95 ; theory of its mode of action and i-ules for its use, 108 ; cannot be prepared in quantity INDEX. 389 by adding Nitrate of Potash to Sulphate of Iron, 317 ; some- times requires the addition of Nitrate of Silver, 315. Protosulphate of Iron, its prepara- tion and properties, 39 ; its cha- racteristics as a developer for Collodion Positives, 108 ; not well adapted for developing Col- lodion Negatives, 113, 147 ; pre- paration of the solution for Posi- tives, 313, 214; mode of apply- ing it to the plate, 239 ; to re- move iron stains on glass, 234. Pyrogallic Acid, its preparation and properties, 28 ; solution for developing glass Positives, 213 ; for Negatives, 220 ; cannot be used without Acetic Acid, 96 ; less adapted for developing paper pictures, 184 ; requires addition of Nitric Acid when used for Positives, 107 ; superior to Sul- phate of Iron for developing Negatives, 113, 147 ; mode of obviating the brown discolora- tion of developing solutions, 221, 261. Pyrosyline, its nature and proper- ties, 67 ; preparation of, by Mr. Hadow's formula, 195 ; by a rule of thumb mixture of the acids, 197 ; by the Oil of Vitriol and Nitre process, 199 ; details of immersing, washing, and dry- ing, 201 ; the glutinous variety produced by cold acids, 76 ; the result inferior if the temperature be too high, 197 ; recapitulation of the effects of varying the strength of the acid mixture, 202 ; spontaneous decomposition of Pyi'oxyline, 158. Keduction of Metallic Oxides by developers, theory of, 26 ; of Silver Salts by developers, the- ory of, 80 ; practical details of reducing Silver compounds to metaUic state, 372. Salts, nature of, 313. Salts of SUver, their preparation and properties, 13 ; their Pho- tographic action, 18 ; theory of their reduction by developer, 30 ; directions for obtaining metal from, 372. Sel d'or, toning process by, its the- ory, 132 ; its practical details, 266 ; its advantages, 270 ; gives permanent prints, 160. Sensitiveness of Collodion film, causes influencing, 87 ; superior sensitiveness partially explained, 66 ; lodate of Silver thought to lessen sensitiveness, 81, 207 ; preservation of sensitive film, 389. Sensitiveness, term explained, 87. Sensitive paper, theory of prepara- tion of, 31, 120 ; its darkening by light described, 131; prepara- tion of Albuminized paper, 241 ; of plain paper, 245 ; of Ammo- nio-Nitrate paper, 246, 257 ; of paper for Negative processes, 259 ; causes which affect the sensitiveness of Positive paper, 122 ; which alter the colour of the image, 125 ; spots and mark- ings on, 284 ; a large excess of Nitrate of Silver essential, 133 ; the paper should not be kept too long, 139, 284. Serum of Milk, preparation of, 355 . Shadbolt, Mr., uses Chloroform in Collodion, 76 ; his Honey keep- ing-process, 294 ; fixes Positive prints by Ammonia, 370 ; em- ploys artificial light in Micro- Photograph;^, 303. 390 INDEX. Silver, properties of, 362; estima- tion of, iu Nitrate Baths, 371 ; recovery from waste solutions, 372 ; reduction from Chloride, 374 ; stains, removal of, 377. Size, mode of removing from paper Positives, 255. Solar spectrum, 46, 60.. Soluble paper. See Pyroxyliue. Specific gravity of liquids, mode of finding, 375. Spherical aberration, 55. Spirits of Wine, preparation and properties, 332 ; not always suf- ficiently strong for CoUodion, 77 ; mode of rectifying, 205 ; sometimes contaminated with fusel oil, 94. Spots on Collodion plates, 278 ; on paper Positives, 284, 285 ; on prints obtained by develop- ment, 265 ; on preserved CoUo- dion plates, 292. Stains, Silver, removal of, 377. Stereoscopic Photographs, mode of obtaining, 295. Stops, theory of use of, 57 ; simple mode of making, 236 ; position of the stop often important, 238. Strength of acids, tables of, 378. Subchloride of Silver, its prepara- tion and properties, 16 ; decom- posed by fixing agents, 140. Suboxide of Silver, its properties, 18 ; forms compounds with or- ganic matters. Citric Acid, Al- bumen, etc., 331, 340; exists in the image of Positive prints, 21. Substitution, laws of, explained, 70. Sulphate of Iron. See Protosulphate of Iron. Sulphate of Quinine, absorption of chemical rays by, 64. Sulphuric Acid, table of strength of, 378. Sutton, Mr., theory of Sel d'or toning process, 132 ; practical details of, 266 ; his Negative printing process, 259 ; prepara- tion of Serum of Milk for, 355. Symbols, use of, 320. Syruped CoUodion film, 289. Talbot, Mr., his discoveries, 9 ; theory of his Calotype process, 182. Taupenot, M., his CoUodio-Albn- men process, 188. Temperature, its effect upon de- velopment of CoUodion film, 97 ; upon fogging, 100 ; upon keeping CoUodion, 218 ; upon action of fixing Bath for paper Positives, 128 ; upon Hj'pb- toning Bath, 134. Test-papers, use of, 376. Toning Bath for Positives, with Sel d'or, 132, 266 ; with Hyposul- phite and Gold, see Fixing and Toning Bath ; with Chloride of Iron and Hyposulphite, 165. Toning of Positives, term defined, 119 ; may injure the stability of the proof, 135 ; points to be kept in view to avoid fading after, 174; manipulatory details of, 253 ; by Sel d'or, 267. Transparencies, mode of printing, 273. Varnishes for CoUodion Photo- graphs, 234. View Lenses, directions for using, 238. Vocabulary of Photographic che- micals, 329. Washing Positive prints, rules for, 168 ; details of, 255. Waxed paper process, theory of, 184. Weights and Measures, table of, 3 7 9 . 1 spec I A L 2.803 2,