A MANUAL OF PHOTOGRAPHIC CHEMISTRY. A MANUAL OF PHOTOGRAPHIC CHEMISTRY, INCLUDING THE PRACTICE OF THE COLLODION PROCESS. Br T. FREDERICK HAEDWICH, LECTUEEB ON PHOTOGEAPHT IN KING'S COLLEGE, LONDON; LATE DEMONSTRATOR OF CHEMISTRY IN KINg's COLLEGE. LONDOJNT : JOHN CHURCHILL, NEW BURLINGTON STREET. MDCCCLYII. [T7ie Author reserves to himself the right of translating this Edition.'] PKINTED BY JOHN EDWAED TAYLOR, LITTLE QUEEN STREET, LINCOLN'S INN FIELDS. PREFACE TO THE THIRD EDITION. It is a source of much, gratification to the Author to find himself called upon to prepare a Third Edition of his Manual in less than fourteen months from the date of its first publication. 'No greater proof could have been af- forded 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 utility 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 difiers 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 Pliotogra- phic Printing are entirely re-written, and include the whole VI PEEFACE. of tlie Author's investigations, as published in the Society's Journal. 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. PREFACE TO THE FOURTH EDITION. The Author has endeavoured to keep pace with the im- provements which are daily being introduced in the sci- ence and art of Photography. In the present Edition alterations have been made in the style and general ar- rangement of the work, and additional matter has been inserted. Since the pubhcation of the Third Edition, a series of experiments have been made on the manufacture of Collo- dion, the results of which have thrown further light upon the conditions affecting the sensitiveness of the excited film, and have enabled the writer to introduce an organic substance, " Glycyrrhizine," which will be found of service in making Photographic copies of Engravings and similar works of Art. Dr. Norris, of Birmingham, has within the last few months communicated a paper on dry Collodion, which places the theory of that subject upon a better footing than before. The Oxymel preservative process is now also thoroughly understood, and may be considered certain. In addition to the above, the "Albuminized CoUodion" of M. Taupenot, which experience proves to be one of the best dry processes at present known, is included in this Edition. King's College^ London, April Qth, 1857. EERATA. Page 24, line ^,for conditions read condition. Page 115, line 32, for Iodide read Iodine. Page, 194, line 15, /or p. 88 read p. 188. CONTENTS. PART 1. THE SCIENCE OE PHOTOGHAPHY. Page Introduction 1 CHAPTER I. Historical Sketch of Photography 6 CHAPTER II. THE SALTS OF SILYER 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. — The 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 X CONTENTS. CHAPTER III. ON THE DEVELOPMENT OF AN INVISIBLE IMAGE. Page Simple experiments illustrating tlie process 25 Section I. — Chemistry of the Substances, employed as Develo- pers. — Development shown to be a process of reduction. — The chemistry of the principal reducing agents, Gallic Acid, Pyrogallic 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 Sil- ver when finely divided. — The reduction of the Iodide and other Salts of Silver containing no Oxygen 30 Section III. — Hypothesis on the Formation and Development of the Latent Image. — Explanation of the terms under- and over-exposure. — Diagram of Molecular change. — Mo- ser's experiments on development. — Curious perversions of development 34 CHAPTER IV. ON " fixing" the PHOTOGRAPHIC IMAGE. Chemistry of the various substances which may be employed as Eixing Agents. — Ammonia, Alkaline Chlorides, Bromides, and Iodides. — Hyposulphite of Soda. — Cyanide of Potas- sium » 41 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 46 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. xi Page The Foci of Lenses. — Formation of a Luminous Image by a Lens 49 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 54 Section IV. — The Photographic Action of Coloured Light. — Diagram of Chemical Spectrum. — Illustrative experiments. — Superior sensibility of Bromide of Silver to coloured light. — Mode in which dark-coloured objects are Photo- graphed 60 Section V. — On Binocular Vision and the Stereoscope. — Phenomena of Binocular Vision. — Theory of the Stereo- scope. — Wheatstone's reflecting Stereoscope. — Brewster's Stereoscope. — Rules for taking Stereoscopic pictures . . 66 CHAPTER VI. THE PHOTOGRAPHIC PROPERTIES OF IODIDE OF SILVER UPON COLLODION. Sectionl. — Collodion. — Pyroxyline — itsvarieties — 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 75 Section 11. — The 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. — Changes in the Nitrate Bath by use 86 Section III. — The Conditions which influence the Formation and Development of the Latent Image. — Causes which increase or diminish the sensitiveness of the film to Light. — Conditions which hasten or retard development ... 91 Section IV. — On certain irregularities in the Developing Pro- xii CONTENTS. Page cess. — Effect of particular states of the Bath, and of the Collodion, in producing clouding of the Image, and of acids in obviating it 103 CHAPTER VII. ON POSITIVE AND NEGATIVE COLLODION PHOTOGRAPHS. Definition of the terms Positive and Negative. — The same Photograph often capable of representing both varieties . 106 Section I. — On Collodion Positives. — The Collodion and Ni- trate Bath best suited for Positives. — Peculiarities of Py- rogallic Acid, and the Protosalts of Iron employed to develope Collodion Positives. — The colour of Positives affected by the length of exposure to light. — A Process for whitening Glass Positives by means of Bichloride of Mercury 108 Section II. — On Collodion Negatives. — The Collodion and Ni- trate Bath best adapted for Negatives. — Use of Glycyrrhi- zine to increase the intensity. — Developing solutions for Negatives. — Conversion of finished Positives into Nega- tives M13 CHAPTER VIII. ON THE THEORY OF POSITIVE PRINTING. Section I. — The Preparation of the Sensitive Paper. — Its dark- ening by light. — The conditions which affect its sensitive- ness and the vigour of the Image. — The colour of the print influenced by the preparation of the paper . . . 121 Section II. — The Processes for Fixing and Toning the Proof. — Conditions of a proper fixing. — The Salts of Gold used as toning agents. — The properties of the single fixing and toning Bath, with the conditions which affect its action . 128 Section III. — The Author's Photographic Researches. — The chemical composition of the Photographic Image. — The various agencies destructive to Photographs. — The action CONTENTS. xiii Page of damp air upon Positive Prints. — The change in proper- ties of the fixing Bath by constant use 140 Section IV. — The Fading of Photographic Prmts. — The causes which produce it. — The comparative permanency of Prints. — The mode of testing permanency 160 CHAPTER IX. ON THE DAGUERREOTYPE AND TALBOTYPE PROCESSES. Section I. — The Daguerreotype. — The nature of the sensitive fihn. — Development of the Latent Image. — The strength- ening of the Image by means of Hyposulphite of Gold . 171 Section II. — The Processes of Talbot, etc. — The Calotype. — The Waxed Paper Process of Le Grey. — The Albumen Negative Process. — Taupenot's Collodio -Albumen Process 176 PART 11. PRACTICAL DETAILS OF THE COLLODION PROCESS. CHAPTER I. PREPARATION OF COLLODION. Mode of preparing soluble Pyroxyline by the mixed Acids — by the Nitre process. — Purification of the Ether and Alcohol. — Preparation of the iodizing compounds in a state of purity 185 CHAPTER II. FORMULiE FOR SOLUTIONS REQUIRED FOR COLLODION PHOTOGRAPHS. Section I. — Formula for direct Positive Solutions. — The Col- xiv CONTENTS. Page lodion. — The Nitrate Bath. — Developing fluids. — Fixing liquids. — Whitening solution 201 Section II. — Formula for Negative Solutions. — The Collodion. — The Nitrate Bath. — Developing fluid. — Fixing liquid . 208 CHAPTER III. MANIPULATIONS OP THE COLLODION PROCESS. Section I. — 3Ianipulations with moist Collodion. — Cleaning the Plates. — Coating with Collodion. — Exciting. — Ex- posing. — Developing. — Fixing 213 Section II. — Simple directions for the use of Photographic Lenses. — Portrait Lenses. — View Lenses. — Mode of find- ing the chemical Focus 227 Section III. — On copying Engravings, Etchings, Diagrams, etc. — Mode of intensifying the Collodion 231 Section IV. — Rules for talcing Stereoscopic Photographs. — Mr. Latimer Clark's arrangement for working with a single Camera 232 Section V. — The Photographic delineation of Microscopic Ob- jects. — Arrangement of the apparatus. — Mode of finding the chemical Focus. — Use of artificial light 235 CHAPTER IV. THE DETAILS OF PHOTOGRAPHIC PRINTING. Section I. — Positive Printing hy the ordinary process. — Selec- tion of the paper. — Preparation of Albuminized paper — of plain paper — of Ammonio-Nitrate Paper. — Preparation of the fixing and toning Bath. — Manipulatory details of Photographic Printing. — Mode of washing and mounting the Proofs 240 Section II. — Positive Printing hy Bevelopment. — A process on Chloride and Citrate of Silver. — On Iodide of Silver. — On Bromide of Silver 259 Section III. — The Seld'Or Process for toning Positives. — Pre- paration of the toning Bath. — Manipulatory details . . 267 CONTENTS. XV Page Section IV. — On Printing enlarged Positives, Transparencies, etc., wpon Collodion 272 CHAPTER V. CLASSIFICATION OF CAUSES OF FAILURE IN THE COLLODION PROCESS. Section I, — Imperfections in Negative and' Positive Collodion Photographs. — Pogging. — Transparent and opaque spots. — Markings of all kinds — under- and over-exposure, etc. . 276 Section II. — Imperfections in Paper Positives 285 CHAPTER VI. LANDSCAPE PHOTOGRAPHY BY THE COLLODION PRESERVATIVE AND COLLODIO-ALBUMEN PROCESSES. The Honey Keeping Process. — The Oxymel Process. — Photo- graphy on dry Collodion. — Taupenot's Collodio-Albumen Process 288 PART III. OUTLINES OE GENERAL CHEMISTRY. CHAPTER I. THE CHEMICAL ELEMENTS AND THEIR COMBINATIONS. The more important Elementary Bodies, with their Symbols and Atomic Weights. — The Compounds formed by their union. — The class of Salts. — Illustrations of the nature of Chemical Affinity. — Chemical Nomenclature. — Symbo- lic Notation. — The Laws of Combination. — The Atomic Theory. — The Chemistry of Organic Bodies 305 xvi CONTENTS. CHAPTER II. Page Vocabulary of Photographic Chemicals 327 APPENDIX. Quantitative testing of Nitrate Baths. — Recovery of Silver from waste solutions. — Reduction of Chloride of Silver. — • Mode of taking the Specific Gravity of liquids. — On Fil- tration. — The use of Test-papers. — The removal of Silver stains from the hands, etc. — Dr. lire's tables of the strength of Sulphuric and Nitric Acid of different densi- ties.— Table of Weights and Measui'es 371 A MANUAL OF PHOTOGRAPHIC CHEMISTRY. INTRODUCTION. In attempting to impart knowledge on any subject, it is not sufficient tliat the writer sliould himself be acquainted with that which he professes to teach. Even supposing such to be the case, yet much of the success of his effort must depend upon the manner in which the information is conveyed ; for as, on the one hand, a system of extreme brevity always fails of its object, so, on the other, a mere compilation of facts imperfectly explained tends only to confuse the reader. A middle course between these extremes is perhaps the best to adopt ; that is, to make selection of certain funda- mental points, and to explain them with some minuteness, leaving others of less importance to be dealt with in a more summary manner, or to be altogether omitted. But independently of observations of this kind, which apply to educational instruction in general, it may be re- marked, that there are sometimes difficulties of a more formidable description to be overcome. For instance, in- treating of any science, such as that of Photography, which may be said to be comparatively new and unex- B 2 INTEODUCTION. plored, there is great danger of erroneously attributing effects to tlieir wrong causes ! Perhaps none but lie who has himself worked in the laboratory can estimate this point in its proper light. In an experiment where the quantities of material acted upon are inlinitesimally small, and the chemical changes involved of a most refined and subtle description, it is soon discovered that the slightest variation in the usual conditions will suffice to alter the result. Nevertheless Photography is truly a science, governed by fixed laws ; and hence, as our knowledge increases, we may fairly hope that uncertainty will cease, and the same precision at length be attained as that with which chemical operations are usually performed. The intention of the author in writing this work, is to impart a thorough knowledge of what may be termed the "First Principles of Photography," that the amateur may arm himself with a theoretical acquaintance with the sub- ject before proceeding to the practice of it. To assist this object, care will be taken to avoid needless complexity in the formulae, and all ingredients will be omitted which are not proved to be of service. The impurities of chemicals will be pointed out as far as possible, and special directions given for their removal. Amongst the variety of Photographic processes devised, those only will be selected which are correct on theoretical grounds, and are found in practice to succeed. As the work is addressed to one supposed to be unac- quainted both with Chemistry and Photography, pains will be taken to avoid the employment of all technical terms of which an explanation has not previously been given. A SKETCH OF THE MAIN DIVISIONS TO BE ADOPTED, WITH THE PRINCIPAL SUBJECT-MATTEE OF EACH. The title given to the Work is "A Manual of Photo- graphic Chemistry," and it is proposed to include in it a INTRODUCTION. 3 familiar explanation of the nature of the various chemical agents employed in the Art of Photography, with the rationale of the manner in which they are thought to act. The division adopted is threefold : — Part I. enters minutely into the theory of Photographic processes ; Part II. treats of the 'practice of Photography upon Collodion ; Part III. embraces a simple statement of the main laws of Chemistry, with the principal proper- ties of the various substances, elementary or compound, which are employed by Photographers. Part I., or "the Science of Photography," includes a full description of the chemical action of Light upon the Salts of Silver, wdth its application to artistic purposes ; all mention of manipulatory details, and of quantities of in- gredients, being, as a rule, omitted. In this division of the "Work will be found nine Chap- ters, the contents of which are as follows : — Chapter I. is a sketch of the history of Photo2:raphy, intended to convey a general notion of the origin and progress of the Art, without dwelling on minute parti- culars. Chapter II. describes the Chemistry of the Salts of Silver employed by Photographers ; their preparation and properties ; the phenomena of the action of Light upon them, with experiments illustrating it. Chapter III. leads us on to the formation of an invisible image upon a sensitive surface, with the development or bringing out to view of the same by means of chemical re-agents. This point, being of elementary importance, is described carefully ; — the reduction of metallic oxides, the jjroperties of the bodies employed to reduce, and the hypotheses which have been entertained on the nature of the Light's action, are all minutely explained. Chapter IV. treats of the fixing of Photographic im- pressions, in order to render them indestructible by dif- fused light. Chapter V. contains a sketch of the Optics of Photo- 4 INTEODUCTION. graphy — the decomposition of white Light into its elemen- tary rays, the Photographic properties of the different co- lours, the refraction of Light, and construction of Lenses. In the last Section of the same Chapter will be found a short sketch of the history and use of the Stereoscope. Chapter YI. embraces a more minute description of the sensitive Photographic processes upon Collodion. In it is explained the chemistry of Pyroxyline, with its solution in Alcoholized Ether, or Collodion ; also the Photographic properties of Iodide of Silver upon Collodion, with the causes which affect its sensitiveness to Light, and the ac- tion of the developing solutions in bringing out the image. Chapter YII. continues the same subject, describing the classification of Collodion Photographs as Positives and J^egatives, with the distinctive peculiarities of each. Chapter YIII. contains the theory of the production of Positive Photographs upon paper. In this Chapter will be found an explanation of the somewhat complex chemical changes involved in printing Positives, with the precau- tions which are required to ensure the permanency of the proofs. Chapter IX. is supplementary to the others, and a brief notice of it will suffice. It explains the theory of the Pho- tographic processes of Daguerre and Talbot ; especially noticing those points in which they may be contrasted with Photography upon Collodion, but omitting all description of manipulatory details, which if included would extend the Work beyond its proposed limits. The title of the second principal division of the Work, viz. "The practice of Photography upon Collodion," ex- plains itself. Attention however may be invited to the fifth Chapter, in which a classification is given of the prin- cipal imperfections in Photographs, with short directions for their removal ; and to Chapter YL, which describes the preservation of the sensitiveness of Collodion plates and the mode of operating upon films of Albumenized Collo- dion. INTRODUCTION. 5 In Part III. will be found, in addition to a statement of the laws of cliemical combination, etc., a list of Photo- graphic chemicals, alphabetically arranged, including their preparation and properties as far as required for their em- ployment in the Art. The reader will at once gather from this sketch of the contents of the volume before him, that whilst the general theory of every Photographic process is described, with the preparation and properties of the chemicals employed, minute directions in the minor points of manipulation are restricted to Photography upon Collodion, that branch of the Art being the one to which the time and attention of the author have been especially directed. Collodion is allowed by all to be the best vehicle for the sensitive Silver Salts which is at present known, and successful results can be obtained with a very small expenditure of time and trouble, if the solutions employed in the process are pre- pared in a state of purity. 6 CHAPTER 1. HISTOKICAL SKETCH Or PHOTOGEAPHY. The Art of Photography, which has now attained such perfection, and has become so popular amongst all classes, is one of comparatively recent introduction. The word Photography means literally "writing by means of Light;" and it includes all processes by which any kind of picture can be obtained by the chemical agency of Light, Avithout reference to the nature of the sensitive surface upon which it acts. The philosophers of antiquity, although chemical changes due to the influence of Light were continually passing be- fore their eyes, do not appear to have directed their atten- tion to them. Some of the Alchemists^ indeed noticed the fact that a substance which they termed " Horn Silver," which was probably a Chloride of Silver which had under- gone fusion, became blackened by exposure to Light ; but their ideas on such subjects being of the most erroneous nature, nothing resulted from the discovery. The first philosophical examination of the decomposing action of Light upon compounds containing Silver was made by the illustrious Scheele, no longer than three-quar- ters of a century ago, viz. in 17.77. It was also remarked by him that some of the coloured rays of Light were pecu- liarly active in promoting the change. Earliest application of these facts to purposes of Art. — The first attempts to render the blackening of Silver Salts HISTORICAL SKETCH OF PHOTOGRAPHY. 7 by Light availabie for artistic purposes were made by Wedgwood and Davy about a.d. 1802. A sheet of white paper or of white leather was saturated with a solution of JN^itrate of Silver, and the shadow of the figure intended to be copied projected upon it. Under these circumstances the part on which the shadow fell remained white, whilst the surrounding exposed parts gradually darkened under the influence of the sun's rays. Unfortunately these and similar experiments, which ap- peared at the outset to promise well, were checked by the experimentalists being unable to discover any means of fixing the pictures, so as to render them indestructible by difiused Light. The unchanged Silver Salt being per- mitted to remain in the white portions of the paper, na- turally caused the proofs to blacken in every part, unless carefully preserved in the dark. Introduction of the Camera Obscura, and other Improve- ments in Photography. — The " Camera Obscura," or dark- ened chamber, by means of which a luminous image of an object may be formed, was invented by Baptista Porta, of Padua ; but the preparations employed by Wedgwood were not sufficiently sensitive to be easily afiected by the sub- dued light of that instrument. In the year 1814, however, twelve years subsequent to the publication of Wedgwood's paper, M. Niepce, of Cha- lons, having directed his attention to the subject, succeeded in perfecting a process in which the Camera could be em- ployed, although the sensibility was still so low that an ex- posure of some hours was required to produce the effect. In the process of M. Niepce, which was termed " Helio- graphy," or " sun-drawing," the use of the Silver Salts was discarded, and a resinous substance, known as " Bitu- men of Judsea," substituted. This resin was smeared on the surface of a metal plate, and exposed to the luminous image. The light in acting upon it so changed its pro- perties, that it became insoluble in certain essential oils. Hence, on subsequent treatment with the oleaginous sol- S HISTOEICAL SKETCH OF PHOTOGEAPHY. vent, tlie shadows dissolved away, and tTie ligJits were repre- sented by the unaltered resin remaining on the plate. The Discoveries of 3f. Daguerre. — MM. iNiepce and Dagnerre appear at one time to have heen associated as partners, for the purpose of mutually prosecuting their re- searches ; but it was not until after the death of the for- mer, viz. in 1839, that the process named the Daguerreo- type was given to the world. Daguerre was dissatisfied with the slowness of action of the Bitumen sensitive sur- face, and directed his attention mainly to the use of the Salts of Silver, which are thus again brought before our notice. Even the earlier specimens of the Daguerreotype, al- though far inferior to those subsequently produced, pos- sessed a beauty which had not been attained by any Pho- tographs prior to that time. The sensitive plates of Daguerre were prepared by ex- posing a silvered tablet to the action of the vapour of Iodine, so as to form a layer of Iodide of Silver upon the surface. By a short exposure in the Camera an effect was produced, not visible to the eye, but appearing when the plate was subjected to the vapour of Mercury. This fea- ture, viz. the production of a latent image upon Iodide of Silver, with its subsequent development by a chemical re- agent, is one of the first importance. Its discovery at once reduced the time of taking a picture from hours to minutes, and promoted the utility of the Art. Daguerre also succeeded m fixing his proofs, by removal of the unaltered Iodide of Silver from the shadows. The processes employed however were imperfect, and the mat- ter was not set at rest until the publication of a paper by Sir John Herschel, on the property possessed by *' Hypo- sulphites" of dissolving the Salts of Silver insoluble in water. On a means of M.ulti])lying Photographic Impressions ^ and other Discoveries of Mr. Fox Talbot. — The first com- munication made to the Hoyal Society by Mr. Fox Talbot, HISTOEICAL SKETCH OF PHOTOGKAPHY. 9 in Jamiary, 1839, included only the preparation of a sen- sitive paper for copying objects by application. It was di- rected that the paper should be dipped first in solution of Chloride of Sodium, and then in Nitrate of Silver. In this way a white substance termed Chloride of Silver is formed, more sensitive to light than the Nitrate of Silver originally employed by Wedgwood and Davy. The object is laid in contact with the prepared paper, and, being exposed to light, a copy is obtained, which is Negative, — id est, with the light and shade reversed. A second sheet of paper is then prepared, and the first, or Negative impression, laid upon it, so as to allow the sun's light to pass through the transparent parts. Under these circumstances, when the Negative is raised, a natural representation of the object is found below ; the tints having been again reversed by the second operation. This production of a Negative Photograph, from which any number of Positive copies may be obtained, is a car- dinal point in Mr. Talbot's invention, and one of great im- portance. The patent issued for the process named Talhoty^e or Calotype dates from Tebruary, 1841. A sheet of paper is first coated with Iodide of Silver by soaking it alter- nately in Iodide of Potassium and Nitrate of Silver ; it is then washed with solution of Gallic Acid containing Nitrate of Silver (sometimes termed Gallo-Nitrate of Sil- ver), by which the sensibility to light is greatly augmented. An exposure in the Camera of some seconds or minutes, according to the brightness of the light, impresses an in- visible image, which is brought out by treating the plate with a fresh portion of the mixture of Gallic Acid and Nitrate of Silver employed in exciting. On the use of Glass Plates to retain Sensitive Films. — The principal defects in the Calotype process are attribu- table to the coarse and irregular structure of the fibre of paper, even when manufactured with the greatest care, and expressly for Photographic purposes. In consequence of 10 HISTORICAL SKETCH OF PHOTOGEAPHY. this, the same amount of exquisite definition and sharpness of outline as that resulting from the use of metal plates, cannot be obtained. We are indebted to Sir J ohn Herschel for the first em- ployment of glass plates to receive sensitive Photographic films. The Iodide of Silver may be retained upon the glass by means of a layer of Albumen or white of egg, as proposed by M. Niepce de Saint- Victor, nephew to the original dis- coverer of the same name. A more important improvement still is the employment of " Collodion " for a similar purpose. Collodion is an ethereal solution of a substance almost identical with Gun-cotton. On evaporation it leaves a trans- parent layer, resembling goldbeater's skin, which adheres to the glass with some tenacity. M. Le Grey of Paris originally suggested that this substance might possibly be rendered available in Photography, but our own coun- tryman, Mr. Archer, was the first to carry out the idea practically. In a communication to * The Chemist' in the autumn of 1851, this gentleman gave a description of the Collodion process much as it now stands ; at the same time proposing the substitution of Pi/ro-gaWic acid for the Gallic acid previously employed in developing the image. At that period no idea could have been entertained of the stimulus which this discovery would render to the pro- gress of the Art ; but experience has now abundantly de- monstrated, that, as far as all qualities most desirable in a Photographic process are concerned, none at present known can excel, or perhaps equal, the Collodion process. 11 CHAPTER 11. THE SALTS OE SILYER EMPLOYED IN PHOTOGEAPHT. By the term Salt of Silver we understand that the com- pound in question contains Silver, but not in its elemen- tary form ; the metal is in fact in a state of chemical union with other elements which disguise its physical properties, 80^ that the Salt possesses none of the external characters of the Silver from which it was produced. Silver is not the only metal which forms Salts ; there are Salts of Lead, Copper, Iron, etc. Sugar of Lead is a familiar instance of a Salt of Lead. It is a white crystal- line body, easily soluble in water, the solution possessing an intensely sweet taste ; chemical tests prove that it con- tains Lead, although no suspicion of such a fact could be entertained from a consideration of its general properties. Common Salt, or Chloride of Sodium, which is the type of the salts generally, is constituted in a similar manner ; that is to say, it contains a metallic substance, the cha- racters of which are masked, and lie hid in the compound. The contents of this Chapter may be arranged in three Sections : the first describing the Chemistry of the Salts of Silver ; the second, the action of Light upon them ; the third, the preparation of a sensitive surface, with ex- periments illustrating the formation of the Photographic image. 12 THE SALTS OF SILVER SECTIOJN" I. Chemistry of the Salts of Silver. The principal Salts of Silver employed in the Pho- tographic processes are four in number, viz. JN'itrate of Silver, Chloride of Silver, Iodide of Silver, and Bromide of Silver. In addition to these, it will be necessary to de- scribe the Oxides of Silver. THE PREPARATION AND PROPERTIES OF THE NITRATE OF SILVER. Nitrate of Silver is prepared by dissolving metallic Silver in Nitric Acid. Nitric Acid is a powerfully acid and cor- rosive substance, containing two elementary bodies united in definite proportions. These are Nitrogen and Oxygen ; the latter being present in greatest quantity. Nitric Acid is a powerful solvent for the metallic bodies generally. To illustrate its action in that particular, as contrasted with other acids, place pieces of silver-foil in two test-tubes, the one containing dilute Sulphuric, the other dilute Nitric Acid ; on the application of heat a vio- lent action soon commences in the latter, but the former is unaffected. In order to understand this, it must be borne in mind that when a metallic substance dissolves in an acid, the nature of the solution is different from that of an aqueous solution of salt or sugar. If salt water be boiled down until the whole of the water has evaporated, the salt is recovered with properties the same as at first ; but if a similar experiment be made with a solution of Silver in Nitric Acid, the result is different : in that case metallic Silver is not obtained on evaporation, but Silver combined with Oxygen and Nitric Acid, both of which are strongly retained, being in fact in a state of chemical com- bination with the metal. If we closely examine the effects produced by treating Silver with Nitric Acid, we find them to be of the following EMPLOYED IN PHOTOGRAPHY. 13 nature : — first, a certain amount of Oxygen is imparted to the metal, so as to form an Oxide, which Oxide dissolves in another portion of the Nitric Acid, producing Nitrate of the Oxide, or, as it is shortly termed, Nitrate of Silver."^ It is the instability of Nitric Acid therefore — its prone- ness to part with Oxygen — which renders it superior to the Sulphuric and to most acids in dissolving Silver and various other substances, both organic and inorganic. Properties of Nitra te of Silver. — In preparing Nitrate of Silver, when the metal has dissolved, the solution is boiled down and set aside to crystallize. The salt however as so obtained is still acid to test-paper, and requires either re-crystallization, or careful heating to about 300° Fahren- heit. It is this retention of small quantities of Nitric Acid, and sometimes probably of Nitrous Acid, which renders much of the commercial Nitrate of Silver useless for Pho- tography, until rendered neutral by fusion and a second crystallization. Pure Nitrate of Silver occurs in the form of white crys- talline plates, which are very heavy and dissolve readily in an equal weight of cold water. The solubility is much lessened by the presence of free Nitric Acid, and in the concentrated Nitric Acid the crystals are almost insoluble. Boiling Alcohol takes up about one-fourth part of its weight of the crystallized Nitrate, but deposits nearly the whole on cooling. Nitrate of Silver has an intensely bitter and nauseous taste ; acting as a caustic, and corroding the skin by a prolonged application. Its aqueous solution does not redden blue litmus -paper. Heated in a crucible the salt melts, and when poured into a mould and solidified, forms the white lunar caustic of commerce. At a still higher temperature it is decom- posed, and bubbles of Oxygen Gas are evolved : the melted mass cooled and dissolved in water leaving behind a black powder, and yielding a solution, which is faintly alkaline * The preparation of JN'itrate of Silver from the standard coin of the realm is described in Part III., Art. Silver." 14 THE SALTS OF SILVER to test-paper, from the presence of minute quantities of Nitrite or basic Nitrite of Silver.* THE CHEMISTRY OF THE CHLORIDES OF SILVER. JPreparation of JProtocJiloride of Silver. — The ordinary white Chloride of Silver may be prepared in two ways, — by the direct action of Chlorine upon metallic Silver, and by double decomposition between two salts. If a plate of polished silver be exposed to a current of Chlorine Gas,t it becomes after a short time coated on the surface with a superficial film of white powder. This powder is Chloride of Silver, containing the two elements Chlorine and Silver united in single equivalents. Preparation of CJiloride of Silver hy double decomposi- tion. — In order to illustrate this, take a solution in water of Chloride of Sodium or " common salt," and mix it with a solution containing Nitrate of Silver ; immediately a dense, curdy, white precipitate falls, which is the substance in question. • In this reaction the elements change places ; the Chlo- rine leaves the Sodium with which it was previously com- bined, and crosses over to the Silver ; the Oxygen and Nitric Acid are released from the Silver, and unite with the Sodium ; thus Chloride of S odium j9Zw5 Nitrate of Silver equals Chloride of Silver plus Nitrate of Soda. This interchange of elements is termed by chemists double decomposition ; further illustrations of it, with the conditions necessary to the proper establishment of the process, are given in the first Chapter of Part III. The essential requirements in two salts intended for the * Mtrite of Silver differs from the IS'itrate in containing less Oxygen, and is formed from it by the abstraction of two atoms of that element ; it is described in the vocabulary, Part III. t For the properties of the element " Chlorine," see the third division of the Work. EMPLOYED IN PHOTOGRAPHY. 15 preparation of Chloride of Silver, are simply tliat tlie first should contain Chlorine, the second Silver, and that both should be soluble in water ; hence the Chloride of Potas- sium or Ammonium may be substituted for the Chloride of Sodium, and the Sulphate or Acetate for the Nitrate of Silver. In preparing Chloride of Silver by double decomposition, the white clotty masses which first form must be washed repeatedly with water, in order to free them from soluble Nitrate of Soda, the other product of the change. When this is done, the salt is in a pure state, and may be dried, etc., in the usual way. Properties of Chloride of Silver. — Chloride of Silver differs in appearance from the Nitrate of Silver. It is not usually crystalline, but forms a soft white powder resem- bling common chalk or whiting. It is tasteless and inso- luble in water; unaffected by boiling with the strongest Nitric Acid, but sparingly dissolved by concentrated Hy- drochloric Acid. Ammonia dissolves Chloride of Silver freely, as do solu- tions of Hyposulphite of Soda and Cyanide of Potassium. Concentrated solutions of alkaline Chlorides, Iodides, and Bromides are likewise solvents of Chloride of Silver, but to a limited extent, as will be more fully shown in Chapter IV., when treating of the modes of fixing the Photographic proofs. Dry Chloride of Silver carefully heated to redness fuses, and concretes on cooling into a tough and semitransparent substance, which has been termed Jiorn silver or luna cornea. Placed in contact with metallic Zinc or Iron acidified with dilute Sulphuric Acid, Chloride of Silver is reduced to the metallic state, the Chlorine passing to the other metal under the decomposing influence of the galvanic current which is established. Preparation and Properties of the Suhchloride of Silver. — If a plate of polished Silver be dipped in solution of Per- 16 THE SA.LTS OF SILVER cliloride of Iron, or of Bichloride of Mercury, a hlaclc stain is produced, tlie Iron or Mercury Salt losing a portion of Chlorine, which passes to the Silver and converts it super- ficially into Subchloride of Silver. This compound differs from the white Chloride of Silver in containing less Chlo- rine ; the composition of the latter being represented by the formula AgCl, that of the former may perhaps be written as AggCl (?). Subchloride of Silver is interesting to the 'Photographer as corresponding in properties and composition with the ordinary Chloride of Silver blackened by light. It is a pulverulent substance of a bluish-black colour not easily affected by Nitric Acid but decomposed by fixing agents such as Ammonia, Hyposulphite of Soda, or Cyanide of Potassium, into Chloride of Silver which dissolves, and in- soluble metallic Silver. THE CHEMISTRY OF IODIDE OF SILVER. The properties of Iodine are described in the third divi- sion of the Work : they are analogous to those of Chlorine and Bromine, the Silver Salts formed by these elements bearing also a strong resemblance to each other. Preparation and Properties of Iodide of Silver, — Iodide of Silver may be formed in an analogous manner to the Chloride, viz. by the direct action of the vapour of Iodine upon metallic Silver, or by double decomposition, between solutions of Iodide of Potassium and ]N"itrate of Silver. When prepared by the latter mode it forms an impal- pable powder, the colour of which varies slightly with the manner of precipitation. If the Iodide of Potassium be in excess, the Iodide of Silver falls to the bottom of the vessel nearly white ; but with an excess of Nitrate of Silver it is of a straw-yellow tint. This point may be noticed, because the yellow salt is the one adapted for Photographic use, the other being insensible to the influence of light. Iodide of Silver is tasteless and inodorous ; insoluble in EMPLOYED IN PHOTOGRAPHY. 17 water and in dilute Nitric Acid. It is scarcely dissolved by Ammonia, which serves to distinguish it from the Chlo- ride of Silver, freely soluble in that liquid. Hyposulphite of Soda and Cyanide of Potassium both dissolve Iodide of Silver ; it is also soluble in solutions of the alkaline Bromides and Iodides, as will be further explained in Chapter IV. Iodide of Silver is reduced by Metallic Zinc in the same manner as the Chloride of Silver, forming soluble Iodide of Zinc and leaving a black powder. THE PREPAEATION AND PEOPEETIES OF BROMIDE OF SILVER. This substance so closely resembles the corresponding salts containing Chlorine and Iodine, that a short notice of it will suffice. Bromide of Silver is prepared by exposing a silvered plate to the vapour of Bromine, or by adding solution of Bromide of Potassium to Nitrate of Silver. It is an in- soluble substance, slightly yellow in colour, and distin- guished from Iodide of Silver by dissolving in strong Ammonia and in Chloride of Ammonium. It is freely soluble in Hyposulphite of Soda and in Cyanide of Potas- sium. The properties of the element Bromine are described in Part III. CHEMISTRY OF THE OXIDES OF SILVER. The Protoxide of Silver (Ag O). — If a little Potash or Ammonia be added to solution of Nitrate of Silver, an olive-brown substance is formed, which, on standing, col- lects at the bottom of the vessel. This is Oxide of Silver, displaced from its previous state of combination with Nitric Acid by the stronger oxide. Potash. Oxide of Silver is soluble to a very minute extent in pure water, the solution possessing an alkaline reaction to Litmus ; it is easily dis- solved by Nitric or Acetic Acid, forming a neutral Nitrate c 18 THE SALTS OF SILVER or Acetate ; also soluble in Ammonia (Ammonio-Nitrate of Silver), and in Nitrate of Ammonia, Hyposulphite of Soda, and Cyanide of Potassium. Long exposure to light con- verts it into a black substance, which is probably a Sub- oxide. The Suboxide of Silver (Ag2 O ?) — This substance was obtained by Faraday on exposing a solution of the Am- monio-Nitrate of Silver to the action of the air. It bears a relation to the ordinary brown Protoxide of Silver similar to that which the Subcliloride bears to Protochloride of Silver. Suboxide of Silver is a black or grey powder, which assumes the metallic lustre on rubbing, and when treated with dilute Acids is resolved into Protoxide of Silver which dissolves, and metallic Silver. SECTIOJVT II. On the JPhotographic Properties of the Salts of Silver. In addition to the Salts of Silver described in the first Section of this Chapter there are many others well known to chemists, as the Acetate of Silver, the Sulphate, the Ci- trate of Silver, etc. Some occur in crystals which are solu- ble in water, whilst others are pulverulent and insoluble. The Salts of Silver formed by colourless Acids are white when first prepared, and remain so if kept in a dark place ; but they possess the remarkable peculiarity of being darkened in colour by exposure to Light. Action of Light upon the Nitrate of Silver. — The Nitrate of Silver is one of the most permanent of the Silver salts. It may be preserved unchanged in the crystalline form, or in solution in distilled water, for an indefinite length of time, even when constantly exposed to the difiused light of day. This is partly explained by the nature of the acid with which Oxide of Silver is associated in the Salt ; Nitric Acid, possessing strong oxidizing properties, being EMPLOYED IN PHOTOGRAPHY. 19 opposed to the darkening influence of Light upon the Silver compounds. Nitrate of Silver may, however, be rendered susceptible to the influence of Light, by adding to its solution organic matter, vegetable or animal. The plienomena produced in this case are well illustrated by dipping a pledget of cotton- wool, or a sheet of white paper, in solution of JSTitrate of Silver, and exposing it to the direct rays of the sun ; it slowly darkens, until it becomes nearly black. The stains upon the skin produced by handling Nitrate of Silver are caused in the same way, and are seen most evidently when the part has been exposed to light. The varieties of organic matter which especially facilitate the blackening of Nitrate of Silver are such as tend to ab- sorb Oxygen ; hence pure vegetable fibre, free from Chlo- rides, such, for instance, as the Swedish filtering-paper, is not rendered very sensitive by being simply brushed with solution of the Nitrate, but a little grape sugar added soon determines the decomposition. Decomposition of Chloride, Bromide, and Iodide of Silver hy Light. — Pure moist Chloride of Silver"^ changes slowly from white to violet on exposure to light. Bromide of Silver becomes of a grey colour, but is less affected than the Chloride. Iodide of Silver (if free from excess of Nitrate of Silver) does not alter in appearance by exposure even to the sun's rays, but retains its yellow tint unchanged. Of these three compounds therefore Chloride of Silver is the most readily acted on by light, and papers prepared with this salt will become far darker on exposure than others coated with Bromide or Iodide of Silver. There are certain conditions which accelerate the action of light upon the Chloride of Silver. These are, first, an excess of Nitrate of Silver, and second, the presence of organic matter. Pure Chloride of Silver would be useless * The Chloride here spoken of is the compound prepared by adding a soluble Chloride to a solution of Nitrate of Silver : the product of the direct action of Chlorine upon metallic Silver is sometimes insensitive to light. 20 THE SALTS OF SILVER as a Pliotograpllic agent, but a Chloride witli excess of Ni- trate is very sensitive. Even Iodide of Silver, ordinarily unaffected, is blackened by light when moistened with a solution of the Nitrate of Silver.* Organic matter combined with Chloride and JSTitrate of Silver gives a still higher degree of sensibility, and in this way the Photographic papers are prepared. The hlaclcening of Chloride of Silver hy Light explained, — This may be studied by suspending pure Chloride of Sil- ver in distilled water, and exposing it to the sun's rays for several days. When the process of darkening has pro- ceeded to some extent, the supernatant liquid is found to contain /ree Chlorine^ or, in place of it. Hydrochloric Acid (H CI), the result of a subsequent action of the Chlorine upon the water. The luminous rays appear to loosen the affinity of the elements Chlorine and Silver for each other ; hence a por- tion of Chlorine is separated, and the white Protochloride IS converted into the violet SuhQ\Aov\Ae of Silver. If an atom of Nitrate of Silver be present, the liberated Chlorine unites with it, displacing Nitric Acid, and forming again Chloride of Silver, which is decomposed in its turn. The excess of Nitrate of Silver thus exerts an accelerating in- fluence upon the darkening of Chloride of Silver, by ren- dering the chain of chemical affinities more complete, and preventing an accumulation of Chlorine in the liquid, which would be a check to the continuance of the action. Action of Light upon organic Salts of Silver. — On adding diluted Albumen, or white of egg, to solution of Nitrate of Silver, a flocculent deposit forms which is a compound of the animal matter with Protoxide of Silver, and is known as " Albuminate of Silver." This substance is at first quite white, but on exposure to light it turns to a brick- red colour. The change which takes place is one of de- * The reader will understand that the Acetate, Sulphate, or any other soluble Salt of Silver, might be substituted for the Nitrate in this experi- ment. EMPLOYED IN PHOTOGRAPHY. . 21 oxidation, the Protoxide of Silver losing a portion of its Oxygen, and a Suboxide of Silver, tke product of tlie re- duction, remaining in union vidtli tlie oxidized Albumen. The red compound may therefore be loosely designated as an Albuminate of Suboxide of Silver. Gelati?ie does not precipitate Nitrate of Silver in the same manner as Albumen : but if a sheet of transparent Gela- tine be allowed to imbibe a solution of the Nitrate, it be- comes of a clear ruby-red tint on exposure to light, and a true chemical compound of Gelatine, or a product of its oxidation, with a low Oxide of Silver, is produced. Casein e, the animal principle of milk, is coagulated by Nitrate of Silver, and the red substance formed on expos- ing the curds to light may be viewed as analogous in com- position to the corresponding compounds with Albumen and Gelatine. Many other organic salts of Silver are darkened by light. The white Citrate of protoxide of Silver changes to a red substance, reacting with chemical tests in the same manner as Wohier's Citrate of suboxide of Silver, which he ob- tained by reducing the ordinary Citrate in Hydrogen Gas. Glycyrrhizin, the Sugar of Liquorice, also forms a white compound with Oxide of Silver which becomes brown or red in the sun's rays.^ SIMPLE EXPERIMENTS ILLUSTRATING THE ACTION OF LIGHT UPON A SENSITIVE LAYER OF CHLORIDE OF SILVER ON PAPER. In the performance of the most simple experiments on the decomposition of Silver Salts by Light, the student may employ ordinary test-tubes, in which small quantities of the two liquids required for the double decomposition may be mixed together. When however concentrated solutions are used in this * For further particulars on the action of light upon the Salts of Silver associated with organic matter, see the Author's paper on the composition of the photographic image, in the eighth Chapter. 22 THE SALTS OF SILVER way, tlie insoluble Silver Salt falls in dense and clotted masses, which, exposed to the sun's rays, quickly blacken on the exterior, but the inside is protected, and remains white. It is of importance therefore in Photography that the sensitive material should exist in the form of a sitrface, in order that the various particles of which it is composed may each one individually be brought into relation with the disturbing force. Eull directions for the preparation of sensitive Photo- graphic paper are given in the second division of this work. The following is the theory of the process : — A sheet of paper is treated with solution of Chloride of Sodium or Ammonium, and subsequently with Nitrate of Silver; hence results a formation of Chloride of Silver in a fine state of division, with an excess of JSTitrate of Silver, the Silver bath having been purposely made stronger in pro- portion than the salting solution. Illustrative Experiment No. I. — Place a square of sen- sitive paper (prepared according to the directions given in the Second Part of the work) in the direct rays of the sun, and observe the gradual process of darkening which takes place ; the surface passes through a variety of changes in colour until it becomes of a deep chocolate-brown. If the Light is tolerably intense, the brown shades are pro- bably reached in from three to five minutes ; but the sen- sibility of the paper, and also the nature of the tints, will vary much with the character of the organic matter pre- sent. Experiment No. II. — Lay a device cut from black paper upon a sheet of sensitive paper, and compress the two to- gether by means of a sheet of glass. After a proper length of exposure the figure will be exactly copied, the tint however being reversed : the black paper protecting the sensitive Chloride beneath, produces a loJiite figure upon a dark ground. Experiment No. III. — Hepeat the last experiment, sub- stituting a piece of lace or gauze-wire for the paper device. EMPLOYED IN PHOTOGRAPHY. 23 This is intended to sliow the minuteness with which objects can be copied, since the smallest filament will be distinctly represented. JExperiment No. IV. — Take an engraving in which the contrast of light and shade is tolerably well marked, and having laid it closely in contact with the sensitive paper, expose as before. This experiment shows that the surface darkens in degrees proportionate to the intensity of the light, so that the half shadows of the engraving are ac- curately maintained, and a pleasing gradation of tone pro- duced. In the darkening of Photographic papers, the action of the light is quite superficial, and although the black colour may be intense, yet the amount of reduced Silver which forms it is so small that it cannot conveniently be estima- ted by chemical reagents. This is well shown by the re- sults of an analysis performed by the Author, in which the total weight of Silver obtained from a blackened sheet measuring nearly 24 by 18 inches amounted to less than half a grain. It becomes therefore of great importance in preparing sensitive paper to attend to the condition of the surface layer of particles, the action rarely extending to those beneath. The use of Albumen, Gelatine, etc., which will be explained in the eighth Chapter, has refer- ence to this amongst other advantages, and secures a better and more sharply defined print. 24 CHAPTER III. OT^ THE DEYELOPMEIS^T OF A'N INYISIBLE IMAGE ET MEAIVS OF A EEDUCIFG AGENT. It lias been shown in the previous Chapter that the ma- jority of the Salts of Silver, both organic and inorganic, are darkened in colour on exposure to light, and, by the loss of Oxygen, Chlorine, etc., become reduced to the conditions of Suhsnlts. Many of the same compounds are also susceptible of a change under the influence of light, which is even more remarkable. This change takes place after a comparatively short exposure, and as it does not affect the appearance of the sensitive layer, for some time it escaped notice : but it was afterwards discovered that an impression, be- fore invisible, might be brought out by treating the plate with certain chemical agents which are without effect on the original unchanged salt, but quickly blacken it after exposure. It is a remarkable fact that the Silver compounds most readily affected by light alone, are not the most sensitive to the reception of the invisible image. Thus, of Photo- graphic papers prepared with Chloride, Bromide, or Iodide of Silver, the former assume the deepest shade of colour under the influence of the sun's rays, but if all be exposed momentarily, and then removed, the greatest amount of DEVELOPMENT OF AN INVISIBLE IMAGE. 25 effect will be developed upon the Iodide paper. Iodide of Silver therefore is the salt commonly used when sensi- bility is an object, but it should be noted that images nearly or quite latent can be impressed upon many other of the compounds of Silver, including those belonging to the animal and vegetable kingdoms. Experiments illustrating tlie Formation of an Invisible Image. — Take a sheet of sensitive paper, prepared with Iodide of Silver by the method given in the fourth Chapter of Part II., and having divided it into two parts, expose one of them to the luminous rays for a few seconds. ISTo visible decomposition takes place, but on removing the pieces to a room dimly illuminated, and brushing with a solution of Gallic Acid, a manifest difference will be ob- served ; the one being unaffected, whilst the other darkens gradually until it becomes black. Experiment II. — A prepared sheet is shielded in certain parts by an opaque substance, and then after the requisite exposure, which is easily ascertained by a few trials, treated with the Gallic Acid as before ; in this case the protected part remains white, whilst the other darkens to a greater or less extent. In the same way, copies of leaves, engravings, etc. may be made, very correct in the shading and much resembling those produced by the prolonged action of light alone upon the Chloride of Silver. The object of employing a substance like G-allie Acid to develope or bring out to view an invisible image, in prefer- ence to forming the picture by the direct action of light, unassisted by a developer, is the economy of time thereby effected. This is well shown in the results of some ex- periments conducted by M. Claudet in the Daguerreotype process : he found that with a sensitive layer of Bromo- lodide of Silver, an intensity of light three thousand times greater was required if the use of a developer was omitted, and the exposure continued until the picture became visible upon the plate. 26 ON THE DEVELOPMENT OF To increase tlie sensitiveness of Photographic prepara- tions is a point of great consequence ; and indeed, when the Camera is used, from the low intensity of the luminous image formed in that instrument, no other plan than the one above described would be practicable. Hence the ad- vancement, and indeed the very origin, of the Photographic Art, may be dated from the first discovery of a process for bringing out to view an invisible image by means of a re- ducing agent. The present Chapter is divided into three Sections: — first, the chemical properties of the substances usually employed as developers ; — second, their mode of action in reducing the Salts of Silver ; — third, hypotheses on the action of light in impressing a latent image. SECTION I. Chemistry of the various Substances employed as Developers. Development is essentially a process of reduction, or, in other words, of deoxidation. If we take a certain metal, we can, by means of Nitric Acid, impart Oxygen to it, so that it becomes first an Oxide, and afterwards, by solution of the Oxide in the excess of acid, a salt. When this salt is formed, by a series of chemical operations the reverse of the former it may be deprived of all its Oxygen, and the metallic element again isolated. The degree of facility with which oxidation as well as reduction is performed, depends upon the affinity for Oxy- gen which the particular metal under treatment possesses. In this respect there is considerable difierence, as may be shown by a reference to the two well-known metals, Iron and Grold. How speedily does the first become tarnished and covered with rust, whilst the other remains bright even in the fire ! It is indeed possible, by a careful process, to form Oxide of Gold ; but it retains its Oxygen so loosely AN INVISIBLE IMAGE. 27 that the mere application of heat is sufficient to drive it off, and leave the metal in a pure state. Silver, Gold, and Platinum all belonsr to the class of nohle metals, having the least affinity for Oxygen : hence their Oxides are unstable, and any body tending strongly to absorb Oxygen will reduce them to the metallic state. Observe, therefore, that the substances employed by the Photographer to assist the action of the light, and to deve- lope the picture, act by removing Oxygen. The sensitive Salt of Silver is thus reduced, more or less completely, in the parts touched by light, and an opaque deposit results which forms the image. ^ The most important of the developers are as follows : — Gallic Acid, Pyrogallic Acid, and the Pro/^osalts of Iron. CHEMISTEY OF GALLIC AND PYEOGALLIC ACIDS. a. Of Gallic Acid. — Gallic Acid is obtained from Gall Nuts, which are peculiar excrescences formed upon the branches and shoots of the Quercus infectoria by the punc- ture of a species of insect. The best kind is imported from Turkey, and sold in commerce as Aleppo Galls. Gall Nuts do not contain Gallic Acid ready formed, but an ana- logous chemical principle termed Tannic Acid, well known for its astringent properties and employment in the process of tanning raw hides. Gallic Acid is produced by the decomposition and oxi- dation of Tannic Acid when powdered galls are exposed for a long time in a moist state to the action of the air. By boiling the mass with water and filtering whilst hot, the acid is extracted, and crystallizes on cooling, on ac- count of its sparing solubility in cold water. Gallic Acid occurs in the form of long silky needles, soluble in 100 parts of cold and 3 of boiling water ; they are also readily soluble in Alcohol, but sparingly in Ether. * These remarks do not apply to the vapour of Mercury employed as a developing agent in the Daguerreotype. The chemistry of that process will be explained in a separate Chapter. 28 ON THE DEVELOPMENT OF TKe aqueous solution becomes mouldy on keeping, to ob- viate which, the addition of Acetic Acid or a drop or two of Oil of Cloves is recommended. Gallic Acid is a feeble acid, scarcely reddening litmus ; it forms salts with the alkaline and earthy bases, such as Potash, Lime, etc., but not with the oxides of the noble metals. When added to Oxide of Silver the metallic ele- ment is separated and the Oxygen absorbed.' b. Pyrogallic Acid. — The term pyro prefixed to Gallic Acid implies that the new substance is obtained by the action of heat upon that body. At a temperature of about 410° Fahr., Gallic Acid is decomposed, and a white subli- mate forms, which condenses in lamellar crystals ; this is Pyrogallic Acid. Pyrogallic Acid is very soluble in cold water, and in Al- cohol and Ether ; the solution decomposes and becomes brown by exposure to the air. It gives an indigo blue colour with Protosulphate of Iron, which changes to dark green if any Persulphate be present. Although termed an acid, this substance is strictly neu- tral ; it does not redden litmus-paper, and forms no salts. The addition of Potash or Soda decomposes Pyrogallic Acid, at the same time increasing the attraction for Oxy- gen; hence this mixture may conveniently be employed for absorbing the Oxygen contained in atmospheric air. The compounds of Silver and Gold are reduced by Pyro- gallic Acid even more rapidly than by Gallic Acid, the re- ducing agent absorbing the Oxygen, and becoming con- verted into Carbonic Acid and a brown matter insoluble in water. Commercial Pyrogallic Acid is often contaminated with empyreumatic oil, and also with a black insoluble sub- stance known as Metagallic Acid, which is formed when the heat is raised above the proper temperature in the pro- cess of manufacture. AN INVISIBLE IMAGE. 29 CHEMISTRY OF THE PROTOSALTS OF IKON. The combinations of Iron with Oxygen are somewhat numerous. There are two distinct Oxides which form Salts, viz. the Protoxide of Iron, containing an atom of Oxygen to one of metal ; and the Peroxide, with an atom and a half of Oxygen to one of metal. As half atoms however are not allowed in chemical language, it is usual to say that the Peroxide of Iron contains three equivalents of Oxygen to two of metallic Iron. Expressed in symbols, the composition is as follows : — Protoxide of Iron, Pe O. * Peroxide of Iron, FcgOg. The Proto- and Persalts of Iron do not resemble each other in their physical and chemical properties. The for- mer are usually of an apple-green colour, and the aqueous solutions almost colourless, if not highly concentrated. The latter, on the other hand, are dark, and give a yellow or even blood-red solution. The Protosalts of Iron are alone useful in Photography ; but the following experiment will serve to illustrate the properties of both classes of salts : — Take a crystal of Pro- tosulphate of Iron, and, having reduced it to powder, pour a little Nitric Acid upon it in a test-tube. On the appli- cation of heat, abundance of fumes will be given off, and a red solution obtained. The Nitric Acid in this reaction imparts Oxygen, and converts the Pro^osulphate entirely into a Persulphate of Iron. It is this feature, viz. the tendency to absorb Oxygen, and to pass into the state of Persalts, which makes the Protosalts of Iron useful as developers. There are two Protosalts of Iron commonly employed by Photographers : the Protosulphate and the Protonitrate of Iron. a. JProtosulplia te of Iron. — This salt, often termed Cop- peras or Green Vitriol, is an abundant substance, and used for a varietv of purposes in the arts. Commercial 30 ON THE DEVELOPMENT OF Sulphate of Iron however, being prepared on a large scale, requires recrystallization to render it sufficiently pure for Photographic purposes. Pure Sulphate of Iron occurs in the form of large trans- parent, prismatic crystals, of a delicate green colour : by exposure to the air they gradually absorb Oxygen and be- come rusty on the surface. Solution of Sulphate of Iron, colourless at first, afterwards changes to a red tint, and deposits a brown powder ; this powder is a basic Persul- phate of Iron, that is, a Persulphate containing an excess of the oxide or base. By the addition of Sulphuric or Acetic Acid to the solution, the formation of a deposit is prevented, the brown powder being soluble in acid liquids. The Crystals of Sulphate of Iron include a large quan- tity of water of crystallization, a part of which they lose by exposure to dry air. By a higher temperature, the salt may be rendered perfectly anhydrous, in which state it forms a white powder. b. Protonitrate of Iron. — This salt is prepared by double decomposition between Nitrate of Baryta or of Lead and Protosulphate of Iron. It is an unstable substance and crystallizes w^ith great difficulty ; its aqueous solution is pale green at first, but very prone to decomposition, even more so than the corresponding Sulphate of Iron. SECTION II. TJie Reduction of Salts of Silver hy Developing Agents. The general theory of the reduction of metallic oxides having been explained, it may be desirable to enter more minutely into the exact nature of the process as applied to the compounds of Silver. First, the Reduction of the Oxide of Silver will be taken, as the most simple illustration; then that of Salts of Silver formed by Oxygen-acids ; and lastly, of the Chloride, Iodide, and Bromide of Silver containing no Oxygen. AN INVISIBLE IMAGE. 31 Reduction of Oxide of Silver. — To illustrate this conve- niently, the Oxide of Silver should be in a state of solution ; water dissolves Oxide of Silver very sparingly, but it is freely soluble in Ammonia, forming the liquid known as Ammonio-Nitrate of Silver. If, therefore, a little of the Ammonio-JN^itrate of Silver be placed in a test-tube, and solution of Sulphate of Iron be added to it, immediately it becomes discoloured, and a deposit settles to the bottom. This deposit is metallic Silver, produced by the redu- cing agent appropriating to itself the Oxygen previously combined with the metal. As metallic Silver does not dissolve in Ammonia, the liquid becomes turbid, and the metal subsides in the form of a bulky precipitate. deduction of the Oxyacid Salts of Silver. — The term Oxyacid includes those salts which contain the Oxide of Silver intimately combined with Oxygen-acids ; as e. g. the Nitrate of Silver, the Sulphate, the Acetate of Silver, etc. These salts, soluble in water, are reduced by developing agents in the same manner as Oxide of Silver, but more slowly. The presence of an acid united with the base is a hindrance to the process and tends to keep the oxide in solution, especially when that acid is powerful in its affini- ties. To illustrate the effect of the acid constituent of the salt in retarding reduction, take two test-tubes, the one containing Ammonio-Nitrate, and the other ordinary Nitrate of Silver — a single drop of solution of Sulphate of Iron added to each will indicate an evident difference in the rapidity of deposition. The precipitate of metallic Silver obtained by the action of reducing agents upon the Nitrate, varies much in colour and in general appearance. If Grallic or Pyrogallic Acid be employed, it is a black powder whilst the salts of Iron, and especially the same with free Nitric Acid add- * Silver precipitated by Gallic or Pyrogallic Acid does not appear to be free from organic matter, and x^robably contains also a small proportion of Oxygen. 32 ON THE DEVELOPMENT OF ed, produce a sparkling precipitate, resembling wliat is termed frosted silver. Grape Sugar and many of the es- sential oils, such as the Oil of Cloves, etc., separate the metal from Ammonio-JSTitrate of Silver in the form of a brilliant mirror film, and are often employed in silyering glass. In remarking upon these peculiarities in the molecular condition of precipitated Silver, it should be observed that the appearance of a metal whilst in mass is no indication of its colour when in the state of fine powder. Platinum and Iron, both bright metals, and susceptible of a high polish, are dull and intensely black when in a fine state of division ; Gold is of a purple or yellowish brown ; Mercury a dirty grey. Reduction of the Hydracid Salts of Silver. — By the term Sydracid is meant Salts of Silver which contain no Oxygen or Oxygen-acids, but simply elements like Chlorine or Iodine combined with Silver. These elements are charac- terized by forming acids with Hydrogen, which acids are hence called SydrdiCidi^. Hydrochloric Acid (HCl) is an example ; so also is Hydriodic Acid (HI). The reduction of the Hydracid Salts requires to be dis- cussed separately, because it is evidently difierent from that already described ; the reducing agent tending only to absorb Oxygen, which is not present in these salts. The explanation is as follows : "When a Chloride of a noble metal is reduced by a developer, an atom of water, com- posed of Oxygen and Hydrogen, takes a part in the re- action. The Oxygen of the water passes to the developer, the Hydrogen to the Chlorine. To illustrate this, take a solution of Chloride of Gold, and add to it a little Sulphate of Iron. A yellow deposit of metallic Gold soon forms, and the supernatant liquid is found, by testing, to be acid from free Hydrochloric Acid. The following simple diagram, in which however the num- ber of the atoms concerned is omitted, may assist the com- prehension of the change. AN INVISIBLE IMAGE. 33 Compound Atom of Compound Atom Atom of Chloride of Gold. of Water. Sulphate of Iron. The symbol Au represents Gold, CI Chlorine, H Hy- drogen, and O Oxygen. Observe that the molecules H and O separate from each other and pass in opposite di- rections : the latter unites with the Sulphate of Iron ; the former meets CI, and produces Hydrochloric Acid (HCl), whilst the atom of Gold is left alone. Hence there is no theoretical difficulty in supposing a reduction of Iodide of Silver by a developer, if we asso- ciate with the Iodide an atom of water to furnish the Oxy- gen. Unless the sensitive plate however has been exposed to the light, the reduction does not readily take place ; nor can it be produced under any circumstances, with or with- out light, when the whole of the free Nitrate of Silver has been washed away from the plate. Pure Iodide of Silver is therefore unaffected by a developer, and tlie compound which blackens on the application of Sulphate of Iron or Pyrogallic acid is an Iodide with excess of Nitrate of Silver. The mode in which a Salt of Silver, such as the Nitrate, Compound Atom of Compound Atom of Atom of Iodide of Silver. Nitrate of Silver. Sulphate of Iroa. 34 ON THE DEVELOPMENT OF soluble in water, may act in facilitating the reduction of ^ Iodide of Silver, is shown in the preceding diagram, which corresponds closely with the last. Notice that the compound atom of ^^itrate of Silver contains a molecule of Oxygen for the developer, one of Silver (Ag) for the separated Iodine, and an atom of Ni- tric Acid (NO5), which is liberated, and takes no further part in the change. The chain of chemical affinities is more complete in this diagram than in the last, where an atom of water only was present, the affinity of Iodine for Silver being greater than that of Iodine for Hydrogen. Hence it is possible that an excess of Nitrate of Silver may, by furnishing an ele- mentary basis for which Iodine has an attraction, assist in drawing off that element, so to speak, from the original particle of Iodide of Silver touched by light.* SECTION III. The formation and development of the Latent Image. It was shown in the second Chapter that the continued action of white light upon certain of the Salts of Silver re- sulted in the separation of elements like Chlorine and Oxy- gen and the partial reduction of the compound. We have also seen that bodies possessing affinity for Oxygen, such as Sulphate of Iron and Pyrogallic Acid, tend to produce a similar effect ; acting in some cases with great energy and precipitating metallic Silver in a pure state. In forming an extemporaneous theory on the production * The reader must not suppose from the remarks which have been made in this Section that images obtained by development consist invariably of pure metallic Silver. It can be shown that such is not the case, — that the process of reduction is in many cases suspended when a part only of the Oxygen has been removed ; and hence results a subsali similar to that pro- duced by the direct action of light upon organic compounds of Silver, and differing in properties from metallic Silver. For further particulars see the Author's Photographic researches in the eighth Chapter. AN INVISIBLE IMAGE. 35 of the latent image in the Camera, it would therefore be natural to suppose that the process consisted in setting up a reducing action upon the sensitive surface by means of light, afterwards to be continued by the application of the developing solution. This idea is to a certain extent cor- rect, but it requires some explanation. The effects pro- duced by the light and the developer are not so precisely similar that the one agency can always be substituted for the other : an insufficient exposure in the Camera cannot be remedied by prolonging the development of the image. In the Photographic processes on paper it is indeed found that a certain latitude may be allowed ; but, as a rule, it should be stated that a definite time is occupied in the formation of the invisible image, which may not be short- ened or extended beyond its proper limits with impunity. There is a maximum point beyond which no advance is made ; hence if the plate be not then removed from the Camera, those portions of the image formed by the bright- est Hghts are speedily overtaken by the "half tones," so that, on developing, an image appears without that contrast between lights and shadows which is essential to the ar- tistic effect. On the other hand, in a case of insufficient exposure, the feeble rays of light not having been allowed time to impress the plate, the half shadows cannot be brought out on subsequent treatment with the developing agent. A careful study of the phenomena involved in this part of the process cannot fail to show that the ray of Light determines a molecular change of some kind in the par- ticles of Iodide of Silver forming the sensitive surface. This change is not of a nature to alter the composition or the chemical properties of the salt. The Iodine does not leave the surface, or there would be a difference in the ap- pearance of the film, or in its solubility in Hyposulphite of Soda. The following diagrams may perhaps be useful in me- chanically illustrating what is meant by a molecular change. Fig. 1 represents a compound molecule of Iodide of 36 ON THE DEVELOPMENT OF Silver, the component atoms of wliich are closely asso- ciated. Fig. 2. The same after the action of a disturbing force. The simple molecules have not altogether separated, but they are prepared to do so, touching only at a single point. Fig. 1. Fig. 3. Now the effect produced on this combination by a de- veloper is understood, if we suppose that in the first case the affinity of the Iodine for Silver is too great to allow of its separation ; but in the second, this affinity having been loosened, the structure gives way, and metallic Silver is the result. This hypothesis has the merit of simplicity, and is not opposed to known facts ; it may therefore for the present be received. The point however on which a doubt must rest is — whether the molecular disturbance produced by light upon Iodide of Silver leads to a reduction of that Salt by the developer. No image can be produced on the application of Pyrogallic Acid unless the ]p articles of Iodide are in contact with Nitrate of Silver ; and hence it may be the Nitrate and not the Iodide which is reduced — that is, the impressed molecule of Iodide may determine the decomposition of a contiguous particle of Nitrate, itself remaining unchanged. This view is supported to some extent by Moser's experiments, shortly to be quoted ; and also by the fact that the delicate image first formed can be intensified by treating it with a mixture of the deve- loping solution and Nitrate of Silver, even after the Iodide has been removed by a fixing agent. The following experiment will serve to illustrate this. — AN INVISIBLE IMAGE. 37 Take a sensitive Collodion plate, and having impressed an invisible image upon it by a proper exposure in the Camera, remove it to the dark room, and pour over it the solution of Pyrogallic Acid. When the picture has fully appeared, stop the action by washing the plate with water, and remove the unaltered Iodide of Silver by Cyanide of Potassium. An examination of the image at this stage will show that it is perfect in the details, but pale and translucent. The plate is then to be taken back again to the dark room and treated with fresh Pyrogallic Acid, to wJiich Nitrate of Silver has been added ; immediately the picture becomes much blacker, and continues to darken, even to complete opacity, if the supply of Nitrate be kept up. Now in this experiment it is evident that the additional deposit upon the image is produced from the Nitrate of Silver, the whole of the Iodide having been previously re- moved. Observe also, that it forms only upon the image, and not upon the transparent parts of the plate. Even if the Iodide, untouched by light, be allowed to remain, the same rule holds good ; — the Pyrogallic Acid and Nitrate of Silver react upon each other and produce a metallic de- posit ; this deposit however has no affinity for the unal- tered Iodide upon the part of the plate corresponding to the shadows of the picture, but attaches itself in preference to the Iodide already blackened by light. This second stage of the development, by which a feeble image may be strengthened and rendered more opaque, is sometimes termed "development by precipitation," and should be correctly understood by the practical operator. Researches of M. Moser. — The papers of M. Ludwig Moser * On the Formation and Development of Invisible Images,' published in 1842, explain so clearly many re- markable phenomena of occasional occurrence in the Col- lodion and paper processes, that no apology need be offered for referring to them somewhat at length. His first proposition may be stated thus : — " If a po- 38 ON THE DEVELOPMENT OF lisked surface has been touclied in particular parts by any- body, it acquires the property of precipitating certain va- pours on these spots differently to what it does on the other untouched parts." To illustrate this, take a thin plate of metal, having characters excised ; warm it gently, and lay it upon the surface of a clean mirror glass for a few mi- nutes : then remove, allow to cool, and breathe upon the glass, when the outlines of the device will be distinctly seen. A plate of poHshed Silver may be substituted for the glass, and in place of developing the image by the breath, it may be brought out by Mercurial vapour. The second proposition of M. Moser is as follows: — " Light acts on bodies, and its influence may be tested by vapours that adhere to the substance." — A plate of mirror glass is exposed in the Camera to a bright and intense light; it is then removed and breathed upon, when an image before invisible will be developed, the breath set- tling most strongly upon the parts where the light has acted. A plate of polished Silver may be used as before instead of glass, the vapour of Mercury or of water being employed to develope the image. An iodized Silver plate is still more sensitive to the influence of the light, and re- ceives a very sharp and perfect impression under the ac- tion of the Mercury. It seems therefore from these experiments and others not quoted, that the surfaces of various bodies are capable of being modified by contact with each other, or by con- tact with a ray of light, in such a way as to impart an affinity for a vapour ; and further, that many of the Salts of Silver are in the list of substances admitting of such modification. But it is also evident that the same condi- tion of surface which causes a vapour to settle in a pecu- liar manner also affects the behaviour of the Silver Salt when treated with a reducing agent. Thus, if a clean glass plate be touched in certain spots by the warm finger, the impression soon disappears, but is again seen on breathing upon the glass ; and if this same plate be coated with a AN INVISIBLE IMAGE. 39 very delicate layer of Iodized Collodion and passed tlirougli the Nitrate bath, the solution of Pyrogallic acid will com- monly produce a well-defined outline of the figure even before the plate has been exposed to the light. This ex- periment, although it does not invariably succeed, is ne- vertheless an instructive one, and shows the necessity of cleaning the plates used in Photography with care. If there be any irregularity in the manner in which the breath settles upon the glass when it is breathed on, a condition of surface exists at that point which will pro- bably so modify the layer of Iodide of Silver, that the ac- tion of the developing fluid will be in some way interfered with. One more remarkable fact observed by M. Moser may be quoted. He finds that the action of light upon the Da- guerreotype plate is of an alternating kind : it first gives an affinity for Mercury, and then removes it. " If light acts on Iodide of Silver," he says, "it imparts to it the power of condensing mercurial vapours ; but if it acts be- yond a certain time, it then diminishes this power and at length takes it away altogether." This is precisely in ac- cordance with phenomena observed also in the Collodion process, where the deposit of metallic Silver is sometime less marked than usual if the plate has been exposed in the Camera beyond the proper period of time, A curious perversion of the developing process is occa- sionally met with, in which on the application of the Py- rogallic Acid, the deposit of Silver takes place upon the shadows of the picture, and not upon the lights ; hence on viewing the image by transmitted light, the usual appear- ance is reversed. This may perhaps be explained by an alternating action of the light as above suggested. A phenomenon at first sight even more remarkable has occurred, in which, on developing the plate, two images start out instead of one. The secondary image in such a ease is probably the remains of a previous impression wJiich, although apparently removed by washing, had 40 DEVELOPMENT OF AN INVISIBLE IMAGE. nevertheless modified the surface of the glass so as to aflPect the layer of Iodide of Silver ; and if the glass were hreatlied upon before again coating it with Collodion, there is every reason to suppose that the outlines of the acci- dental image would be seen.* * Since writing the above, the Author has perused with pleasure a paper by Mr. Grove on the production of latent images by electricity, with a mode of fixing them. In the experiments described, a plate of glass, electrized in certain portions only, was breathed upon, or exposed to the fumes of Hydro- fluoric Acid. In either case the vapour settled exclusively upon the non- electrical part of the glass, thus developing a latent image. When the plate was first submitted to electrization, and then coated with Iodide of Silver iipon Collodion, and exposed to light, — solution of Pyrogallic Acid produced a reduction of Silver only upon the parts of the glass corresponding to those on which the breath settled in the previous experiment ; thus indicating that the electricity neutralized the effect of light upon the sensitive Iodide of Silver. 41 CHAPTER IV. 01^ FIXT1S-& THE PHOTO GRAPHIC IMAGE. A SENSITIVE layer of Chloride or Iodide of Silver on wliich an image lias been formed, either with or with- out the aid of a developing agent, must pass through further treatment in order to render it indestructible by diffused light. It is true that the image itself is sufficiently permanent, and cannot be said, in correct language, to need fixing ; but the unchanged Silver Salt which surrounds it, being still sensitive to light, tends to be decomposed in its turn, and so the picture is lost. It is therefore necessary to amove this salt by applying some chemical agent capable of dissolving it. The list of solvents of Chloride and Iodide of Silver has been given in Chapter II., but some are better adapted for fixing than others. In order that any body may be employed with success as a fixing agent, it is required not only that it should dissolve unchanged Chloride or Iodide of Silver, but that it should produce no injurious effect upon the same salts reduced by light. This solvent action u^on the image, as well as upon the parts which surround it, is most liable to happen when the agency of light alone, without a developer, has been em- ployed. In that case the darkened surface, not being reduced perfectly to the metaUic state, remains soluble to a certain extent in the fixing liquid. 42 ON FIXING THE PHOTOGEAPHIC IMAGE. CHEMISTEY OP THE VAEIOUS PIXING AGENTS. The following will be mentioned : — Ammonia — Alkaline Ctilorides — Alkaline Iodides — Alkaline Hyposulphites — Alkaline Cyanides. AMMONIA. The properties of the alkaline liquid " Ammonia" are given in Part III. Ammonia dissolves Chloride of Silver readily, but not Iodide of Silver : hence its use is neces- sarily confined to the paper proofs upon Chloride of Silver. Even these however cannot advantageously be fixed in Ammonia unless a deposit of Gold has been previously produced upon the surface by a process of " toning," pre- sently to be explained : a peculiar and unpleasant red tint is always caused by Ammonia acting upon the dark- ened material of a sun picture as it comes from the print- ing-frame : but this is obviated by the employment of the Gold. ALKALINE CHLOEIDES, IODIDES, AND BEOMIDES. The Chlorides of Potassium, Ammonium, and Sodium possess the property of dissolving a small portion of Chlo- ride of Silver. In the act of solution a double salt is formed ; that is, a compound of Chloride of Sodium with Chloride of Silver, which may be crystallized out by al- lowing the liquid to evaporate spontaneously. The earlier Photographers employed a saturated solu- tion of common Salt for fixing paper prints ; but the fixing action of the Alkaline Chlorides is slow and imperfect, and their use may now be said to be obsolete. The Iodide and Bromide of Potassium have both been used as fixing agents. They dissolve Iodide of Silver, forming with it a double salt in the manner before de- scribed. It is important to remark in the solution of the insolu- ble Silver Salts by Alkaline Clilorides, Iodides, etc., that ON FIXING THE PHOTOGEAPHIC IMAGE. 43 the amount dissolved is not in proportion to the quantity of the solvent, but to the degree of concentration of its aqueous solution. This is not usual with solvents which act by entering into chemical combination with the sub- stance dissolved. Commonly a given weight of the one salt dissolves a given weight of the other, independent of the amount of water present. The peculiarity in the case before us depends upon the fact that the double salt formed is decomposed by a large quantity of water. Hence it is a saturated solution of Chloride of Sodium which possesses the greatest power of fixing paper prints ; and with the Bromide or Iodide of Potassium the same rule holds good — the stronger the solution the more Iodide of Silver will be taken up. The addition of water pro- duces milkiness and a deposit of the silver Salt previously dissolved. ALKALINE HYPOSULPHITES. Hyposulphurous Acid is one of the Oxides of Sulphur. It is, as its name implies, of an acid nature, and takes its place upon the list immediately below Sulphurous Acid (" upoy' under). The Hyposulphite of Soda commonly employed by Pho- tographers is a neutral combination of Hyposulphurous Acid and the alkali Soda. It is selected as being more economical in preparation than any other Hyposulphite adapted for fixing. Hyposulphite of Soda occurs in the form of large trans- lucent groups of crystals, which include five atoms of water. These crystals are soluble in water almost to any extent, the solution being attended with the production of cold ; they have a nauseous and bitter taste. In the solution of Silver compounds by Hyposulphite of Soda a double decom;position always takes place ; thus : — Hyposulphite of Soda + Chloride of Silver = Hyposulphite of Silver -j- Chloride of Sodium. 44 ON FIXING THE PHOTOGEAPHIC IMAGE. The Hyposulphite of Silver with an excess of Hypo- sulphite of Soda forms a soluble double salt, which may be crystallized out by evaporating the solution. It pos- sesses an intensely sweet taste, and contains one atom of Hyposulphite of Silver, chemically combined with two of Hyposulphite of Soda. In addition to this there is a second double Salt, differing from the first in being very sparingly soluble in water. It is formed by acting upon Chloride of Silver with a solution of Hyposulphite of Soda already saturated, or nearly so, with Silver Salts ; and contains single atoms of each constituent. The fact that the Silver contained in an ordinary fixing Bath is present in the state of Hyposulphite must be borne in mind, because this salt is liable to undergo pe- culiar chemical changes, as will be better shown in Chap- ter YIII. Iodide of Silver is dissolved by Hyposulphite of Soda more slowly than Chloride of Silver, and the amount eventually taken up is less. This is explained as follows : — During the solution of Iodide of Silver, Iodide of Sodium is formed, and this alkaline Iodide has a prejudicial effect upon the continuance of the process. Chloride of Sodium has not the same action, neither has Bromide of Sodium, consequently the corresponding Silver Salts dissolve to a greater extent than the Iodide. ALKALINE CYANIDES. The chemistry of Cyanogen is sketched in Part III» The Cyanide of Fotassium is the salt most frequently employed in fixing. It occurs in commerce in the form of fused lumps of considerable size. In this state it is usually contaminated with a large percentage of Carbonate of Potash, amounting in some cases to more than half its weight. By boiling in proof Spirit the Cyanide may be extracted and crystallized, but this operation is scarcely required as far as its use in Photography is concerned. Cyanide of Potassium absorbs moisture on exposure to ON FIXING THE PHOTOGEAPHIC IMAGE. 45 the air. It is very soluble in water, but the solution de- composes on keeping ; changing in colour and evolving the odour of Prussic Acid, which is a Cyanide of Hydrogen. Cyanide of Potassium is highly poisonous, and must be used with caution. Solution of Cyanide of Potassium is a most energetic agent in dissolving the insoluble Silver Salts : far more so, in proportion to the quantity used, than the Hyposulphite of Soda. The Salts are in all cases converted into Cyanides, and exist in the solution in the form of soluble double Salts, which, unlike the double Iodides, are not affected by dilution with water. Cyanide of Potassium is unadapted for fixing positive proofs upon Chloride of Silver ; and even when a developer has been used, unless the solution is tolerably dilute, it is apt to attack the image and dis- solve it. 46 CHAPTER V. ON THE NATUEE AND PEOPEETIES OP LIGHT. The present Chapter is devoted to a discussion of tlie more remarkable properties of Liglit ; the object being to select certain prominent points, and to state them as clearly as possible, referring, for information of a more complete Mnd, to acknowledged works on the subject of Optics. The Chapter will be divided into five Sections : — first, the compound nature of Light ; second, the laws of refrac- tion of Light; third, the construction of Lenses and of the Camera ; fourth, the Photographic action of coloured Light ; fifth, on Binocular Vision and the Stereoscope. SECTION 1. Tlie Compound Nature of Light. The ideas entertained on the subject of Light, before the time of Sir Isaac Newton, were vague and unsatisfactory. It was shown by that eminent philosopher, that a ray of sunlight was not homogeneous, as had been supposed, but consisted of several rays of vivid colours, united and in- termingled. This fact may be demonstrated by throwing a pencil of Sunlight upon one angle of a jprism, and receiving the ob- long image, so formed, upon a white screen. The space illuminated and coloured by a pencil of rays NATUEE AND PEOPEETIES OF LIGHT. 47 analyzed in this way is called " the Solar Spectrum." The action of a prism in decomposing white light will be more fully explained in the next Section. At present we notice only that seven principal colours may be distinguished in i Violet. Indigo. Orange. Eed. the Solar Spectrum, viz. red, orange, yellow, green, blue, indigo, and violet. Sir David Brewster has made observa- tions which lead him to suppose that the primary colours are in reality but three in number, viz. red, yellow, and blue, and that the others are compound, being produced by two or more of these overlapping each other ; thus the red and yeUow spaces intermingled constitute orange ; the yellow and blue spaces, green. The composition of white light from the seven prismatic colours may be roughly proved by painting them on the face of a wheel, and causing it to rotate rapidly ; this blends them together, and a sort of greyish- white is the result. The white is imperfect, because the colours em- ployed cannot possibly be obtained of the proper tints or laid on in the exact proportions. The decomposition of light is effected in other ways be- sides that already given : — First, hj reflection horn the surfaces of coloured bodies. All substances throw off rays of light, which impinge upon the retina of the eye and produce the phenomena of vision. Colour is caused by a portion only, and not the whole, of the elementary rays, being projected in this way. Surfaces 48 NATURE AND PROPERTIES OF LIGHT. termed white reflect all tlie rays ; coloured surfaces absorb some and reflect others : thus red substances reflect only- red rays, yellow substances, yellow rays, etc, the ray which is reflected in all cases deciding the colour of the substance. Secondly, light may be decomposed by transmission through media which are transparent to certain rays, but opaque to others. Ordinary transparent glass allows all the rays constitu- ting white light to pass ; but by the addition of certain metallic oxides to it whilst in a state of fusion, its proper- ties are modified, and it becomes coloured. Glass stained by Oxide of Cobalt is permeable only to blue rays. Oxide of Silver imparts a pure yellow tint; Oxide of Gold or Suboxide of Copper a ruby red, etc. DIVISION OF THE ELEMENTARY RAYS OF WHITE LIGHT INTO LUMINOUS, HEAT-PRODUCING, AND CHEMICAL. RAYS. The agency of Light produces a variety of distinct effects upon the bodies which surround us. These may be classed together as the properties of light. They are of three kinds — the phenomena of colour and vision, of heat, and of chemical action. By resolving white light into its constituent rays, we find that these properties are associated each one with cer- tain of the elementary colours. The yellow is decidedly the most luminous ray. On ex- amining the Solar Spectrum, it is seen that the brightest part is that occupied by the yellow, and that the light di- minishes rapidly on either side. So again, rooms glazed with yellow glass always appear abundantly illuminated, whilst the effect of red or blue glass is dark and sombre. The yellow colour therefore constitutes that portion of white light by which surrounding objects are rendered visible ; it is essentially the visual ray. NATURE AND PROPERTIES OF LIGHT. 49 The heating properties of the sunlight reside principally in the red ray, as is shown by the expansion of a mercu- rial thermometer placed in that part of the spectrum. The chemical action of light corresponds more to the indigo and violet rays, and is wanting, as regards its in- fluence upon Iodide of Silver, both in the red and yellow\ Strictly speaking however it cannot be localized in either of the coloured spaces, as will be more fully shown in the Fourth Section of this Chapter, to which the reader is re- ferred. SECTION II. The Refraction of Light. A ray of light, in its passage through any transparent medium, travels in a straight line as long as the density of the medium continues unchanged. But if the density varies, becoming either greater or less, then the ray is re- fracted, or bent out of the course which it originally pur- sued. The degree to which the refraction or bending takes place depends upon the nature of the new medium, and in particular upon its density as compared with that of the medium which the ray had previously traversed. Hence Water refracts light more powerfully than Air, and Glass more so than Water. The following diagram illustrates the refraction of a ray of light. The dotted line is drawn perpendicularly to the surface, E So NATUEE AND PEOPEETIES OF LIGHT. and it is seen that tlie ray of light on entering is bent towards this line. On emerging, on the other hand, it is bent to an equal extent away from the "perpendicular, so that it proceeds in a course parallel to, but not coincident with, its original direction. If we suppose the new me- dium, in place of being more dense than the old, to be less dense, then the conditions are exactly reversed, — the ray is bent away from the perpendicular on entering, and towards it on leaving. It must be observed that the laws of refraction apply only to rays of light which fall upon the medium at an angle : if they enter perpendicularly — in the direction of the dotted lines in the last figure — they pass straight through without suffering refraction. JN'otice also, that it is at the surfaces of bodies that the deflecting power acts. The ray is bent on entering, and bent again on leaving ; but whilst within the medium it continues in a straight line. Hence it is evident that by variously modifying the surfaces of refractive media the rays of light may be diverted almost at pleasure. This will be rendered clear by a few simple diagrams. In the figures given below, and in the following page, the dotted lines represent perpendiculars to the surface at the point where the ray falls, and it is seen that the usual Eig. 1, Fig. 2. law of bending towards the perpendicular on entering, and away from it on leaving the dense medium, is in each case correctly observed. NATUEE AND PEOPERTIES OF LIGHT. 51 Fig. 2, termed a prism, bends the ray permanently to one side; fig. 3, consisting of two prisms placed base to base, causes rays before parallel to meet in a point ; and conversely, fig. 4, having prisms placed edge to edge, di- verts them further asunder. Fig. 3. Fig. 4. The various forms of Lenses. — The phenomena of the refraction of light are seen in the case of curved surfaces in the same manner as with those which are plane. Glasses ground of a curvilinear form are termed Lenses, The following are examples. Fig. 1. Fig. 2. Fig. 3. Fig. 1 is a biconvex lens ; fig. 2, a biconcave lens ; and fig. 3, a meniscus lens. 52 NATURE AND PROPERTIES OF LIGHT. As far as regards their refractive powers, such figures may be represented, nearly, by others bound by straight lines, and thus it becomes evident that a biconvex lens tends to condense rays of light to a point, and a biconcave to scatter them. A meniscus combines both actions, but the rays are eventually bent together, the convex curve of a meniscus lens being always greater than the concave. The Foci of Lenses. — It has been shown that convex lenses tend to condense rays of light and bring them to- gether to a point. This point is termed " the focus " of the Lens. The following laws as regards the focus may be laid down ; — That rays of light which are pursuing a parallel course at the time they enter the Lens are brought to a focus at a point nearer to the Lens than diverging rays. The rays proceeding from very distant objects are parallel; those from objects near at hand diverge. The sun's rays are always parallel, and the divergence of the others becomes greater as the distance from the Lens is less. The focus of a Lens for parallel rays is termed the " prin- cipal focus," and is not subject to variation ; this is the point referred to when the focal length of a Lens is spoken of. When the rays are not parallel, but diverge from a point, that point is associated with the focus, and the two are termed " conjugate foci." In the above diagram A is the principal focus, and B and C are conjugate foci. Any object placed at B has its focus at G, and conversely when placed at C it is in focus at B. NATURE AND PEOPERTIES OF LIGHT. 53 Therefore, although the principal focus of a Lens (as de- termined by the degree of its convexity) is always the same, yet the focus for objects near at hand varies, being longer as they are brought closer to the Lens. Formation of a Luminous Image hy a Lens. — As the rays of light proceeding from a ;point are brought to a focus by means of a Lens, so are they when they proceed from an object, and in that case an image of the object is the result. The above figure illustrates this. The size of the image varies with the distance of the arrow from the glass — being larger and formed at a point further from the Lens as the object is brought nearer. The refracting power of the Lens also influences the result — lenses of short focal length, i. e. more convex, giving a smaller image. In order that the course pursued by pencils of rays pro- ceeding from an object may be easily traced, the lines from the barb of the arrow in the last figure are dotted. Observe that the object is necessarily inverted, and also that those rays which traverse the central point of the Lens, or the centre of the axis, as it is termed, are not bent away, but pursue a course either coincident with, or parallel to, the original, as in the case of refracting media with parallel surfaces. 54 NATUEE AND PEOPEETIES OF LIGHT. SECTION III. TJie T^liotograjpTiic Camera, The Photograpliic Camera is in its essential nature an extremely simple instrument. It consists merely of a dark chamber, having an aperture in front in which a Lens is inserted. The accompanying figure shows the simplest form of Camera. The body is represented as consisting of two portions which slide within each other; but the same object of lengthening or shortening the focal distance may be at- tained by making the Lens itself movable. A luminous image of any object placed in front of the Camera is formed by means of the Lens, and received upon a surface of ground glass at the back part of the instrument. When the Camera is required for use, the object isfocussed upon the ground glass, which is then removed, and a slide con- taining the sensitive layer inserted in its place. The luminous image, as formed upon the ground glass, is termed the " Eield " of the Camera ; it is spoken of as being flat or curved, sharp or indistinct, etc. These and other peculiarities which depend upon the construction of the Lens will now be explained. Chromatic Aberration of Lenses. — The outside of a bi- convex lens is strictly comparable with the sharp edge of a prism, and therefore necessarily produces decomposition in the white light which passes through it. The action of a prism in separating white light into its constituent rays may be simply explained ; — all the co- NATUEE AND PEOPERTIES OF LIGHT. 55 loured rays are refrangible, but not to the same extent. The indigo and violet are more so than the yellow and red, and consequently they are separated from them, and oc- cupy a higher position in the Spectrum. (See the diagram at p. 47.) A little reflection will show that in consequence of this unequal refrangibility of the coloured rays, white light must invariably be decomposed on entering any dense medium. This is indeed the case ; but if the surfaces of the medium are parallel to each other the effect is not seen, because the rays recombine on their emergence, being bent to the same extent in the opposite direction. Hence light is transmitted colourless through an ordinary pane of glass, but yields the tints of the Spectrum in its passage through a prism or a lens, where the two surfaces are inclined to each other at an acute angle. Chromatic aberration is corrected by combining two lenses cut from varieties of glass which differ in their power of separating the coloured rays. These are the dense flint-glass containing Oxide of Lead, and the light crown-glass. Of the two lenses, the one is biconvex, and the other biconcave; so that when fitted together they produce a compound Achromatic lens of a meniscus form, thus : — The first Lens in this figure is the flint- and the second the crown-glass. Of the two the biconvex is the most powerful, so as to overcome the other, and produce a total of refraction to the required extent. Each of the Lenses produces a spectrum of a different length ; and the effect 56 NATUEE AND PEOPERTIES OF LIGHT. of passing the rays through both, is, by overlapping the coloured spaces, to unite the complementary tints, and to form again white light. Spherical Aberration of Lenses. — The field of a Camera is not often equally sharp and distinct at every part. If the centre be rendered clear and well defined, the outside is misty ; whilst, by slightly altering the position of the ground glass, so as to define the outside portion sharply, the centre is thrown out of focus. Opticians express this by saying that there is a want of proper flatness of field ; two causes may be mentioned as concurring to produce it. The first is " spherical aberration," by which is meant the property possessed by Lenses which are segments of spheres, of refracting rays of light unequally at dif- ferent parts of their surfaces. The following diagram shows this : — Observe that the dotted lines which fall upon the cir- cumference of the Lens are brought to a focus at a point nearer to the Lens than those passing through the centre ; in other words, the outside of the Lens refracts light the most powerfully. This causes a degree of confusion and indistinctness in the image, from various rays crossing, and interfering with, each other. Spherical aberration may be avoided by increasing the convexity of the centre part of the Lens, so as to add to its refracting power at that particular point. The surface is then no longer a segment of a sphere, but of an ellipse, and refracts light more equally. The difficulty of grinding Lenses to an elliptical form however is so great, that the NATURE AND PEOPERTIES OF LIGHT. 57 spherical Lens is still used, the aberration being corrected in other ways. A second cause interfering with the distinctness of the outer portions of the image in the Camera is the obliquity of some rays proceeding from the object ; in consequence of which the image has a curved form, with the concavity inwards, as may be seen by referring to the figure given at page 53. The following diagram is meant to explain cur- vature of the image. The centre line running at right angles to the general direction of the Lens is the axis ; an imaginary line, on which the Lens may be said to rotate as a wheel turns on its axle. The lines A A represent rays of light falling parallel to the axis ; and the dotted lines, others which have an oblique direction ; B and C show the points at which the two foci are formed. Observe that these points, although equidistant from the centre of the Lens, do not fall in the same vertical plane, and therefore they cannot both be received distinct upon the ground glass of the Camera, which would occupy the position of the perpen- dicular double line in the diagram. Hence it is that with most lenses, when the centre of the field has been fo- cussed, the glass must be shifted forwards a little to define the outside sharply. The Use of Stops in Lenses. — Curvature of the image and indistinctness of outline from spherical aberration are both remedied to a great extent by fixing in front of the A A B 58 NATUEE AND PEOPEETIES OF LIGHT. Lens a diaphragm having a small central aperture. The diagram gives a sectional view of a Lens with a "stop" attached ; the exact position it should occupy with refer- ence to the Lens is a point of importance, and influences the flatness of the field. By using a diaphragm the quantity of light admitted into the Camera is diminished in proportion to the size of the aperture. The image is therefore less brilliant, and a longer exposure of the sensitive plate is required. In other respects however the result is improved ; the spheri- cal aberration is lessened by cutting off* the outside of the Lens, and a portion of the oblique rays being intercepted, the focus of the remainder is lengthened out, and the image is rendered flatter, and improved in distinctness. Hence also, when a small stop is affixed to a Lens, a variety of objects, situated at different distances, are all in focus at once ; whereas, with the full aperture of the Lens, objects near at hand cannot be rendered distinct upon the ground glass at the same time Avith distant objects, or vice versa. The Double or Portrait Combination of Achromatic Lenses. — The brightness of illumination of an image formed by a Lens is in proportion to the diameter of the Lens, that is, to the size of the aperture by which the Light is ad- mitted. The clearness or distinctness of outline however is independent of this, being improved by using a stop, which lessens the diameter. . The Portrait combination of Lenses is constructed to ensure rapidity of action by admitting a large volume of light. The following diagram gives a sectional view. NATURE AND PEOPEETIES OF LIGHT. 59 In this combination tlie front Lens is an Achromatic plano-convex, with, the convex side turned toward the ob- ject ; and the second, which takes up the rays and refracts them further, is a compound Biconvex Lens ; there are therefore in all four distinct glasses concerned in forming the image, which may appear at first to be an unnecessarily complex arrangement. It is found however that a good result cannot be secured by using a single Lens, when a "stop" is inadmissible. By combining two glasses of different curves, the aberrations of one correct those of the other to a great extent, and the field is both flatter and more distinct than in the case of an Achromatic Meniscus employed without a diaphragm. The manufacture of Portrait Lenses is a point of great difficulty, the glass ^ ^5^5 The amount of Oxygen in all is the same, that of the other ele- ment increases progressively ; hence it is at once evident that the highest member of the series might hy losing Sulphur descend gradually until it reached the condition of the lowest. This transition is not only theoretically possible, but there is an actual tendency to it, all the acids being unstable with the excep- tion of the Hyposulphuric. The Alkaline Salts of these acids are more unstable than the acids themselves ; a solution of Tetra- thionate of Soda becomes milky in the course of a few days from deposition of Sulphur, and, if tested, is then found to contain Tr^'thionate and eventually Z)^thionate of Soda. The cause of the change in properties of the fixing Bath being thus clearly traced to a decomposition of Hyposulphite of Silver, and a consequent generation of unstable principles capable of im- parting Sulphur to the immersed proofs, it seemed desirable to continue the experiments. — There is a peculiar acid condition commonly assumed by old fixing Baths, which could not be satisfactorily explained, since it was known that acids do not exist long in a free state in solution of Hyposulphite of Soda, but tend to neutralize themselves by dis- placing Hyposulphurous Acid spontaneously decomposable into Sulphurous Acid and Sulphur. This point is set at rest by the discovery of a peculiar reaction which takes place between certain salts of the Polythionic acids and Hyposulphite of Soda. A so- 158 THEOEY OF POSITIVE PRINTING. lution of Tetrathionate of Soda may be preserved for many hours unchanged ; but if a few crystals of Hyposulphite of Soda be dropped in, it begins very shortly to deposit Sulphur, and con- tinues to do so for several days. At the same time the liquid acquires an acid reaction to test-paper, and produces efferves- cence on the addition of Carbonate of Lime. It is evident that a Sulphur acid exists which has not hitherto been described, and that this acid is formed as one of the products of the decomposition of the Hyposulphite of Silver contained in the fixing Bath. The subject is an important one to Photo- graphers, because it is found that Hyposulphite Baths which have acquired the acid reaction, although toning quickly, yield Positives which fade on keeping. The acid may perhaps combine with the reduced Silver Salt, which, if the image be allowed to contain Suboxide of Silver, is theoretically probable. The experiments were next directed towards ascertaining more carefully the efiect of the acid fixing Bath upon the Positive proofs. Tetrathionate of Soda added to solution of Hyposul- phite of Soda produces, at the expiration of twelve hours, a liquid which, when filtered from the deposited Sulphur, reddens blue litmus -paper slowly. Positive prints immersed in the Bath pass from red to black, dissolving in the half-tones, and becom- ing yellow and faded if the action be too long continued. On adding Carbonate of Soda in quantity sufficient to remove the acid reaction, the power of toning is much diminished, but dark colours can still be obtained by continuing the action. The solvent efiect upon the half-tones, evidently caused in great measure by the acid, is lessened; whilst the tendency to yellowness in the white parts of the proof, almost disappears. These effects are more particularly manifested when the prints are immersed in the Bath immediately on their removal from the printing frame ; and it is found almost impossible to preserve the whites of the impression clear, in the acid Bath, unless the Nitrate of Silver has been washed away. Solution of half-tones and yellowness in the lights, both a source of annoyance to the operator, are thus traced in great measure to an acid condition of the fixing and toning Bath; and the remedy is obvious. The Author's experiments upon the Tetrathionates and their re- THEOEY or POSITIVE FEINTING. 159 action with Hyposulphite of Soda Hkewise ehcited the important fact tliat alkalies decompose the unstable sulphuretted principle. If the Bath be treated with Potash or Carbonate of Soda, an alka- line Sul]pliuret appears to be gradually formed, which precipitates Sulphui'et of Silver, and in the course of a few days the Hquid re- turns to its original condition and ceases to act as a toning agent upon the proof. The same effect takes place to a great extent when the solution is set aside for several weeks or months ; a pro- cess of spontaneous change going forward, which issues in a de- position of Sulphur and Sulphuret of Silver, and a partial loss of sulpliuretting properties in the liquid. It may be interesting to the scientific investigator to describe the mode of preparing a fixing and toning Bath, illustrating the above remarks : — Take of Nitrate of Silver 3 drachms. Hyposulphite of Soda 4 ounces. Water 8 ounces. Dissolve the Nitrate of Silver in 2 ounces of the water, then from the total quantity of Hyposulphite of Soda, weigh out Hyposulphite of Soda 2 drachms ; dissolve this likewise in 2 ounces of water, and the remainder of the Hyposulphite in the other 4 ounces. Then, having the three solutions in separate vessels, pour the Nitrate of Silver at once into the 2-ounce solution of Hyposidphite, agitating the precipitated Hyposulphite of Silver rapidly. In a short time it will begin to decompose, passing from white to canary-yellow, and then to orange-yellow. When the orange-yellow begins to verge towards hrown^ add the 4-ounce concentrated solution of Hyposulphite, which will at once complete the decomposition, a part of the precipitate dissolving and the remainder becoming perfectly black. After filtering out the black Sulphuret of Silver, the solution is ready for use. A Bath prepared by this formula is not usually very active, but it shows clearly the process by which an ordinary fixing Bath may be converted into a toning Bath by the immersion of positives having free Nitrate of Silver upon the surface. The following formula is more economical and gives a better 160 THEOEY OF POSITIVE PRINTING. result, but it cannot be used for " Ammonio-Nitrate" prints ; tbe addition of an alkali precipitating Sulphuret of Iron. Strong solution of Percliloride of Iron . 6 fluid drachms. Dissolve the Hyposulphite of Soda in seven ounces of the water, the Nitrate of Silver in the remaining one ounce ; then pour the Perchloride of Iron into the solution of Hyposulphite, by degrees, stirring all the time. The addition of the Iron Salt strikes a fine purple colour, but this soon disappears. When the liquid has become again colourless, whicli it does in a few minutes, add the Nitrate of Silver, stirring briskly. Perfect so- lution will take place without any formation of black Sulphuret. A toning Bath prepared with Chloride of Iron will be ready for use twelve hours after mixing, but it will be more active at the expiration of a week. The solution is acid to test-paper, and milky from a deposit of Sulphur, which must be filtered out. The Percliloride of Iron should be prepared by boiling Per- oxide of Iron with Hydrochloric Acid, in preference to dissolving Iron wire in Aqua Regia. The addition of the Nitrate of Silver is made in order to pro- duce a portion of Hyposulphite of Silver in the bath ; the pre- sence of a Silver Salt having been found to modify the tint of the Positives, and to prevent their quickly turning yellow. For many years subsequent to the discovery of the pro- cess of Photographic Printing by Mr. Fox Talbot, it was not generally known that pictures so produced were easily susceptible of injury from various causes, and in particular from traces of the fixing-agent remaining in the paper. Hence, due care not being taken in the proper cleansing and preservation of the proofs, the majority of them faded. Hyposulphite of Soda Water Nitrate of Silver . . 4 ounces. 8 ounces. 30 grains. SECTIOJN IV. On the Fading of JPhotogra^liic Prints. THEORY OF POSITIVE PRINTING. 161 This matter became at last one of such importance that the Council of the Photographic Society decided upon forming a Committee for the purpose of examining the sub- ject. The Author was honoured by being placed upon this Committee, and the researches of which an abstract has been given in the previous Section, were undertaken at the request of the Society. The present Section is intended to explain practically and in a concise manner the causes of the fading of Photo- graphic Prints, and the precautions which should be taken to ensure their permanency. The chemistry of the sub- ject having been fully explained in the last Section, it will suffice to refer the reader to its pages for more detailed in- formation. Historical evidence of the permanence of Photographs. — It is a point of interest to collect information as to the existence of old Photographs which have remained many years unchanged. There are numerous instances of Posi- tives printed more than ten years ago, which have not per- ceptibly altered up to the present time. These prints are mostly on plain paper, Albumen not having come into use at so early a date. The general impression of practical operators however is, that fading has occurred less fre- quently since the introduction of Albuminized paper. Positives printed by development on paper prepared by Talbot's method seem as a rule to have stood remarkably well, and instances of Talbotype Negatives having faded are rare. Of the prints which have proved to be permanent, some are red or brown in colour, but many, being of a dark or purple shade, have evidently been toned, although not with Gold, the use of which was unknown to the earlier Photo- graphers. It is plain from data thus collected, that Photographs do not necessarily fade by time ; and the fact that in one and the same portfolio are constantly seen prints which appear permanent, and others in an advanced state of M 162 THEOEY OF POSITIVE PEINTING. cliange, cannot but lead to the inference tliat tlie main causes of deterioration are intrinsic, depending upon some injurious matters left in the paper; which is confirmed by experiment. Causes of fading. — The Author believes that the fading of Photographic Prints may almost invariably be referred to one or other of the following conditions : — a. Imperfect washing. — This is perhaps the most impor- tant of all, and the most frequent. When Hyposulphite of Soda is allowed to remain in the paper, even in minute quantity, it gradually decomposes, with liberation of Sul- phur, and destroys the print in the same way and quite as efiectually as a solution of Sulphuretted Hydrogen or an alkaline sulphuret. Imperfect washing may be suspected, if the Photograph, within a few months from the date of its preparation, he- gins to get darlcer in colour: the half tints, which are the first to show the action, afterwards passing into the yellow stage, whilst the dark shadows remain black or brown for a longer time. The proper mode of washing Photographs is sometimes misunderstood. The length of time during which the print lies in the water is a point of less importance, than that the water should be continually changed. When a number of Positives are placed together in a pan, and a tap turned upon them, the circulation of fluid does not necessarily ex- tend to the bottom. This is proved by the addition of a little colouring matter, which shows that the stream flows actively above, but at the lower part of the vessel, and between the prints, there is a stationary layer of water which is of little use in washing out the Hyposulphite. Care should therefore be taken that the pictures are kept as far as possible separate from each other, and when run- ning water cannot be had, that they are frequently moved and turned over, fresh water being constantly added. When this is done, and especially if the size be removed from the paper in the manner presently to be advised, ybz^r THEORY OF POSITIVE PRINTING. 163 or jive hours washing will be sufficient. It is a mistake to allow the pictures to remain in the water for several days ; which produces no good effect, and may tend to encourage a putrefactive fermentation, or the formation of a white deposit upon the image when the water contains Carbonate of Lime. b. Acid matters left in the Taj^er. — Upon examining collections of old Photographs, it is not uncommon to find prints which are stated to have remained unaltered for a long time after their first production, but in the course of time to have lost their brilliancy, and become pale and in- distinct. This kind of fading often commences at the cor- ners and edges of the paper, and works inwards towards the centre. The Author's experiments have shown that it is principally caused by a slow process of oxidation. The Photographic Image does not appear readily suscep- tible of oxidation unless it be previously darkened by the action of Sulphur, or placed in contact with acids or bodies which act as solvents of Oxide of Silver (p. 146). The materials often used in sizing papers, such as Alum and Eesin, being of an acid nature, are directly injurious to the image ; and the removal of the size, which may easily be effected by means of a dilute alkali or an alkaline carbo- nate, without injury to the tint, has the additional advan- tage of carrying out the last traces of Hyposulphite of Soda, and also the germs of fungi, which if allowed to re- main would vegetate and produce a destructive mouldiness on exposure to damp (Chap. III. Part II.). The fact that acids facilitate oxidation of the image is likewise a hint that Photographic Prints should not be handled too frequently, or touched with the finger more than is necessary ; the warm hand may leave behind a trace of acid* which would tend in time to produce a yellow mark. * The writer lias seen blue litmus-paper immediately reddened by being laid upon the arm of a person suffering from acute Rheumatism. This acid is probably Lactic Acid ! 164 THEORY OF POSITIVE FEINTING. c. Moisture as a cause of fading. — Althongh. PJioto- graplis properly printed are not readily injured by damp air (p. 153), yet as there are impurities of varions kinds constantly floating in the atmosphere, a state of comparative dryness may be said to be essential to the preservation of all Photographs. In collecting evidence upon the subject, "wet" and damp" are frequently alleged as having been causes of fading — the prints were hung against a damp wall during frosty weather, in a room without a fire : or the rain had been allowed to penetrate the frame ! JSTo pictures will long survive such treatment, and Photographs, like engravings and water-colour paintings, require com- mon care to be exercised in their preservation. d. TJie modes of Mounting the JProof — This subject has been alluded to in the abstract of the Author's papers at p. 155. All cements which are of an acid nature, or which are liable to become sour by acetous fermentation, should be avoided. Plour paste is especially injurious, and many cases of fading have been traced to this cause. The addi- tion of Bichloride of Mercury, which is often made to pre- vent the paste from becoming mouldy, would still more unfit it for Photographic use (p. 151). Starch is not much preferable. No substance appears better than Gelatine, which does not readily decompose, and shows no tendency to absorb atmospheric moisture. The deliquescent nature of many bodies is a point which should be borne in mind in mounting Pliotographs, and hence the use of a salt like Carbonate of Potash, which the writer has known to be added to paste to prevent the formation of acid, would be unadvisable. e. The effect of Imperfect Fixation. — The earlier Photo- graphers did not always succeed in properly fixing their prints, since old Photographs are often found thickly studded with spots and blotches in the tissue of the paper. These prints however are not invariably faded upon the surface, and hence it cannot be said that imperfect fixation will certainly end in the total destruction of the picture. THEOEY OF POSITIVE PEINTING. 165 Still a notice of the subject may properly be introduced in this place, and the attention of the reader be once more drawn to the importance of washing the print in water on > removing it from the printing frame ; a decomposition invariably occurring when paper Positives saturated toitJi free Nitrate of Silver are plunged in a dilute solution of Hyposulphite of Soda, containing an insufficient quantity of the salt to dissolve away the Hyposulphite of Silver before it begins to undergo spontaneous change. f. Exposure to an impure Atmosphere as a cause of Fading. — The five causes of fading which precede, have mostly reference to an intrinsically faulty condition of the print. This, the sixth, explains the mode in which a Pho- tograph carefully prepared may yet suffer injury from de- leterious matters often present in the atmosphere. The air of large cities, and particularly that emanating from sewers and drains, contains Sulphuretted Hydrogen, and hence articles of silver-plate become tarnished unless placed beneath glass. The injury which a print sustains by expo- sure to air contaminated with Sulphuretted Hydrogen, is less than the tarnish produced upon the bright surface of a silver plate (see p. 148) ; but it is recommended as a pre- cautionary measure, that Photographic Pictures be pro- tected by glass or kept in a portfolio, and that they be not exposed too freely to the air. The products of the combustion of coal-gas are probably more likely than the cause last named, to be a source of injury to Photographs suspended without any covering. The sulphur compounds in gas burn into Sulphurous and Sulphuric Acids, the latter of which, in combination with Ammonia, produces the sparkhng crystals often observed upon the shop windows. The question as to the manner in which the Photographic Image may best be protected from these extraneous causes of fading has been mooted, and many plans of coating prints with some impervious material have been devised. If the pictures are to be glazed or kept in a portfolio, this 166 THEOEY OF POSITIVE FEINTING. of itself will be sufficient, but in other cases it may perhaps be useful to apply a layer of spirit or gutta-perclia varnish. The use of wax, resin, and such bodies is likely, by intro- ducing impurities, to act injuriously rather than otherwise. g. Decomjoosition of Pyroxyline a source of Injury to Collodion Photographs. — Collodion Positives and Nega- tives are usually esteemed permanent ; but some have been exhibited which, having been put away in a damp place, gradually became pale and indistinct. The change com- mences at rough edges and isolated points, leaving the centre, as a rule, the last affected. On examination, nu- merous cracks are often visible, thus seeming to indicate that the Collodion film has undergone decomposition. The result of this would be the liberation of corrosive Oxides of Nitrogen, which destroy the image. Substitution com- pounds containing Peroxide of Nitrogen are known to be liable to spontaneous change. The bitter resin produced by acting upon white sugar with Nitro- Sulphuric Acid, if not kept perfectly dry, will sometimes evolve enough gas to destroy the cork of the bottle in which it is kept ; the solution of the resin has then a strong acid reaction, and rapidly fades an ordinary Positive Print. These facts are interesting, and indicate that Collodion Pictures, containing in themselves the elements of their destruction, should be protected from moisture by a coat- ing of varnish. Comparative JPermanence of Photographic Prints. — There is every reason to think that the Photographic Image, however formed, is permanent, if certain injurious conditions are avoided; — in other words, that prints do not necessarily fade, in the same manner as fugitive colours, by a simple exposure to light and air. But supposing a case, which is the common one, of injurious influences which cannot altogether be removed, it may be useful to inquire what mode of printing gives the greatest amount of stability. Positives produced by a short exposure to light and sub- THEORY OF POSITIVE PRINTING. 167 sequent development with Gallic Acid, may be expected to be more permanent tban ordinary sun-prints ; not that there is any reason to suppose the chemical composition of a developed image to be peculiar, but that the use of the Gallic Acid enables us to increase the intensity of the red picture first formed, and to add to its stability by preci- pitating fresh Silver upon it. This point has not always been attended to. It has been recommended to remove the print from the developing solution whilst in the red and early stage of development, and to produce the dark tones subsequently by means of gold ; but this plan, although giving very good results as regards colour and gradation of tone, appears to lessen the advantage which would other- wise accrue from the adoption of a Negative process, and to leave the picture, as regards permanency, much in the condition of an ordinary print obtained by direct action of light. The original Talbotype process, in which the latent image is formed upon Iodide of Silver, produces, next to Collo- dion, the most stable image; but the difficulty of obtaining bright and warm tints on Iodide of Silver, will stand in the way of its adoption. The toning of Paper Positives is the part of the process which is likely to injure their stability ; inasmuch as the finest results cannot easily be obtained without incurring sulphur ation, and the action of Sulphur, if carried to any extent, has been shown to be detrimental. The point to be kept in view, is to alter the original structure of the image as little as possible in toning ; and it is best to use Gold in preference to Sulphur as the colouring agent. On theo- retical grounds, toning by an alkaline solution of Chloride of Gold (p. 132), and fixing by Ammonia, is the best pro- cess ; but the employment of Sel d'or, which gives a more agreeable colour and has not been found practically to in- jure the image, will be generally preferred. In using a single fixing and toning Bath the same object of working by Gold rather than by Sulphur may be best attained by 168 THEOEY OF POSITIVE PEINTING. maintaining tlie activity of the Bath by constant additions of Chloride of Gold. The prints which are least stable are such as have been toned in acid Hyposuljpliite Baths, without Gold ; and the difficulty of preserving such pictures from becoming yellow in the half-tones is very great. Possibly a portion of the Sul- phuretted Acid may unite with the Suboxide of Silver and * cannot be removed by washing (see p. 158); but even if this be not the case, it is certain that no ordinary amount of care will obviate the occasional occurrence of fading, unless the Hyposulphite Bath be kept neutral to test-paper. And all those plans of toning in which Acetic or Hydrochloric Acid is mixed with Hyposulphite of Soda, and the Positive im- mersed whilst the liquid is in a milky state from precipi- tation of Sulphur, ought studiously to be avoided. It will be well also to avoid pushing the action of the fixing and toning Bath to its utmost limits, since practice and theory both teach us that the Positives which have been long in the Hyposulphite, and consequently show a tendency to yellowness in the light parts, are most liable to lose their half-tones on keeping. Photographic Prints are found often to darken slightly in the course of years ; and therefore by suspending the toning action at an earlier stage a margin is left for what some have termed " an im- provement by time." The use of Albuminized in preference to plain paper gives an advantage in protecting the image from oxidation ; but if constantly exposed to moisture, a putrefactive de- composition of the animal matter may occur. The proper colour of the Albumen image being a pale red, the black tones should not be sought for on that variety of paper : their production, if Hyposulphite of Soda were used in toning, would probably imply an amount of Sulphuration which would more than counterbalance any advantage otherwise derivable from the Albumen. Permanent Positives of a black colour may easily be ob- tained by sensitizing plain paper, free from animal matters. THEORY OF POSITIVE PRINTING. 169 with Oxide of Silver in place of Nitrate. The simply fixed image being in that case of a sepia tint, requires a less amount of toning to change it to black. An impression was at one time prevalent that Ammonio-Nitrate prints were unstable ; but so far from such being the case, they are proved to withstand the action of all destructive tests better than pictures prepared upon the same kind of paper sensitized with plain Nitrate of Silver. Mode of testing the joermanence of Positives. — The tests for Hyposulphite of Soda are not sufficiently delicate to indicate with certainty when the process of wasliing has been properly performed. The quantity of that salt left in the paper is usually so small and so much mixed up with organic matter, that the application of Protonitrate of Mercury or of Nitrate of Silver to the liquid which drains from the corner of the print, would probably mislead the operator. A dilute solution of Permanganate of Potash, prepared by dissolving from half a grain or two grains of the salt, according to its purity, in one gallon of distilled water, affords a convenient mode of testing Positives as regards their power of resisting oxidation ; and to an experienced eye it will prove the presence or absence of Hyposulphite of Soda, the smallest trace of which is sufficient to remove the pink colour of the Permanganate. The most available and simple plan of testing perma- nence is to enclose the pictures in a stoppered glass bottle with a small quantity of water. If they retain their half- tones after a course of three months of this treatment, and do not become mouldy, the mode of printing followed is satisfactory. Boiling water will also be found useful in distinguishing the unstable colours produced by Sulphur from those fol- io wdng the judicious employment of Gold ; in all cases the image will at first be reddened by the hot water, but if toned without Sulphur it will, as a rule, recover much of its dark colour on drying. 170 THEORY OF POSITIVE PRINTING. The characteristic appearance of prints which have been I much sulphuretted in the toning Bath, and are very liable to fade, should be known. A yellow colour in the lights is a bad sign ; and if the half-tones are at all faint and indistinct, with an aspect of commencirig yellowness, it is almost certain that the Positive will not last for any con- siderable length of time. 171 CHAPTER IL OTf THE THEOET OE THE DAGUEEEEOTTPE AKD TALBOTYPE PEOCESSES, ETC. SECTION 1. The Daguerreotype. It was not tlie original intention of the Author to include a description of the Daguerreotype Process within the li- mits of the present Work. The Daguerreotype is a branch of the Photographic Art so distinct from the others, thafc, in manipulatory details, it bears very little analogy to them; a slight sketch of the theory of the process may not how- ever be unacceptable. All necessary remarks will fall under three heads : — The preparation of the Daguerreotype film ; — the means by which the latent image is developed ; — and the strength- ening of the image by Hyposulphite of Gold. The Preparation of the Daguerreotype Film, — The sensitive film of the Daguerreotypist is in many respects different from that of the Calotype or Collodiotype. The latter may be termed wet processes, in contradistinction to the former, where aqueous solutions are not employed. The Daguerreotype film is a pure and isolated Iodide of Silver, formed by the direct action of Iodine upon the metal. Hence it lacks one element of sensitiveness pos- sessed by the others, viz. the presence of soluble Nitrate of Silver in contact with the particles of Iodide of Silver. 172 THE THEOEY OF THE It is important to remember that tlie Iodide of Silver prepared by acting with vapour of Iodine upon metallic Silver, is different in its Photographic action from the yellow salt obtained by double decomposition between Iodide of Potassium and Nitrate of Silver. A Daguerreo- type film, when exposed to a bright light, first darkens to an ash-grey colour and then becomes nearly white; the solubility in Hyposulphite of Soda being at the same time lessened. A Collodion film, on the other hand, if the ex- cess of Nitrate of Silver be washed off, although it is still capable of receiving the radiant impression in the Camera, does not alter either in colour or in solubility, by exposure even to the sun's rays. Details of the process for preparing a Daguerreotype Plate. — A copper plate of moderate thickness is coated upon the surface with a layer of pure Silver, either by the electrotype or in any other convenient manner. It is then polished with great care, until the surface assumes a bril- liant metallic lustre. This preliminary operation of po- lishing is one of great practical importance, and the trou- blesome details attending it constitute one of the main difficulties to be overcome. After the polishing is complete, the plate is ready to receive the sensitive coating. This part of the process is conducted in a peculiar manner. A simple piece of card- board or a thin sheet of wood, previously soaked in solu- tion of Iodine, evolves enough of the vapour to attack the silver plate ; which being placed immediately above, and allowed to remain for a short time, acquires a pale violet hue, due to the formation* of an excessively delicate layer of Iodide of Silver. By prolonging the action of the Iodine the violet tint disappears and a variety of pris- matic colours are produced, much in the same way as when light is decomposed by thin plates of mica or the surface of mother-of-pearl. From violet the plate becomes of a straw-yellow, then rose-colour, and afterwards steel-grey. By continuing the exposure, the same sequence of tints is DAGUERREOTYPE AND TALBOTYPE PROCESSES. 173 repeated; the steel-grey disappears, and the yellow and rose-colours recur. The deposit of Iodide of Silver gra- dually increases in thickness during these changes ; but to the end it remains excessively thin and delicate. In this respect it contrasts strongly with the dense and creamy layer often employed in the Collodion process, and shows that a large proportion of the Iodide of Silver must in such a case be superfluous, as far as any influence pro- duced by the light is concerned. An inspection of a sen- sitive Daguerreotype plate reveals the microscopic nature of the actinic changes involved in the Photographic Art, and teaches a useful lesson. Increase of sensibility obtained by combining the joint action of Bromine and Iodine. — The original process of Daguerre was conducted with the vapour of Iodine only ; but in the year 1840 it was discovered by Mr. John God- dard that the sensibihty of the plate was greatly promoted by exposing it to the vapours of Iodine and Bromine in succession, — the proper time for each being regulated by the tints assumed. The composition of this JBromo -Iodide of Silver, so called, is uncertain, and has not been proved to bear any analogy to that of the mixed salt obtained by decompos- ing a solution of Iodide and Bromide of Potassium with JN'itrate of Silver. Observe also that the Bromo-Iodide of Silver is more sensitive than the simple Iodide only token the vapour of Mercury is employed as a developer. M. Claudet proves that if the image be formed by the direct action of light alone (see page 174), the usual condition is reversed, and that the use of Bromine under such circum- stances retards the effect. The Development and Properties of the Image. — The la- tent image of the Daguerreotype is developed in a manner different from that of the humid processes generally, — viz. by the action of Mercurial vapour. Mercury, or Quick- silver, is a metallic fluid which boils at 662° Fahrenheit. We are not however to suppose that the iodized plate is 174 THE THEOEY OF THE subjected to tlie vapour of Mercury at a temperature at all aproacMng to 662°. The cup containing the Quicksilver is previously heated by means of a spirit-lamp to about 140°, a temperature easily borne by the hand, in most cases, without inconvenience. The amount of Mercurial vapour evolved at 140° is very small, but it is sufficient for the purpose, and after continuing the action for a short time the image is perfectly developed. There are few questions which have given rise to greater discussion amongst chemists than the nature of the Da- guerreotype image. Unfortunately, the quantity of mate- rial to be operated on is so small, that it becomes almost impossible to ascertain its composition by direct analysis. Some suppose it to consist of Mercury alone. Others have thought that the Mercury is in combination with metalHc Silver. The presence of the former metal is certain, since M. Claudet shows that, by the application of a strong heat, it can actually be volatilized from the image in sufficient quantity to develope a second impression immediately su- perimposed. It is a remarkable fact that an image more or less re- sembling that developed by Mercury can be obtamed by the prolonged action of light alone upon the iodized plate. The substance so formed is a white powder, insoluble in solution of Hyposulphite of Soda ; amorphous to the eye, but presenting the appearance of minute reflecting crys- tals when highly magnified. Its composition is uncertaiu. For all practical purposes the production of the Daguer- reotype image by light alone is useless, on account of the length of time required to effect it. This was alluded to in the third Chapter, where it was shown that in the case of the Bromo-Iodide of Silver an intensity of light 3000 times greater is required, if the use of the Mercurial va- pour be omitted. M.. JEd. JBecquereV s discovery of the continuing action of rays of yellow light. — Pure homogeneous yellow light has no action upon the Daguerreotype plate ; but if the iodized DAGUERREOTYPE AND TALBOTTPE PROCESSES. 175 surface be first exposed to white light for a sufficient time to impress a latent image, and then afterwards to the yel- low light, the action already commenced is continued, and even to the extent of forming the peculiar white deposit, insoluble in Hyposulphite of Soda, already alluded to. Yellow light may therefore in this sense be spoken of as a developing agent, since it produces the same effect as the Mercurial vapour in bringing out to view the latent image. A singular anomaly however requires notice, viz. that if the plate be prepared with the mixed vapours of Bromine and Iodine, in place of Iodine alone, then the yellow light cannot be made to develope the image. In fact, the same coloured ray which continues the action of white light upon a surface of Iodide of Silver, actually destroys it, and restores the particles to their original condition, with a surface of ^romo-Iodide of Silver. These facts, although not of great practical importance, are interesting in illustration of the delicate and complex nature of the chemical changes produced by light. The Strengthening of the Daguerreotype Image hy means of Hyposulphite of Gold. — The use of the Hyposulphite of Gold to whiten the Daguerreotype image, and render it more lasting and indestructible, was introduced by M. Fizeau, subsequent to the original discovery of the process. After removal of the unaltered Iodide of Silver by means of Hyposulphite of Soda, the plate is placed upon a levelling stand and covered with a solution of Hyposul- phite of Gold, containing about one part of the salt dis- solved in 500 parts of water. The flame of a spirit-lamp is then applied until the liquid begins to boiL Shortly a change is seen to take place in the appearance of the image ; it becomes whiter than before, and acquires great force. This fact seems to prove conclusively that metallic Mercury enters into its composition, since a surface of Silver — such, for instance, as that of the CoUodion image — is darkened by Hyposulphite of Gold. 176 THE THEOEY OF THE The difference in the action of the gilding solution upon the image and the pure Silver surrounding it illustrates the same fact. This Silver, which appears of a dark colour, and forms the shadows of the image, is rendered still darker ; a very delicate crust of metallic Gold gradually forming upon it, whereas with the image the whitening effect is immediate and striking. SECTION 11. Theory of the Talhotype and Albumen Processes. TJie Talhotype or Calotype. — This process, as practised by many at the present time, is almost identical with that originally described by Mr. Pox Talbot. The object is to obtain an even and finely divided layer of Iodide of Silver upon the surface of a sheet of paper ; the particles of the Iodide being left in contact with an excess of Nitrate of Silver, and usually with a small proportion of Gallic Acid, to heighten, still further, the sensibility to light. The English papers sized with Gelatine are commonly used for the Calotype process ; they retain the film more per- fectly at the surface, and the Gelatine in all probability assists in forming the image. With a foreign starch-paper, unless it be re-sized with some organic substance, the so- lutions sink in too deeply, and the picture is wanting in clearness and definition. There are two modes of iodizing and sensitizing the sheets : first, by floating alternately upon Iodide of Potas- sium and Nitrate of Silver, in the same manner as in the preparation of papers for Positive Printing ; and second, by what is termed "the single wash," which is thought by many to give superior results as regards sensitiveness and intensity of image. To iodize by this mode, the yellow Iodide of Silver, prepared by mixing solutions of Iodide of Potassium and Nitrate of Silver, is dissolved in a strong solution of Iodide of Potassium ; the sheets are floated for DAGUERREOTYPE AND TALBOTYPE PROCESSES. 177 an instant upon tliis liquid and dried; they are tlien re- moved to a dish of water, by the action of which the Iodide of Silver is precipitated upon the surface of the paper in a finely divided state. The properties of a solution of Iodide of Silver in Iodide of Potassium, or of the double Iodide of Potassium and Silver, are described at page 43, a reference to which will show that the double salt is decomposed by a large quantity of water, with precipitation of the Iodide of Silver, this substance being insoluble in a dilute solution of Iodide of Potassium, although soluble in a strong solution. Paper coated with Iodide of Silver by this mode, after proper washing in water to remove soluble salts (which if allowed to remain would attract damp), will keep good for a long time. The layer of Iodide appears of a pale prim- rose colour, and is perfectly insensitive to light. Even ex- posure to the sun's rays produces no change, thus indicat- ing that an excess of Nitrate of Silver is essential to the visible darkening of Iodide of Silver b}^ light. The paper is also insensitive to the reception of an invisible image, differing in this respect from the washed Collodion plate, which receives an impression in the Camera, although ap- parently freed from Nitrate of Silver. To render Calotype paper sensitive to light, it is brushed with a solution of Nitrate of Silver containing both Acetic and Gallic Acids, termed Aceto-Nitrate" and " Gallo- Nitrate" solution. The Gallic Acid lessens the keeping qualities of the paper, but increases the sensitiveness. The Acetic Acid prevents the paper from blackening all over during the development, and preserves the clearness of the white parts ; its employment is indispensable. The paper is commonly excited upon the morning of the day upon which it is intended to be used ; and the longer it is kept, the less active and certain it becomes. An ex- posure of five to eight minutes in the Camera is the ave- rage time with an ordinary view lens. The picture is developed with a saturated solution of 178 THE THEORY OF THE Gallic Acid, to which a portion of Aceto-Nitrate of Silver is added to heighten the intensity. Both Sulphate of Iron, and Pyrogallic Acid have also been used, but they are unne- cessarily strong, the invisible image being more easily de- veloped upon paper than upon Collodion (see page 143). After fixing the Negative by removing the unaltered Iodide of Silver with Hyposulphite of Soda, it is well washed and dried. White wax is then melted in with a hot iron, so as to render the paper transparent, and to facilitate the after-process of printing. The Calotype cannot be compared with the Collodion process for sensitiveness and delicacy of detail, but it pos- sesses advantages for tourists and those who do not wish to be encumbered with large glass plates. The principal difficulty appears to be in obtaining a uniformly good paper, many samples giving a speckled appearance in the black parts of the Negative. The Waxed Paper process of Le Grey. — This is a useful modification of the Talbotype introduced by M. Le Grey. The paper is waxed before iodizing, by which, \^'ithout in- volving any additional operation, a very fine surface layer of Iodide of Silver can be obtained. The Waxed Paper Process is well adapted for tourists, from its extreme sim- plicity and the length of time which the film may be kept in a sensitive condition. Both English and foreign papers are employed : but the former take the wax with difficulty. Mr. Crookes, who has devoted his attention to this process, gives clear direc- tions for waxing paper ; it is essential that pure white wax should be obtained direct from the bleachers, since the flat cakes sold in the shops are commonly adulterated. The temperature must also be carefully kept below that point at which decomposition of the wax takes place ; the use of too hot an iron being a common source of failure fsee * Photographic Journal,' vol. ii. p. 231). The sheets of paper, having been properly waxed, are soaked for tico hours in a solution containing Iodide and DAGUERREOTYPE AND TALBOTYPE PROCESSES. 179 Bromide of Potassium, with, enough free Iodine to tinge the liquid of a port-wine colour. The greasy nature of wax impedes the entry of liquids, and hence a long immer- sion is required. The iodizing formulae of the French Photographers have been encumbered by the addition of a variety of substances which appear to introduce compli- cations without giving proportional advantage, and Mr. Townshend has done the art a service by proving that the Iodide and Bromide of Potassium, with free Iodine, are sufficient. This latter ingredient w^as first used by Mr. Crookes ; it seems to add to the clearness and sharpness of the Negatives ; and as the papers are coloured by the Iodine, air-bubbles cannot escape detection. The process of exciting with Nitrate of Silver is also rendered more certain by the employment of free Iodine, the action of the Bath being continued until the purple colour gives place to the characteristic yellow tint of the Iodide of Silver. Waxed Paper is rendered sensitive by immersion in a Bath of Nitrate of Silver containing Acetic Acid; the quan- tity of which latter ingredient should be increased when the papers are to be long kept. As the excess of Nitrate is subsequently removed, the solution may be used weaker than in the Calotype or Collodion process. After exciting, the papers are washed with water to re- duce the amount of free Nitrate of Silver to a minimum. This lessens the sensitiveness, but greatly increases the keeping qualities, and the paper will often remain good for ten days or longer. It is a very important point, in operating with Waxed Paper, to keep the developing dishes clean. The develop- ment is conducted by immersion in a Bath of Gallic Acid containing Acetic Acid and Nitrate of Silver ; and being retarded by the superficial coating of wax, there is al- ways a tendency to an irregular reduction of Silver upon the white portions of the Negative. When the developer becomes brown and discoloured, this is almost sure to happen ; and it is well known to chemists that the length 180 THE THEORY OF THE of time during whicli Gallic Acid and Nitrate of Silver may remain mixed without decomposing, is mwli lessened by using vessels which are dirty from having been before em- ployed for a similar pur]30se. The black deposit of Silver exercises a catalytic {KaraXvcns, decomposition by contact) action upon the freshly-mixed portion, and hastens its dis- coloration. The Waxed Paper process is exceedingly simple and in- expensive, — very suitable for tourists, as requiring but little experience, and a minimum of apparatus. It is however slow and tedious in all its stages, the sensitive papers fre- quently taking an exposureof twenty minutes in the Camera, and the development extending over an hour or an hour and a half. Several Negatives however may be developed at the same time ; and as the removal of the free Nitrate of Silver gives the process a great advantage during hot wea- ther, it will in all probability continue to be extensively followed. The prints which have been sent to the Exhi- bition of the Photographic Society, show that waxed paper in the hands of a skilful operator may be made to delineate architectural subjects with great fidelity, and also to give the details of foliage and landscape Piiotography with dis- tinctness. The Albumen process upon Glass, — The process with Albumen originated in a desire to obtain a more even sur- face layer of Iodide of Silver than the coarse structure of the tissue of paper will allow. It is conducted with simple Albumen, or " white of eggs," diluted with a conve- nient quantity of water. In this glutinous liquid Iodide of Potassium is dissolved ; and the solution, having been thoroughly shaken, is set aside, the upper portion being drawn oflP for use, in the same manner as in the prepara- tion of Albuminized paper for printing. The glasses are coated with the Iodized Albumen, and are then placed horizontally in a box to dry. This part of the process is considered the most troublesome, the moist Albumen easily attracting particles of dust, and being apt COLLODIO-ALBUMEN PROCESS. 181 to blister and separate from tlie glass. If an even layer of the dried and Iodized material can be obtained, the chief difficulty of the process has been overcome. The plates are rendered sensitive by immersion in a Bath of Nitrate of Silver with Acetic Acid added, and are then washed in water and dried. They may be kept for a long time in an excited state. The exposure in the Camera must be unusually long ; the free Nitrate of Silver having been removed by wash- ing, and the Albumen exercising a direct retarding influ- ence upon the sensitiveness of Iodide of Silver. The development is conducted in the ordinary way by a mixture of Gallic Acid and Nitrate of Silver, with Acetic Acid added to preserve the clearness of the lights. It usually requires one hour or more, but may be accelerated by the gentle application of heat. Albumen pictures are remarkable for elaborate dis- tinctness in the shadows and minor details, and are ad- mirably adapted for viewing in the Stereoscope ; but they do not often possess the peculiar and characteristic softness of the Photograph upon Collodion. The process is well adapted for hot climates, being very little prone to the cloudiness and irregular reduction of Silver which are often complained of with moist Collodion under such cir- cumstances. M. Taitpenof s Collodio- Albumen process. — This is a re- cent discovery which seems to involve a new principle in the Art, and gives promise of great utility. One of the greatest objections to the Albumen process has been its want of sensitiveness ; but M. Taupenot found that this was obviated to a great extent by pouring the Al- bumen upon di^\2iiQ previously coated with Iodide of Silver. In this way two layers of that sensitive salt are formed, and the sensibility of the surface layer, which alone re- ceives the image, is promoted by its resting upon a sub- stratum of Iodide rather than upon the inert surface of the glass. In this view, if the theory be correct, the lower 182 THE COLLODIO-ALBUMEN PEOCESS. particle of Iodide of Silver promotes the molecular dis- turbance of the upper, itself remaining unchanged. Other experimenters, pursuing the subject further, have asserted that a successful result may be obtained by coat- ing the plate with plain Collodion and subsequently with Iodized Albumen. If this observation should prove correct, the process will be simplified and its utility increased. In the sixth Chapter of Part II. the practical details of the Collodio- Albumen process will be described. END OF PART I. PART II. PRACTICAL DETAILS OF THE COLLODION PROCESS. 185 PRACTICAL DETAILS OF THE COLLODION PROCESS. CHAPTER L PEEPAEATION OF C0LL0DI01S-. This includes — the soluble Paper; — the Alcohol and Ether ; — and the iodizing compounds. The formulce for Negative and Positive Collodion, and for the Nitrate Bath and developing fluids, are given in the second Chapter. THE SOLUBLE PAPEE. Pyroxyline may be prepared either from cotton wool or from Swedish Filtering-paper. Most operators prefer the latter, from its giving a product of constant solubility, and yielding a fluid solution."^ The Cotton Wool however is better adapted for use with the Sulphuric Acid and Nitre, since the Paper, from its closeness of texture, requires a longer immersion in the mixture. Preparation of a Nitro- Sulphuric Acid of the proper strength. — There are two modes of preparing the Nitro- Sulphuric Acid : first, by mixing the acids ; second, by the Oil of Vitriol and Nitre Process. The former is the * Swedish filtering-paper may be procured at the operative chemists', at about five shillings the quire. Each half-sheet has the water-mark " J. H. MunkteU.'* 186 PREPARATION OF COLLODION. best in cases where large quantities of the material are operated on, but the amateur is recommended to begin by trying the JN^itre Process (p. 190) as the most simple. PREPARATION OF NITRO-SULPHURIC ACID BY THE MIXED ACIDS. The operator may proceed in either of two ways ; first, by taking the strength of each sample of acid, and mixing according to fixed rule ; second, by a more ready plan, which may be used when the exact strength of the acids is not known. Each of these will be described in succes- sion. a. Directions for mixing according to fixed ride. — This process is given from Mr. Hadow's original paper in the * Quarterly Journal of the Chemical Society.' It is certain in its results if the strength of both acids be accurately determined. A very perfect process for taking the strength of Nitric Acid is by means of powdered Marble or Carbonate of Lime, as described in various works on practical Chemistry. Sulphuric Acid may be estimated by precipitating with Nitrate of Baryta, and weighing the insoluble Sulphate with the proper precautions. The specific gravity is not a criterion of strength to be perfectly relied on, but if it be adopted as a test, the fol- lowing points must be attended to. 1st. That the temperature of the acid be at or near 60° Fahrenheit ; the density of Sulphuric Acid especially is, from its small specific heat, greatly influenced a change of temperature. 2nd. The sample of Nitric Acid must be free from Per- oxide of Nitrogen, or only slightly coloured by it. This substance, when present, increases the specific gravity of the acid without adding to its available properties. A yellow sample of Nitric Acid will therefore be somewhat weaker than is indicated by the specific gravity. 3rd. The Oil of Vitriol should yield no solid residue on PREPAEATION OF COLLODION. 187 evaporation. Sulphate of Lead and Bisulpliate of Potasli are often found in the commercial acid, and add much to its density. Oil of Vitriol containing Sulphate of Lead becomes milky on dilution. The formula for a definite Nitro- Sulphuric Acid, of the proper strength for makmg the soluble Pyroxyline, may be stated thus : — HO NO5, 2 (HO SO3) + 3^ HO or Atoms. Atomic weight. , , 1 54 Sulphuric Acid . . . . 2 80 . . 6J 58 192 Having found the percentage of real acid which is present,^ the following calculation will give the relative weights of the ingredients required to produce the for- mula : — Let ( ^ ~ percentage of real Nitric Acid, \h ~ ,, ,, Sulphuric Acid, then 5400 __ . ^ . , quantity 01 JNitric Acid, a 8000 . b Sulphuric Acid, 5400 8000 Ijj ^ Water. a o Observe that the numbers in the calculation correspond to the atomic weights recently given ; and that the amount of water is derived from the total atomic weight, viz. 192, minus the sum of the weights of both acids. Hence if the samples of acid employed are too weak for the purpose, the formula for the water gives a negative quantity. * Tables are given in the Appendix for calculation by specific gravity ; but direct analysis of the acids is the most certain. 188 PEEPAEATION OF COLLODION. The weiglit of mixed acids produced by the formula is 192 grains, which would measure somewhere about two fluid drachms. Ten times this quantity forms a convenient bulk of liquid, in which about 50 or 60 grains of Paper may be immersed. In weighing corrosive liquids, such as Sulphuric and Nitric Acid, a small glass may be counterbalanced in the scalepan, and the acid poured in carefully. If too much is added, the excess can be removed by a glass rod, or by *'the pipette" commonly employed for such a purpose. The following example of a calculation similar to the above may be given : — 100 parts of the Oil of Vitriol = 76*65 real acid. „ „ Nitric Acid = 65*4 real acid. therefore = 104*3 grains of Oil of Vitriol. 76-65 ^ ^ 82*5 „ Nitric Acid. 65-4 192-104*3-82*5= 5*2 „ Water. Multiplying these weights ten times, we have Oil of Vitriol . . . 1043 grains. Nitric Acid .... 825 „ Water 52 „ Total weight of the Nitro- ^^^0 grains. Sulphuric Acid . . . 3 Having prepared the acid mixture of a definite strength by the above formula, the paper must be immersed accord- ing to directions given at page 191. b. Process for mixing Nitro- SulpJiuric Acid, the strength of tlie two acids not having heen previously determined. — Take a strong sample of Nitric Acid (the yellow Nitrous acid, so called, succeeds well), and mix it with Oil of Vi- triol as follows : — Sulphuric Acid . . 10 fluid drachms, Nitric Acid ... 10 „ PREPARATION OF COLLODION. 189 Now immerse a thermometer and note the temperature it should be from 130^ Fahr. to 150°. If it sinks below 120°, place the mixture in a capsule, and float upon boiling water for a few minutes. A preliminary experiment with a small tuft of Cotton Wool (cotton shows it better than paper) will then indicate the actual strength of the Nitro- Sulphuric Acid. Stir the tuft in the mixture for five minutes. E-emove with a glass rod, and wash with water for a short time, until no acid taste can be perceived. If the Wool becomes matted, and gelatinizes slightly on its first immersion in the acid, or if, in the subsequent washing, the fibres appear to adhere and to be disintegrated by the action of the water, the Nitro- Sidphuric Acid is too weak. In that case add to the acid mixture. Oil of Vitriol, 3 drachms. If the cotton was actually dissolved in the first trial, an addition of half of a fluid ounce of Oil of Vitriol may be required. Supposing the cotton not to be gelatinized and to wash well, then wring it out very dry, pull out the fibres, and treat it in a test-tube with rectified Ether,t to which a few drops of Alcohol have been added. If it be insoluble, dry it by a gentle heat and apply a flame : a brisk explosion indicates that the Nitro- Sulphuric Acid employed is too strong. In that case, add to the twenty drachms of mixed acids, one drachm of water, and test again, repeating the process until a soluble product is obtained. There is a third condition of Pyroxyline, different from either of the above, which may be puzzling : — the fibres of * In the preparation of soluble cotton, and indeed in aU Photographic manipulations, a thermometer is almost indispensable. Instruments of sufficient delicacy for common purposes are sold in Hatton Garden and elsewhere, at a low price. The bulb should be uncovered, to admit of being dipped in acids, etc., without injury to the scale. t Observe that the Ether be pure ; if it contains too much water and Alcohol, it wiU not dissolve the Pyroxyline, or will yield an opalescent so- lution. 190 PREPARATION" OF COLLODION. tlie Cotton mat together very slightly or not at all on im- mersion, and the washing proceeds tolerably well ; the com- pound formed is scarcely explosive, and dissolves imper- fectly in Ether, leaving little nodules or hard lumps. The ethereal solution yields, on evaporation, a film which is opaque instead of transparent. In this case (presuming the Ether to be good) the acid mixture is slightly too weak, or the temperature is too low, being probably about 90°, instead of 130° to 140° (?). When the acid mixture has been brought to the proper strength by a few preliminary trials, proceed according to the directions given at the next page. PREPARATION OF NITRO-SULPHURIC ACID BY OIL OF VITRIOL AND NITRE. This process is recommended, in preference to the other, to the amateur who is unable to obtain Nitric Acid of con- venient strength. The common Oil of Vitriol sold in the shops is often very good for Photographic purposes ; but it is best, if possible, to take the specific gravity, when any doubt exists of its genuineness. At a temperature of 58° to 60°, specific gravity 1*833 is the usual strength, and if it falls below this, it should be rejected. (See Part III. for * Impurities of Commercial Sulphuric Acid.') The Nitre must be the purest sample which can be ob- tained. Commercial Nitre often contains a large quantity of Chloride of JPotassimn, detected on dissolving the Nitre in distilled water, and adding a drop or two of solution of Nitrate of Silver. If a milkiness and subsequent curdy deposit is formed. Chlorides are present. These Chlorides are injurious ; after the Oil of Vitriol is added, they de- stroy a portion of Nitric Acid by converting it into brown fumes of Peroxide of Nitrogen, and so alter the strength of the solution. Nitrate of Potash is an anhydrous salt, — it contains simply Nitric Acid and Potash, without any water of crys- tallization ; still, in many cases, a little w^ater is retained PEEPAEATION OF COLLODION. 191 mechanically between the interstices of the crystals, and therefore it is better to dry it before use. This may be done by laying it in a state of fine powder upon blotting- paper, close to a fire, or upon a heated metallic plate. The sample must also be reduced to a fine powder before adding the Oil of Vitriol ; otherwise portions of the salt escape decomposition. These preliminaries having been properly observed, weigh out Pure ]N"itre, powdered and dried, 600 grains. This quantity is equivalent to IJ ounce Troy or Apotheca- ries' weight ; — and to IJ ounce Avoirdupois weight plus 54 grains. Place this in a teacup or any other convenient vessel, and pour upon it. Water . . . IJ fiuid drachms mixed with Oil of Vitriol .12 „ Stir well with a glass rod for two or three minutes, until all efiervescence has ceased, and an even, pasty mixture, free from lumps, is obtained. During the whole process, abundance of dense fumes of Nitric Acid will be given off*, which must be allowed to escape up the flue or into the open air. A modification of tJie formula. — The above formula will invariably succeed with a good sample of acid and pure Nitre. When tried however with Oil of Vitriol rather weaker than ordinary, and commercial Nitre, it may fail, the cotton being gelatinized and dissolved. When such is the case, the addition of water must be omitted or the quantity reduced from one drachm and a half to half a drachm. GENEEAL DIEECTIONS FOE IMMEESING, WASHING, AND DEYING THE PYEOXYLINE. The mixture of Sulphuric Acid and Nitre requires to be used immediately after its preparation, as it solidifies into 192 PEEPAEATION OF COLLODION. a stiff mass on cooling ; but tbe mixed acids may be kept for any length of time in a stoppered bottle. When Cotton is used, the fibres should be well pulled out, and small tufts added one by one to the acid mixture, stirring with a glass rod in order to keep up a constant change of particles. The Paper is cut into squares or strips, which are introduced singly. In either case the quantity must not be too great, or some portions will be imperfectly acted upon ; about 20 grains to each fluid ounce of the mixture will be sufficient. The time of immersion required varies from ten minutes with Cotton, to twenty minutes or even half an hour with the Paper. When an unusually large proportion of Sul- phuric Acid is used, as in the case of a weak sample of I^itric Acid, the Cotton should be removed at the expira- tion of six or seven minutes, as there is a tendency to par- tial solution of the Pyroxyline in the acid mixture under tliose circumstances. It is an advantage in some cases to prepare the material at a high temperature, but unless the proportions of the Acids are strictly according to Mr. Hadow's formula, so- lution of the Cotton may take place if the thermometer in- dicates more than 140^. After the action is complete, the Nitro- Sulphuric Acid is left weaker than before, from addition of various atoms of water necessarily formed during the change. Hence, if the same portion be used more than once, an addition of Sulphuric Acid will be required. Directions for Washing. — In removing the Pyroxyline from the Nitro- Sulphuric Acid, press out as much of the liquid as possible, and wash it rapidly in a large quantity of cold water, using a glass rod to preserve the fingers from injury. If it were simply thrown into a small quan- tity of water and allowed to remain, the rise in tempera- ture and weakening of the acid mixture might do mis- chief. The washing should be continued for at least a quarter PEEPARATION OF COLLODION. 193 of an hour, or longer in the case of Paper, as it is essential to get rid of every trace of acid. When the Nitre plan has been adopted, a portion of the Bisulphate of Potash formed adheres to the fibres, and if not carefully washed out, an opalescent appearance is seen in the Collodion, re- sulting from the insolubility of this salt in the ethereal mixture. If no acid taste can be perceived, and a piece of blue litmus -paper remains in contact with the fibres for five minutes without changing in colour, the product is tho- roughly washed. It is however a safe plan to place the Pyroxyline in running water and allow it to remain for se- veral hours. Lastly, wring it out in a cloth, pull out the fibres, and dry slowly, by a moderate heat. After drying, it may be kept for any length of time in a stoppered bottle. EECAPITULATION OF THE GENERAL CHARACTERS OF PY- ROXYLINE PREPARED IN NITRO-SULPHURIC ACID OF VARIOUS DEGREES OF CONCENTRATION. The acid mixture too strong. — The appearance of the cotton is not much altered on its first immersion in the mixture. It washes well, without any disintegration. On drying, it is found to be strong in texture, and produces a peculiar crackling sensation between the fingers, like starch. It explodes on the application of flame, without leaving any ash. It is insoluble in the mixture of Ether and Alcohol, but dissolves if treated with Acetic Ether. The acid mixture of the jpro^per strength, — JNTo aggluti- nation of the fibres of the cotton on immersion, and the product washes well ; soluble in the ethereal mixture, and yields a transparent film on evaporation. The acid mixture too weah. — The fibres of the cotton agglutinate, and the Pyroxyline is washed with difficulty. On drying, the texture is found to be short and rotten. It does not explode on being heated, but either burns quietly with a flame, leaving behind a black ash — in which case o 194 PEEPAEATION OF COLLODION. it consists simply of unaltered cotton, — or is only slightly combustible, and not explosive. It dissolves more or less perfectly in glacial Acetic Acid. When treated with the ethereal mixture, it is acted on partially, leaving behind lumps of unchanged cotton ; the solution does not form an even transparent layer on evaporation, but becomes opaque and cloudy as it dries. This opacity however may be seen to a small extent with any sample of Pyroxyline, if the sol- vents contain too much water. In using Swedish Paper in place of Cotton, the Pyroxy- line formed in too weak a Nitrosulphuric Acid is usually insoluble in Ether and Alcohol, and burns slowly like un- changed paper. By studying these characters, and at the same time bearing in mind that a drachm and a half of loater in the quantities of acid given in the formula (p. '88) will suffice to cause the difference, the operator will overcome all dif- ficulties. PURIFICATION OF THE SOLVENTS EEQUIEED FOE COLLODION. The purity of the Ether employed is a matter of as much importance in the manufacture of a good Collodion as that of any other ingredient ; this point must be attended to in order to secure a good result. There are four kinds of Ether sold by manufacturing chemists ; first, ordinary rectified Sulphuric Ether, con- taining a certain percentage of Alcohol and of water ; spe- cific gravity about '750. Second, the washed Ether, which is the same agitated with an equal bulk of water, to remove the Alcohol : by this proceeding the specific gravity of the fluid is reduced considerably. Third, Ether both washed and re-rectified from a caustic alkali, so as to contain nei- ther Alcohol nor water ; in this case the specific gravity should not be higher than '720. Eourth, "Methylated" Ether, manufactured at a lower price than the others. Ivectified Ether of 750° is not to be depended on, inas- PREPARATION OF COLLODION. 195 much as tlie specific gravity is often made up by adding water instead of alcohol. Methylated Ether should be used only when economy is an object, as it is prone to acidity and less certain in its properties. Some of the qualities which render Ether unfit for Pho- tographic purposes, are as follows : — a peculiar and dis- agreeable smell, either of some essential oil, or of Acetic Ether ; an acid reaction to test-paper ; a property of turning alcoholic solution of Iodide of Potassium brown with unusual rapidity ; an alkaline reaction to test-paper ; a high specific gravity, from superabundance of Alcoliol and water. The Ether which has been both washed and redistilled is always the most uniform in composition, and especially so if the second distillation be conducted from Quicklime, Carbonate of Potash, or Caustic Potash. These Alkaline substances retain the impurities, which are often of an acid nature, and leave the Ether in a fit state for use. The redistillation of Ether is a simple process : in deal- ing with this fluid however the greatest caution must be exercised, on account of its inflammable nature. Even in pouring Ether from one bottle into another, if a light of any kind be near, the vapour is apt to take fire ; and se- vere injuries have been occasioned from this cause. JPurification of Ether hy redistillation from a caustic or carbonated allcali. — Take ordinary rectified Sulphuric Ether, and agitate it with an equal bulk of water to wash out the Alcohol ; stand for a few minutes until the contents of the bottle separate into two distinct strata, the lower of which — id est, the watery stratum — is to be drawn off and rejected. Then introduce Caustic Potash, finely powdered, in the proportion of about one ounce to a pint of the t washed Ether ; shake the bottle again many times, in order that the water — a small portion of which is still pre-: sent in solution in the Ether — may be thoroughly absorbed. Afterwards set aside for twenty-four hours (not longer, or the Potash may begin to decompose the Ether), when it 196 PEEPAEATION OF COLLODION. will probably be observed that the liquid has become yel- low, and that a flocculent deposit has formed in small quan- tity. Transfer to a retort of moderate capacity, supported in a saucepan of warm water, and properly connected with a condenser. On applying a gentle heat, the Ether distils over quietly, and condenses with very little loss ; care must be taken that none of the alkaline liquid contained in the body of the retort finds its way, by projection or other- wise, into the neck, so as to run down and contaminate the distilled fluid. A more economical plan of purifying Ether is, without previous washing with water, to agitate with Carbonate of Potash or with Quicklime, and redistil at a moderate tem- perature. In order to preserve Ether from decomposition, it must be kept in stoppered bottles, nearly full, and in a dark place. The stoppers should be tied over with bladder and luted, or a considerable amount of evaporation will take place, unless the neck of the bottle has been ground with unusual care. After the lapse of some months, probably a certain amount of decomposition, evidenced by the libera- tion of Iodine on adding Iodide of Potassium, will be found to have taken place. This however is small in amount, and not of a character to injure the fluid. Rectification of Spirits of Wine from Carbonate of Potash. — The object of this operation is to remove a portion of water from the spirit, and so to increase its strength. Alcohol thus purified may be added to Collo- dion almost to any extent, without producing glutinosity and rottenness of film. The salt termed Carbonate of Potash is a deliquescent salt, — that is, it has a great attraction for water ; conse- quently when Spirits of Wine are agitated with Carbo- nate of Potash, a portion of water is removed, the salt dissolving in it and forming a dense liquid, which refuses to mix with the Alcohol, and sinks to the bottom. At the expiration of two or three days, if the bottle has been PREPAKATION OF COLLODION. 197 sliaken frequently, the action is complete, and tlie lower stratum of fluid may be drawn off and rejected. Pure Carbonate of Potash is an expensive salt, and a commoner variety may be taken. It should be well dried on a heated metal plate, and powdered, before use. The quantity may be about two ounces to a pint of spirit ; or more, if an unusually concentrated Alcohol is required. After the distillation is complete, a fluid is obtained con- taining about 90 per cent, of absolute Alcohol, the remain- ing 10 per cent, being water. The specific gravity at 60o Fahrenheit should be from '815 to '825 ; commercial Spirit of Wine being -836 to '840. PEEPARATION OF THE IODIZING COMPOUNDS IN A STATE OF PUEITY. These are the Iodides of Potassium, Ammonium, and Cadmium. The properties of each are more fully described dn Part III. a. The Iodide of Potassium. — Iodide of Potassium, as sold in the shops, is often contaminated with various im- purities. The first and most remarkable is Carbonate of Potash. When a sample of Iodide of Potassium contains much Carbonate of Potash, it forms small and imperfect crj^stals, which are strongly alkaline to test-paper, and be- come moist on exposure to the air, from the deliquescent nature of the Alkaline Carbonate. Sulphate of Potash is also a common impurity ; it may be detected by Chloride of Barium. A third impurity of Iodide of Potassium is Chloride of Potassium ; it is detected as follows : — Precipitate the salt by an equal weight of Nitrate of Silver, and treat the yel- low mass with solution of Ammonia ; if any Chloride of Silver is present, it dissolves in the Ammonia, and, after filtration, is precipitated in white curds by the addition of an excess of pure Nitric Acid. If the Nitric Acid em- ployed is not pure, but contains traces of free Chlorine, 198 PEEPAEATION OF COLLODION. the Iodide of Silver must be well waslied with distilled water before treating it with Ammonia, or the excess of free Nitrate of Silver dissolving in the Ammonia would, on neutralizing, produce Chloride of Silver, and so cause an error. lodate of Potash is a fourth impurity often found in Iodide of Potassium : to detect it, add a drop of dilute Sul- phuric Acid, or a crystal of Citric Acid, to the solution of the Iodide ; when, if much lodate be present, the liquid will become yellow from liberation of free Iodine. The ra- tionale of this reaction is as follows : — The Sulphuric Acid unites with the base of the salt, and liberates Hydriodic Acid (HI), a colourless compound ; but if Iodic Acid (IO5) be also present, it decomposes the Hydriodic Acid first formed, oxidizing the Hydrogen into Water (HO), and set- ting free the Iodine. The immediate production of a yellow colour on adding a weak acid to aqueous solution of Iodide of Potassium, is therefore a proof of the presence of an lodate. As lodate of Potash renders Collodion insensitive, this point should be attended to. Iodide of Potassium may be rendered very pure by recrystallizing from Spirit, or by dissolving in strong Alcohol of sp. gr. '823, in which Sulphate, Carbonate, and lodate of Potash are insoluble. The proportion of Iodide of Potassium contained in saturated Alcoholic solutions varies with the strength of the spirit {vide Part III., article Iodide of Potassium). Solution of Chloride of Barium is commonly used to detect impurities in Iodide of Potassium ; it forms a white precipitate if Carbonate, lodate, or Sulphate be present. In the two former cases the precipitate dissolves on the addition of pure dilute Nitric Acid, but in the latter it is insoluble. The commercial Iodide however is rarely so pure as to remain quite clear on the addition of Chloride of Barium. b. The Iodide of Ammonium. — This salt may be pre- pared by adding Carbonate of Ammonia to Iodide of Iron, PREPAEATION OF COLLODION. 199 but more easily by the following process : — A strong solu- tion of Hydrosulphate of Ammonia is first made, by pass- ing Sulphuretted Hydrogen gas into Liquor Ammonise. To this liquid, Iodine is added until the whole of the Sulphuret of Ammonium has been converted into Iodide. When this point is reached, the solution at once colours brown from solution of free Iodine. On the first addition of the Iodine, an escape of Sulphuretted Hydrogen gas and a dense deposit of Sulphur take place. After the de- composition of the Hydrosulphate of Ammonia is com- plete, a portion of Hydriodic Acid — formed by the mutual reaction of Sulphuretted Hydrogen and Iodine — attacks any Carbonate of Ammonia which may be present, and causes an efiervescence. The efFeryescence being over, the liquid is still acid to test-paper, from excess of Hy- driodic Acid ; it is to be cautiously neutralized with Am- monia, and evaporated by the heat of a water-bath to the crystallizing point. The crystals should be thoroughly dried over a dish of Sulphuric Acid, and then sealed in tubes ; by this means it will be preserved colourless. Iodide of Ammonium is very soluble in Alcohol, but it is not advisable to keep it in solution, from the rapidity with which it decomposes and becomes brown. The most common impurity of commercial Iodide of Ammonium is Sulphate of Ammonia ; it is detected by its sparing solubility in Alcohol. Carbonate of Ammonia is also frequently present to a large extent, in which case an alkaline Collodion and eventually an alkaline Nitrate Bath will be produced. e. Iodide of Cadmium. — This salt is formed by heating filings of metallic Cadmium with Iodine, or by mixing the two together with addition of water. Iodide of Cadmium is very soluble both in Alcohol and Water; the solution yielding on evaporation large six- sided tables of a pearly lustre, which are permanent in the air. The commercial Iodide is sometimes contaminated 200 PEEPAKATION OF COLLODION. with Iodide of Zinc ; tlie crystals being imperfectly formed and slowly liberating Iodine wben dissolved in Ether and Alcohol. Pure Iodide of Cadmium remains nearly or quite colourless in Collodion, if the fluid be kept in a cool and dark place. 201 CHAPTER 11. POEMrL^ FOR SOLTJTIOKS EEQUIRED TN THE COL- LODION PROCESS. Section I. — Solutions for direct Positives. Section II. — Solutions for Negative Pliotograplis. SECTION I. Formulce for Solutions for direct Positives. The solutions are taken in tlie following order; — The Collodion. — The Nitrate Bath. — Developing fluids. — Fix- ing liquids. — Whitening solution. THE COLLODION. Formula No. 1. Purified Ether, sp. gr. '720 ... 5 fluid drachms. Purified Alcohol, sp. gr. -825 . . . 3 „ Pyroxyline 3 to 5 grains. Pure Iodide of Cadmium or Ammo- nium 4 grains. Formula No. 2. Eectified Ether, sp. gr. '750 ... 6 fluid drachms. Spirits of Wine, sp. gr. '836 . . . 2 „ „ Pyroxyline 2 to 4 grains. Iodide of Potassium or Ammonium . 3 to 4 „ 202 FOEMUL^ FOE SOLUTIONS. If tlie operator wislies to prepare a stock of tlie plain Collodion, and to iodize as required, tlie last formula will stand thus : — Dissolve the Pyroxyline, and let the fluid stand for forty-eight hours to subside, then draw off clear, with a siphon. To each fluid ounce of this plain Collodion add about two fluid drachms of the following iodizing mixture : — Alcohol, sp. gr. '836 1 fluid ounce. Iodide of Potassium .... 16 grains. Of the two formulse above given, the first is considered the best, but the second may be substituted for it when highly rectified spirits cannot be obtained. Iodide of Am- monium chemically pure is perhaps superior to any other Iodide for preparing a portrait Collodion, but Iodide of Cadmium, with addition of free Iodine, possesses better keeping properties, and gives very good results. A mix- ture of the two Iodides may also be used advantageously, or Iodide of Potassium may be combined with Iodide of Cadmium : this preparation has been much recommended, but the Collodion will be liable to produce a spotted film unless the salts are quite pure. The exact quantity of Pyroxyline will vary with the temperature at which the preparation was made. The Collodion should flow smoothly on the glass and remain free from crapy lines on setting. When Iodide of Cad- mium is used, the tendency to glutinosity will be a little greater than usual, which must be obviated by the directions given at page 83. The film, after dipping in the Bath, should appear opales- cent and not too yellow and creamy. Pale-blue films yield very good Positives, but with more liability to failure than thicker films (p. 109). Eectified Ether, -750 Alcohol of -836 . . 3 fluid ounces. 2 fluid drachms, 8 to 14 grains. Pyroxyline FOEMUL^ FOR SOLUTIONS. 203 If the Positives are not perfectly clear and transparent in the shadows, dissolve 5 grains of Iodine in an ounce of Spirits of Wine (not methjdated), and add a few drops until the Collodion assumes a golden-yellow colour. 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- liydroiis 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 often become some- what glutinous if the proportions (by measure) of 5 parts of Ether to 3 of Alcohol be exceeded. Collodion prepared by Formula No. 1, and iodized with Iodide of Cadmium, may be kept for weeks or months without much loss of sensitiveness ; but when Alkaline Iodides are employed as in the second Formula, Iodine is liberated, and the fluid becomes at last brown and insen- sitive. Nitrate of Silver which has been melted, in order to expel Oxides of Nitrogen, is always the most certain in its action : but the heat must not be raised too high or the salt will be contaminated with Nitrite of Silver. In the Vocabulary (see Part III.) directions are given for the preparation and purification of Nitrate of Silver ; also for the testing of distilled water, and the best substi- tutes when it cannot be obtained. The Bath must be saturated with Iodide of Silver, and THE NITEATE BATH. Nitrate of Silver 3i Nitric Acid niinim, or Acetic 30 grains. i Acid (glacial) Alcohol . . , Distilled water \ minim. 15 minims. 1 fluid ounce. 204 FOEMUL^ FOE SOLUTIONS. JSTitric Acid neutralized if it be present. JN'itrate of Silver however which has undergone fusion is free from Nitric Acid. Weigh out the total quantity of crystals of JN^itrate re- quired for the Bath, and dissolve in about two parts of water. Then take a quarter of a grain of Iodide of Po- tassium to each 100 grains of JN^itrate, dissolve in half a drachm of water, and add to the strong solution ; a yellow deposit of Iodide of Silver first forms, but on stirring is completely redissolved. When the liquid is clear, test for free Nitric Acid by dropping in a piece of blue litmus- paper. If at the expiration of two minutes the paper appears reddened, JSTitric Acid is present, to neutralize which, add solution of Potash or Carbonate of Soda (not Ammonia) until a distinct turbidity, rem.aining after agi- tation, is produced (an excess does no harm). Then dilute down the concentrated solution with the remaining portion of the water, stirring all the time, and filter out the milky deposit. If the liquid does not at first run clear, it will probably do so on passing it again through the same filter. Lastly, add the Acetic Acid (previously tested for im- purities, see Part III.) and the Alcohol to the filtered liquid. As the bulk of the Bath becomes lessened by use, fill it up with a solution containing 40 grains of JN'itrate to the ounce, which will be found sufficient to maintain the strength nearly at the original point. The common practice of occasionally dropping Am- monia or Potash into the solution, to remove Nitric Acid liberated by free Iodine in the Collodion, is not recom- mended (see p. 89). When the Bath becomes old, and yields Positives which are highly intense or stained, and slightly foggy, with a deficiency of half-tone, it will be advisable to precipitate it with a Chloride and prepare a new one. FORMULA FOE SOLUTIONS. 205 THE DEVELOPING FLUIDS. Either of the three following formulae may be used, ac- cording to the taste of the operator : — FORMULA No. 1. Sulphate of Iron, recrystallized . 12 to 20 grains. Acetic Acid (glacial) .... 20 minims. Alcohol 10 minims. Water 1 fluid ounce. FORMULA No. 2. Pyrogallic Acid 2 grains. jN'itric Acid 1 drop. "Water 1 fluid ounce. FORMULA No. 3. Solutionof Protonitrateof Iron . 1 fluid ounce. Alcohol 20 minims. In all these formulae, if distilled water is not at hand, read the directions in the Vocabulary, Part III., Article "Water," for the best substitute. Itemarlcs upon these Formulce. — Formula No. 1 is the most simple, since the solution can he used as a Bath, the same portion being employed many times successively. If it acts too rapidly, lessen the proportion of Sulphate of Iron. An addition of Nitric Acid, half a minim to the ounce, makes the image whiter and more metallic; but if too much is used, the development proceeds irregularly, and spangles of Silver are formed. The Alcohol and Acetic Acid render the development uniform by causing the solution of Protosulphate to com- bine more readily with the film. The latter also has an effect in whitening the image and increasing its brightness. Solution of Sulphate of Iron becomes red on keeping, from a gradual formation of ^^ersalt. When it is too weak, add more of the Protosulphate. The muddy deposit which 206 FORMULA FOE SOLUTIONS. settles to the bottom of tlie Batli is metallic Silver, reduced from the soluble Nitrate upon the plates. Some operators add pure Nitrate of Potash to this developing solution, to form a small portion of Protoni- trate of Iron. It is said to improve the colour slightly. The proportions are 10 grains of Nitrate of Potash to about 14 or 15 grains of Protosulphate of Iron. formula No. 2. — In this formula, if the colour of the image is not sufficiently white, try the effect of increasing the amount of Nitric Acid slightly. On the other hand, if the development is imperfect in parts, and patches of a green colour are seen, use three grains of Pyrogallic Acid to the ounce, with less Nitric Acid. A few drops of Nitrate of Silver solution added to the Pyrogallic, im- mediately before use, will augment the energy of deve- lopment when blue and green spots occur. Forymda No. 3, or Protonitrate of Iron, does not re- quire any addition of Acid ; but it will be advisable, in some cases, to add to it a few drops of Nitrate of Silver immediately before developing. It gives a bright metallic image, resembling that obtained by adding Nitric Acid to Protosulphate of Iron. The following process is commonly followed for pre- paring Protonitrate of Iron : — Take of Nitrate of Baryta 300 grains ; — powder and dissolve by the aid of heat in three ounces of water. Then throw in by degrees, with constant stirring, crystallized Sulphate of Iron, powder edy, 320 grains. Continue to stir for about five or ten minutes. Allow to cool, and filter from the white deposit, which is the insoluble Sulphate of Baryta. In place of Nitrate of Baryta, the Nitrate of Lead may be used (Sulphate of Lead being an insoluble salt), but the quantity required will be difierent. The atomic weights of Nitrate of Baryta and Nitrate of Lead are as 131 to ] 66 ; consequently 300 grains of the former are equivalent to 380 grains of the latter. FORMULA FOR SOLUTIONS. 207 THE FIXING SOLUTION. Cyanide of Potassium . . . . 2 to 12 grains. Common Water 1 fluid ounce. Cyanide of Potassium is usually preferred to Hj^posul- pliite of Soda for fixing direct Positives ; it is less liable to injure the purity of the white colour. The percentage of Carbonate of Potash in commercial Cyanide of Potassium is so variable that no exact directions can be given for the formula. It is best however to use it rather dilute — of such a strength that the plate is cleared gradually in from half a minute to a minute. The solution of Cyanide of Potassium decomposes slowly on keeping, but it will usually retain its solvent power for several weeks. In order to escape inconvenience from the pungent odour evolved by this salt, many employ a verti- cal Bath to hold the solution ; but in that case the plates must be carefully washed before fixing, as the Iron salts hasten the decomposition of the Cyanide. THE WHITENING SOLUTION. Bichloride of Mercury ... 30 grains. Distilled Water 1 fluid ounce. By a gentle application of heat the corrosive sublimate dissolves and forms a solution as nearly as possible satu- rated at common temperatures. The addition of a portion of Muriatic Acid enables the water to take up a larger quantity of Bichloride ; but this concentrated solution, at the same time that it whitens more quickly than the other, is apt to act unequally upon different parts of the image. Before applying the Bichloride, the image is to be fixed and the plate well washed. Either the Protosulphate of Iron or the Pyrogallic Acid with Acetic (p. 223) may be used for the development ; but the whitening process is more rapid and uniform in the latter case. 208 POEMUL^ FOE SOLUTIONS. SECTIOl^r II. FormulcB, etc., for Negative Solutions.^ THE COLLODION. FOKMULA No. 1. Purified Ether, sp. gr. '720 . . 5 fluid draclims. Purified Alcoliol, sp. gr. '825 . 3 fluid drachms. Soluble Pyroxyline 4 to 8 grains. Pure Iodide of Cadmium or Am- monium 4 to 5 grains. FOEMULA No. 2. Hectified Ether, sp. gr. '750 . . 6 fluid drachms. Alcohol, sp. gr. '836 .... 2 fluid drachms. Soluble Pyroxyline 4 to 8 grains. Iodide of Potassium or Ammo- nium 4 grains. When the Collodion and Iodizing mixture are kept se- parate, the second formula will stand thus : — Rectified Ether '750 .... 3 fluid ounces. Alcohol of '836 ...... 2 fluid drachms. Pyroxyline 15 to 30 grains. To each fluid ounce of this plain Collodion add 2 fluid drachms of the following Iodizing solution : — Alcohol, sp. gr. -836 .... 1 fluid ounce. Iodide of Potassium .... 20 grains. When the temperature of the IN'itro- Sulphuric Acid used in making the Pyroxyline is high (140° to 155°), it often happens that the Collodion is too fluid with 4 grains of soluble paper to the ounce, and forms a blue transparent film of Iodide on dipping the plate in the Bath. In that * The same Collodion and Nitrate Bath may be used both for Positives and Negatives if required ; but there are a few minor points of difference which are included in the following remarks. FORMULJE FOE SOLUTIONS. 209 case, increase the quantity of Pyroxyline from 4 grains to 6, or even to 8 grains to each ounce. If the Collodion is glutinous, and produces a wavy sur- face, with less than 4 grains of Pyroxyline to the ounce, it is probable that the Alcohol is too weak, or that the soluble Cotton is badly made. If flakes of Iodide of Silver are seen loose upon the surface of the film, and falling away into the Bath, the Collodion is over-iodized, and it will be impossible to ob- tain a good picture. After the Collodion has been employed to coat a number of plates, the relative proportions of Alcohol and Ether contained in it become changed, from the superior vola- tility of the latter fluid : when it ceases to flow readily, and gives a more dense film than usual, thin it down by the addition of a little rectified Ether. In dissolving the Pyroxyline, any fibrous or flocculent matter which resists the action of the Ether, must be al- lowed to subside, the clear portion being decanted for use. The Iodide of Potassium is to be finely poiodered, and di- gested with the spirit until dissolved ; it is better not to apply any heat. Both Iodide of Ammonium and Iodide of Cadmium dissolve almost immediately, if the salts are pure. The Collodion must be kept in a cool and dark place. When prepared with Iodide of Ammonium or Potassium it becomes at length high coloured and insensitive. The free Iodine may then be removed by a strip of pure zinc or silver foil, When sensitiveness is not an object, many prefer work- ing with an old, coloured Collodion, finding that it gives more intensity. It has been shown at page 97 that a pe- culiar change takes place in Collodion after iodizing, by which the intensity of the image is increased. Directions for using Glycyrrhizine in Collodion. — The action of this material has been described at page 114. The Collodion should be iodized with the Iodide of Cadmium p 210 eormuljE foe solutions. only, or with a mixture of the Iodides and Bromides of the alkalies. The condition which calls for the employment of Glycyrrhizine is that often found in a newly made and rather glutinous Collodion, viz. sensitiveness of film, with good half-tones, but insufficient intensity in the high lights. Dissolve the Glycyrrhizine in Alcohol (not Methy- lated) in the proportion of 5 grains to the ounce : this so- lution may perhaps keep unchanged for three or four months. To each ounce of the Collodion add from one to four drops, and expose in the Camera a few seconds longer than before. The effect of the Glycyrrhizine upon the Col- lodion may not be fully produced immediately ; if so, the fluid must be set aside for twenty-four hours. Use of Nitro-glucose in Collodion. — Nitro-glucose is a substance analogous to Pyroxyline, but more unstable. When added to Collodion iodized with the alkaline Iodides, it slowly decomposes, liberates Iodine, lessens the sensi- tiveness to a certain extent, and confers intensity. Like Glycyrrhizine, it may be used to remedy feebleness of the image, and to give opacity to the blacks. Prepare the JSTi- tro-glucose by the directions given in the Vocabulary, Part III. Dissolve twenty grains in an ounce of pure spirit, and agitate with powdered chalk to remove free acid. Add from five to eight drops to each ounce of Collodion. In a few days, more or less, according to temperature, the Collodion will deepen in colour, and will be found on trial to produce a more vigorous picture. Collodion for hot Climates. — In this case the Iodide of Ammonium should be avoided, as unstable and prone to change colour. Iodide of Cadmium may be substituted, which has been shown to remain quite colourless when dissolved in Alcohol and Ether. Collodion iodized with the Iodide of Potassium will usually keep for about six weeks or two months ; but no certain rule can be given, much depending upon the con- dition of the Ether and the heat of the weather. Plain Collodion may retain its properties unimpaired for FORMULA FOR SOLUTIONS. 211 five or six months, sometimes mucli longer; but there is a tendency to a formation of the acid principle (p. 85) ; and hence, on the addition of an alkaline Iodide to old Collodion, the coloration is commonly very rapid. The structure of the transparent film may also be injured by keeping plain Collodion for too long a time. Photographers who wish to operate with Collodion in hot climates will find it advantageous to carry with them the prepared Pyroxyline and the spirituous solvents, ob- serving that the bottles are carefully luted, and that a bubble of air is left in the neck of each, to allow for the necessary expansion, which might otherwise burst the glass or force out the stopper. THE NITRATE BATH. This solution may be prepared by the same formula as that given for direct Positives at page 203, acidifying the solution with Acetic Acid in preference to Nitric Acid. THE DEVELOPING SOLUTION. Pyrogallic Acid 1 grain. Acetic Acid (glacial) . . . . 10 to 20 minims, or Beaufoy's Acetic Acid fort. 1 fluid drachm. Alcohol 10 minims. Distilled Water 1 fluid ounce. In place of Distilled Water, pure Eain- Water may be used (see Part III., Art. "Water"). The quantity of Acetic Acid required will vary with the strength of the Acid and the temperature of the atmo- sphere. An excess enables the manipulator to cover the plate more easily before the action begins, but when the picture is taken in a dull light, is apt to give a bluish, inky hue to the image. In cold weather, use less of the Acetic and twice the quantity of Pyrogallic Acid. With Collo- dion prepared from Spirits nearly anhydrous, and iodized with Iodide of Cadmium, the full quantity of Acetic Acid 212 FORMULA FOR SOLUTIONS. will be required, as there is sometimes a little difficulty in making the developer flow up to the edge of the film. If the image cannot be rendered sufficiently black, two or three minims of the Nitrate Bath solution may be added to each drachm towards the end of the development. If the solution be kept for some time after its first pre- paration, it becomes brown and discoloured. In this state it will still develope the image, but is less likely to give a clear and vigorous picture. A solution of Pyrogallic Acid in Acetic Acid will keep for many weeks, and may be di- luted down when required for use. The following is a good formula : — Pyrogallic acid ... 12 grains. Beaufoy's Acetic acid . 1 fluid ounce. To one draclim add seven drachms of water. THE FIXING Cyanide of Potassium . Water or, Hyposulphite of Soda . Water Eor remarks on the Cyanide see the last Section, page 207. LIQUID. 2 to 12 to 20 grains. 1 fluid ounce. ^ ounce. 1 fluid ounce. of Potassium Fixing Bath, 213 CHAPTER III. MAKIPTJLATIOlSrS OF THE COLLODIOIS" PROCESS. These may be classed under five lieads : — Cleaning the Plates. — Coating witli Iodide of Silver. — Exposure in tlie Camera. — Developing tlie image. — Fixing the image. — In addition to this, the present Chapter will include in sepa- rate Sections directions for the choice and management of lenses, for copying engravings, manuscripts, etc., and for taking stereoscopic and microscopic photographs. CLEANING THE GLASS PLATES. Care should be taken in selecting glass for use in Photo- graphy. The ordinary window-glass is inferior, having scratches upon the surface, each of which may cause an irregular action of the developing fluid ; and the squares are seldom flat, so that they are apt to be broken in com- pression during the printing process. The patent plate answers better than any other descrip- tion of glass ; but if it cannot be procured, the " flatted crown glass" may be substituted. Before washing the glasses, each square should be rough- ened on the edges by means of a file or a sheet of emery- paper ; or more simply, by drawing the edges of two plates across each other. If this precaution be omitted, the fingers are liable to injury, and the Collodion film may contract and separate from the sides. 214 MANIPULATIONS OF In cleaning glasses, it is not sufficient, as a rule, to wash tliem simply with water ; other liquids are required to re- move grease, if present. A cream of Tripoli powder and Spirits of Wine, with a little Ammonia added, is commonly- employed. A tuft of cotton is dipped in this mixture, and the glasses are well rubbed with it for a few minutes. They are then rinsed in plain water and wiped dry with a cloth. The cloths used for cleaning glasses should be kept ex- pressly for that purpose ; they are best made of a mate- rial sold as fine " diaper," and very free from flocculi and loosely-adhering fibres. They are not to be washed in soap and water, but always in pure water or in water con- taining a little Carbonate of Soda. After wiping the glass carefully, complete the process by polishing with an old silk handkerchief, avoiding con- tact with the skin of the hand. Some object to silh, as tending to render the glass electrical, and so to attract particles of dust, but in practice no inconvenience will be experienced from this source. Before deciding that the glass is clean, hold it in an an- gular position and breathe upon it. The importance of at- tending to this simple rule will be at once seen by referring to the remarks made at page 39. In the Honey preserva- tive and Collodio-albumen processes it is especially needful that the glasses should be thoroughly cleaned, on account of the tendency which the film has to become loosened or to bhster during the development and washings. Caustic Potash, sold by the druggists under the name of " Liquor Potassse, " is very efficacious, or in place of it, a warm solution of "washing Soda" (Carbonate of Soda). Liquor Potassse, being a caustic and alkaline liquid, softens the skin and dissolves it ; it must therefore be diluted with about four parts of water and applied to the glass by means of a cylindrical roll of flannel. After wetting both sides thoroughly, allow the glass to stand for a time until several have been treated in the same way ; then wash with water and rub dry in a cloth. THE COLLODION PEOCESS. 215 Tlie use of an alkaline solution is usually sufficient to clean the glass, but some plates are dotted on the surface with small white specks, not removable by Potash. These specks may consist of hard particles of Carbonate of Lime, and when such is the case they dissolve readily in a dilute acid, — Oil of Vitriol, with about four parts of water added, or dilute Nitric Acid. The objection to the use of Nitric Acid is, that if al- lowed to come in contact with the dress, it produces stains which cannot be removed unless immediately treated with an alkali. A drop of Ammonia should be applied to the spot before it becomes yellow and faded. When Positives are to be taken, it is advisable to use additional care in preparing the glass, and especially so with pale transparent films and neutral, Nitrate Bath. After a glass has been once coated with Collodion, it is not necessary in cleaning it a second time to use anything but pure water ; but if the film has been allowed to harden and become dry, possibly dilute Oil of Vitriol or Cyanide of Potassium may be required to remove stains. When glasses have been repeatedly used in photography they often become at length so dull and stained, that it is better to reject them. COATING THE PLATE WITH THE COLLODIO-IODIDE OP SILVER. This part of the process, with that which follows, must be conducted in a room from which chemical rays of light are excluded. It is inferred therefore that the operator has provided himself with an apartment of that kind. The most simple plan of preparing the room is to nail a treble thickness of yellow calico completely over the win- dow, or a part of it, the remainder being darkened. To this a single thickness of a waterproof material made by coating linen with gutta-percha may be added as a further security against the entrance of white light, the smallest pencil of which admitted into the room would cause fog- ging. 216 MANIPULATIONS OF It is often convenient to illuminate by means of a candle screened by yellow glass. A dark orange yellow, approach- ing to brown, is more impervious to chemical rays than a lighter canary yellow. Lamps suitable for the purpose are sold by the manufacturers of apparatus and chemicals. Before coating the plate with Collodion, see that the fluid is perfectly clear and transparent, and that all par- ticles have settled to the bottom ; also that the neck of the bottle is free from hard and dry crusts, which, if allowed to remain, would partially dissolve and produce strise npon the film. In taking small portraits and stereoscopic sub- jects, these points are of especial importance, and every picture will be spoiled if they are not attended to. A useful piece of apparatus for clearing Collodion is that represented in the following woodcut. The Collodion, having been iodized some hours previ- ously, is allowed to settle down and become clear in this bottle ; then by gently blowing at the point of the shorter tube, the small glass siphon is filled, and the fluid drawn off more closely than could be done by simply pouring from one bottle to another. When the Collodion is properly cleared from sediment, the operator takes a glass plate, previously cleaned, and wipes it gently with a silk handkerchief, in order to re- move any particles of dust which may have subsequently THE COLLODION TROCESS. 217 collected. If it be a plate of moderate size, it may be beld by the corners in a horizontal position, between the fore- finger and thumb of the left hand. The Collodion is to be poured on steadily until a circular pool is formed, extend- ing nearly to the edges of the glass. By a slight inclination of the plate the fluid is made to flow towards the corner marked 1, in the above diagram, until it nearly touches the thumb by which the glass is held : from corner 1 it is passed to corner 2, held by the forefinger; from 2 to 3, and lastly, the excess poured back into the bottle from the corner marked 'No. 4s. It is then to be held vertically over the bottle for a moment, until it nearly ceases to drip, and then, by raising the thumb a little, the direction of the plate is changed, so as to cause the diagonal lines to coalesce and produce a smooth suface. The operation of coating a plate with Col- lodion must not be done hurriedly, and nothing is required to ensure success but steadiness of hand and a sufficiency of the fluid poured in the first instance upon the plate. In coating larger plates, the 'pneumatic holder, which fixes itself by suction, will be found the most simple and useful. The Proper 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 nor dry, but receives the impression of the finger without ad- hering to it. Photographers term this setting, and when 218 MANIPULATIONS OF it takes place it is a sign that tlie time has come for sub- mitting it to the action of the Bath. If the film be lowered into the Nitrate before it has set, the efiect is the same as that produced by adding Water to Collodion. The Pyroxyline is precipitated in part, and consequently there are cracks, and the developer will not always run up to the edge of the film. On the other hand, if it be allowed to become too dry, the Iodide of Silver does not form perfectly, and the film, on being washed and brought out to the light, exhibits a peculiar iridescent ap- pearance, 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 ; thin Collodion containing but little Alcohol requiring to be immersed more speedily. Twenty seconds in the common way, or ten seconds in hot weather, will be found an average time. When the plate is ready, rest it upon the glass dipper, Collodion side uppermost, and lower it into the solution by a slow and steady movement: if any pause be made, a hori- zontal line corresponding to the surface of the liquid will be formed. Then place the cover upon the vertical trough* and darken the room, if this has not already been done. As the presence of white light does no injury to the plate previous to its immersion in the Bath, it is not necessary to exclude it during the time of coating with Collodion. When the plate has remained in the solution about twenty seconds, lift it partially out two or three times, in order to wash away the Ether from the surface. An immersion of one minute to a minute and a half will usually be sufficient; or two minutes in cold weather, and with Collodion con- taining but little Alcohol. Continue to move the plate until the liquid flows off in a uniform sheet, when the de- * Troughs made of gutta-percha, glass, or porcelain are commonly used ; the latter are the best, being quite opaque and not liable to cracks or leaking. THE COLLODION PEOCESS. 219 composition may be considered to be sufficiently perfect. The principal impediment in this part of the process lies in the difficulty 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 washed away, and a smooth and homogeneous layer obtained. The plate is next removed from the dipper, and held vertically in the liand for a few seconds upon blotting paper, to drain off as much as possible of the solution of Nitrate of Silver.* It is then wiped on the back with filtering-paper, placed in a clean and dry slide, and is ready for the Camera. The amateur is strongly recommended not to proceed to take pictures in the Camera until by a little practice he has succeeded in producing a perfect film which is uniform in every part and will bear inspection when washed and brought out to the light. It should, if properly prepared, present the following ap- pearance : — Smooth and uniform, both by reflected and transmitted light ; free from wavy lines or markings such as would be caused by a glutinous Pyroxyline, and from opaque dots due to small particles of dust or Iodide of Silver in suspension in the Collodion. The evidences of a too rapid 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 the other, and always suffers the most ; horizontal cracks or marks resembhng 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 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. • This blotting-paper must be frequently changed, or stains will be pro- duced at the lower edge of the plate during the development. 220 MANIPULATIONS OF EXPOSUEE OF THE PLATE IN THE CAMEEA. After tlie plate has been rendered sensitive, it should be exposed and developed with all convenient despatch ; the intensity of the JN'egatives being, with some Collodion, materially lessened by neglecting this point (see p. 100). Ascertain that the joints of the Camera are tight in every part — that the sensitive plate, when placed in the slide, falls precisely in the same plane as that occupied by the ground glass — and that the chemical and visual foci of the Lens accurately correspond.^ Supposing the case of a portrait, next proceed to arrange the sitter as nearly as possible in a vertical position, that every part may be equidistant from the lens. Then, an imaginary line being drawn from the head to the knee, point the Camera slightly downwards, so that it may stand at right angles to the line. If this point be neglected the figure will be liable to be distorted in a manner presently to be shown (p. 228). In order to succeed well with portraits, the sitter should be illuminated by an even, diffused light falling horizon- tally. A vertical light causes a deep shadow on the eyes and makes the hair appear grey : it must therefore be cut off by a curtain of blue or white calico suspended over the head. The direct rays of the sun are generally to be avoided, as causing too great a contrast of light and shade. This is a point on which the operator must exercise his judgment. With a feeble Collodion, a better Negative pic- ture may often be obtained by placing the sitter quite in the open air, but when the Collodion and Bath are in the condition for giving great intensity of image, the gradation of tone will be inferior unless the light be prevented from falling too strongly upon the face and hands. In focussing the object, cover the head and the back part of the Camera with a black cloth, and shift the Lens * See the Second Section of tliis Chapter. THE COLLODION PROCESS. 221 gently until the greatest possible amount of distinctness is obtained. Then insert the sensitive plate, and having raised the door of the slide, cover all with a black cloth during the exposure, as a security against white light find- ing entrance at any part excepting through the Lens. With regard to the proper time for the exposure, so much depends upon the brightness of the light and the na- ture of the Collodion, that it must be left almost entirely to experience. The following general rules however may be of use : — In a tolerably bright day in the spring or summer months, and with a newly-mixed Collodion, allow four seconds for a Positive Portrait, and eight seconds for a Negative. With a double -combination Lens of large aper- ture and short focus, perhaps three seconds, and six se- conds, or even less, may be sufficient. Li the dull winter months, in the smoky atmosphere of large cities, or when using an old Collodion brown from free Iodine, multiply these numbers three or four times, which will be an approximation to the exposure required. It is by the appearance presented under the influence of the developer, which will immediately be described, that the operator ascertains the proper time for exposure to light. THE DEVELOPMENT OF THE IMAGE. The details of developing the latent image differ so much in the case of Positive and Negative pictures, that it is better to describe the two separately. The development of direct Positives. — With Sulphate of Iron as a developer, it is most simple to develope the image by immersion. The solution may conveniently be poured into a vertical trough, such as that used for exciting, and the plate immersed by means of a glass dipper in the usual way. Unless the weather be cold, the image makes its ap- pearance in three or four seconds, and the film is then im- mediately washed with clear water. Whilst in the Bath, 222 MANIPULATION'S OF the plate is kept in gentle motion, and the operator must not expect to see the image very distinctly, except the high lights ; the shadows, being faint, are partially con- cealed by the unaltered Iodide, but they come out during the fixing. The action of the Sulphate of Iron is stopped at an early period, or an excess of development will be incurred. The Bath may be used repeatedly. In using Pyrogallic Acid or Nitrate of Iron to develope glass Positives, the plate may be placed upon a levelling- stand, or held in the hand, or by the pneumatic holder, and the solution poured on quickly at one corner ; by blowing gently or inclining the hand, as the case may be, it is scattered evenly over the film before the development commences. If any difficulty is experienced in covering a plate evenly with a strong developer before the action commences, it may be overcome by using a shallow cell formed by ce- menting two or three thicknesses of window-glass on a piece of patent plate to the depth of a quarter of an inch. The size of the cell should be only slightly larger than the plate intended to be developed, that the waste of fluid may be as little as possible. The cell is held in the left hand, and the plate being placed in it, a sufficient quantity of the developer is poured on at one corner. By a slight inclination, the fluid is caused to flow in a uniform sheet over the surface of the film, backwards and forwards. The image starts out quickly, and the developer is then at once poured off*, and the film washed as before. It is very important in developing Positives to use a sufficient quantity of the solution to cover the plate easily ; otherwise oily stains and marks are formed, from the de- veloper not combining properly with the surface of the film. For a plate five inches by four, three or four drachms will be required, and so in proportion for larger sizes. The appearance of the Positive image after developing, as a guide to the proper time of exposure. — When the THE COLLODION PROCESS. 223 plate has been developed, it is waslied, fixed, and laid upon a dark ground, such as a piece of black velvet, for inspection. In the case of a portrait, if the features have an un- naturally black and gloomy appearance, the dark portions of the drapery, etc., being invisible, the picture has been under-exjposed. On the other hand, in an over-exposed plate, the face is usually pale and white, and the drapery misty and indis- tinct. Much however in this respect depends upon the dress of the sitter (see p. 66), and the manner in which the light is thrown ; if the upper part of the figure is shaded too much, the face may perhaps be the last to be seen. The operator should accustom himself to expend pains in the preliminary focussing upon the ground glass, and to ascertain at that time that every part of the object is equally illuminated. For this reason, pictures taken in a room are seldom successful ; the light falls entirely upon one side, and hence the shadows are dark and indis- tinct. The development of Negative Pictures. — This process differs in most respects from that of Positives. In the latter case, there is a tendency to over-develope the image ; but in the former, to stop the action at too early a period ; hence it is common to find ISTegative Pictures which are insufficiently developed, and too pale to print well. In developing Negatives, many operators place the plate upon a levelling- stand, and distribute the fluid by blowing gently upon the surface ; others prefer holding it in the hand and pouring the fluid on and off from a glass measure. The quantity of developer required will be less than that used for Positives, inasmuch as, if the Acetic Acid be pre- sent in sufficient excess, it is easy to cover the plate before the action begins. Some Collodion however, especially the glutinous kind, seems to repel the developer and prevent it from running up to the edge of the plate. When this is the case, or when oiliness and stains are produced, from 224 MANIPULATIONS OF the Bath being old and containing Ether, Alcohol must be added to the solution of Pyrogallic Acid. With ordinary JN'egatiYe Collodion, an addition of Ni- trate of Silver to the developer will often be required ; but the Pyrogallic Acid is to be used alone until the image has reached its maximum of intensity, which it will do in a minute or so, according to the temperature of the deve- loping room. The plate may then be examined leisurely by placing it in front of, and at some distance from, a sheet of white paper. If it is not sufficiently black, add about four drops of the Nitrate Bath to each drachm of developer, stir well with a glass rod, and continue the action until the requisite amount of intensity is obtained. When there is any disposition in the plate to fog towards the end of the development, it may be obviated by fixing with Cyanide of Potassium (not Hyposulphite), and then, after a careful washing, intensifying with Pyrogallic Acid and Nitrate of Silver in the usual way. The glass which contains the mixture of Pyrogallic Acid and Nitrate of Silver must be washed out after each plate, as the black deposit hastens the discoloration of the fresh solution (p. 179). Appearance of the Negative image during and after the reducing process, as a guide to the exposure to light. — An under-exposed plate developes slowly. By continuing the action of the Pyrogallic Acid, the high lights become very hlaclc, but the shadows are invisible, nothing but the yellow Iodide being seen on those portions of the plate. After treatment with the Cyanide, the picture shows weU as a Positive, but by transmitted light aU the minor de- tails are invisible ; the image is black and white, without any half-tone. An over-exposed Negative developes rapidly at first, but soon begins to blacken slightly at every part of the plate. After the fixing is completed, nothing can often be seen by reflected light but a uniform grey surface of metallic Sil- ver, without any appearance (or, at most, an indistinct one) of an image. By transmitted light the plate may appear THE COLLODION PROCESS. 225 of a red or brown colour, and the image is faint and dull. The clear parts of the Negative being obscured by the fogging, and the half- shadows having acted so long as nearly to overtake the lights, there is a want of proper contrast ; hence the over-exposed plate is the exact con- verse of the under-exposed, where the contrast between lights and shadows is too well marked, from the absence of intermediate tints. A Negative which has received the proper amount of exposure, usually possesses the following characters after the development is completed : — The image is partially but not fully seen by reflected light. In the case of a portrait, any dark portions of drapery show well as a Po- sitive, but the features of the sitter are scarcely to be dis- cerned. The plate has a general aspect as of fogging about to commence, but not actually established. By transmitted light the figure is bright, and appears to stand out from the glass : the dark shadows are clear, without any misty deposit of metallic Silver; the high lights black almost to complete opacity. The colour of the image however varies much with the state of the Bath and Collodioif^, and with the brightness of the light. The remarks already made under the head of Positives, apply equally well to Negatives ; that is, it will be difficult to secure gradation of tone, unless the object be equally illuminated, without any strong contrast of light and shade. Hence the direct rays of the sun are, as a rule, to be avoided, and curtains, etc., employed when practicable. EIXINa AND VAENISHING THE IMAGE. After the development is completed, and the plate has been carefully washed by a stream of water, it may be brought out to the light and treated with the Hyposul- phite or Cyanide, until the unaltered Iodide is entirely cleared off. Some use a Bath for the Cyanide ; but it is doubtful whether much saving is effected by doing so. The plate is again to be carefully washed after the fixing ; 226 MANIPULATIONS OF and especially if Hyposulphite of Soda be used. Three or four minutes in running water will not be too long, or the glass may be left in a dish of water for an hour or two. If such precautions are neglected, crystals form on drying, and the image is injured. Collodion pictures should be protected by a coat of var- nish, both Negatives and Positives having been known to fade when exposed to damp air without any covering (see p. 166). To prepare transparent varnish. Amber may be dissolved in Chloroform according to Dr. Diamond's for- mula;— about 80 grains of amber-beads or pipe-stems should be digested with one ounce of the Chloroform, and the clear portion separated by filtration. It may be poured on the plate in the same manner as Collodion, and dries up speedily into a hard and transparent layer. The Spirit Yarnish ordinarily sold for Negatives requires the aid of heat to prevent the gum from chilling as it dries ; the plate is first warmed gently and the varnish poured on and ofi* in the usual way ; it is then, whilst still dripping, held to the fire until the Spirit has evaporated. A few trials will render the operation easy to perform. White Lac dissolved in strong Alcohol or in Benzole has also been recommended for clear varnish. Direct Positives are to be varnished, first with a layer of transparent varnish, and then with black japan. Sug- gett's patent jet is sometimes employed, but it has a dis- agreeable smell, and is apt to crack on drying. The best black japan used by coachmakers is more elastic and less liable to crack. Asphalt (4 oz.) dissolved in mineral Naphtha (10 oz.), with the addition of 30 grains of Caout- chouc dissolved in half an ounce of the same menstruum, is also said to stand well. A third formula contains black sealing-wax dissolved in Alcohol. In either case it will be best to apply first a layer of clear varnish to the film, and afterwards the black varnish, which should combine with the other without dissolving it. Positives whitened with Bichloride of Mercury are in- THE COLLODION PROCESS. 227 jnred by varnishmg; they must therefore be backed up with black velvet, or Japan laid upon the opposite side of the glass. Many prefer taking the picture upon coloured glass, using only a layer of clear varnish ; but in this case the Collodion side being left uppermost, the image is ne- cessarily reversed. SECTION II. Directions for the use of Photograpliic Lenses. Those wlio are comparatively unacquainted with the science of optics require simple rules to guide them in the choice of a photographic lens, and in the proper mode of using it. Two kinds of Achromatic lenses are sold, the Portrait lens and the View lens ; the former of which is constructed to admit a large volume of light, for the purpose of copy- ing living objects, etc. A convenient-sized Camera for small portraits is "the half-plate" with a lens of about 2^ inches diameter, and giving a tolerably flat field on a surface of 5 inches by 4. Much however in this respect will depend upon the quality of the glass and also upon its focal length ; a short focus lens taking a picture more quickly, but giving a smaller image, and a field which is misty towards the edge. There is also a great tendency to distortion of the image in por- trait lenses of large aperture and short focus, such as those employed for operating in a dull light. The "whole plate" portrait lens may be expected to cover 6 J by 4f inches, and has a diameter of about 3^ inches. It will take larger pictures than the last, but not neces- sarily in a shorter time ; since, although the aperture for admitting the light is larger, the focal length is proportion- ately greater and the light less condensed. The " quarter-plate "portrait lens of If inch diameter is useful for stereoscopic subjects and small portraits; which are usually more sharply defined when taken with a small lens. 228 MANIPULATIONS OF The distance at which the Camera is to be placed from the sitter in taking a portrait, will depend upon the focal length of the lens. The effect of bringing the Camera nearer is to add to the size of the image, but at the same time to increase the chance of distortion ; hence with every lens of full aperture, there is a practical limit to the size of picture which can be taken. When it is required to obtain a large image with a small lens, a stop with a central aperture (which may be readily made of a piece of circular cardboard blackened with Indian ink) must be placed in front of the lens. This will diminish the amount of light, but will render the picture more distinct towards the edge, and bring a variety of objects at different distances into focus at the same time. With a stop attached, the lens may also be brought nearer to the object without distorting. With regard to this subject of the distortion often pro- duced by lenses, observe particularly, that with the por- trait combination of full aperture, and especially when the powers of the glass are rather strained by its being ad- vanced too near to the sitter, — all objects near to the lens will be magnified, and those more removed will appear di- minished ; hence, as the position of the sitter is never quite vertical, the Camera must be inclined a little dowmcards, or the hands and feet will be enlarged, the figure in fact becoming pyramidal with the base below ; whereas on the other hand, if the inclination of the Camera be too great, the head and forehead will be enlarged, and the figure be- comes a pyramid with the base above. When groups are taken, arrange the objects as near as possible equidistant from the lens, and use a stop if prac- ticable. Long-focus lenses are the best for this purpose, allowing the Photograph to be taken further off, and giving a greater variety of objects in focus at the same time. Portrait lenses may often be advantageously substituted for View lenses in copying objects of still life which are badly lighted. The aperture of the lens being large, a THE COLLODION PROCESS. 229 Negative can be obtained with an amount of light which would not suffice if a small stop were used. On the other hand, if the light be unusually bright, the lens of full aper- ture is always the most likely, from its extent of reflecting surface, to produce a misty and indistinct image. Hence the object should be well backed up with some neutral colour, or, if that cannot be done, a pasteboard funnel, pro- jecting about a foot and a half, may be fastened in front of the lens, in order to exclude rays of light not immediately concerned in the formation of the image. If the lens were turned towards distant objects brightly illuminated, and a portion of sky included, there would probably be diffused light, and consequent fogging of the plate on the applica- tion of the developer. This effect will also invariably follow if the sun's rays be allowed to fall directly upon the glass. Directions for finding the Plane at which the Sharpest Image can he obtained. — Non- Achromatic Lenses are un- derstood by all to require correction for the chemical focus ; but it is usually said of the compound glasses, that their two foci correspond. The amateur is recommended, in order to avoid disappointment, to test the accuracy of this statement, and also to see that his Camera is constructed with care. To do this, proceed as follows : — First ascertain that the prepared sensitive plate falls precisely in the plane occupied by the ground glass. Sus- pend a newspaper or a small engraving at the distance of about three feet from the Camera, and focus the letters occupying the centre of the field ; then insert the slide, with a square of ground glass substituted for the ordinary plate (the rough surface of the glass looking inwards), and observe if the letters are still distinct. In place of the ground glass, a transparent plate with a square of silver- paper which has been oiled or wetted, may be used, but the former is preferable. If the result of this trial seems to show that the Camera is good, proceed to test the correctness of the Lens. — 230 MANIPULATIONS OF Take a Positive Photograph with the full aperture of the portrait Lens, the central letters of the newspaper being carefully focussed as before. Then examine at what part of the plate the greatest amount of distinctness of outline is to be found. It will sometimes happen, that whereas the exact centre was focussed visually, the letters on a spot midway between the centre and edge are the sharpest in the Photograph. In that case the chemical focus is longer than the other, and by a distance equivalent to, but in the opposite direction of, the space which the ground glass has to be moved, in order to define those particular letters sharply to the eye. When the chemical focus is the shorter of the two, the letters in the Photograph are indistinct at every portion of the plate ; the experiment must therefore be repeated, the lens being shifted an eighth of an inch or less. Indeed it win be proper to take many Photographs at minute varia- tions of focal distance before the capabilities of the lens will be fully shown. The object of finding the point at which the sharpest image is obtained will also be assisted by placing several small figures in different planes and focussing those in the centre. This being done, if the more distant figures come out distinctly in the Photograph, the chemical focus is longer than the Visual, or vice versa when the nearest ones are most sharply defined. Tlie Single Achromatic Lens. — A useful lens for land- scape Photography is one of about 3 inches diameter and 15 inches focal length, which may be expected to cover a field of 10 inches by 8. With the lens, stops are supplied of various diameters, the largest of which will be useful in dull weather ; the smaller when the field is required to be rendered sharp to the very edge. The stop is arranged at a certain distance in front of the lens, and must not be moved. If it were brought close up to the glass, the field would not be so flat ; the effect being then the same as that of a stop placed in front of a THE COLLODION TEOCESS. 231 Portrait Lens, viz. simply to cut off tlie outside portion of the glass. ^ In taking Photographs of architectural and other sub- jects with vertical outlines, it is very important to have the Camera placed perfectly horizontal ; since, if it be inclined either upwards or downwards, the perpendiculars will be destroyed and the object will appear of a pyramidal form, falling inwards or outwards, as before shown. It is con- venient to rule the ground focussing glass with a number of parallel lines in both directions, which enables the ope- rator at once to see that the position of the instrument is correct. SECTION III. * Mode of cojpying Engravings, Etchings, etc. The engraving to be Photographed should be removed from its frame (the glass causing irregular reflection) and suspended vertically and in a reversed position, in a good diffused light. A black cloth may be placed behind the picture with advantage if any surface likely to reflect light be presented to the lens. The Camera must be fixed immovably, so as not to vi- brate in the least degree when the cap of the lens is taken off. It should be pointed at right angles to the picture, and the focus determined in the ordinary way. Either a portrait or a single lens may be used, with a diaphragm sufficiently small to render the image distinct up to the edge. It is not desirable to employ too thin a Collodion, since perfect opacity of the darkest parts of the Negative is essen- tial. An old Collodion containing free Iodine is better than a contractile Collodion, as giving a more intense and clear image. Pure Collodion iodized with Iodide of Cad- mium, if found wanting in intensity, may be at once ren- * See this subject explained in * Photographic Journal/ vol. ii. p. 133. 232 MANIPULATIONS OF dered jfit for use in copying engravings by adding Glycyr- rhizine (p. 209), until the dark parts of the negative become very opaque, and subsequently softening the excessive hardness, if necessary, by dropping alcoholic solution of Iodine into the Collodion until it reaches a straw-yellow tint. A second formula useful in iodizing Collodion for a similar purpose is as follows. This, with addition of Glycyrrhizine, will give a very black image. Etchings, diagrams, and drawings with pencil or ink, without much middle-tint, if on thin paper, are easily co- pied without the aid of the Camera, by simply laying the sketch upon a sheet of Negative Paper, exposing for a brief time to the light, and developing with Grallic Acid. This yields a Negative w^hich is employed for printing Positives in the usual way. Full directions on this subject will be found in the Second Section of the following Chapter. A more simple plan, and one which will succeed when great delicacy is not required, consists in laying the sketch upon a sheet of Positive printing paper (a highly salted paper will be the best, as giving most intensity) and ex- posing to the light until a copy is obtained. All the de- tails are faithfully rendered in this way, but it is some- times difficult to obtain a Negative sufficiently black to yield a vigorous print. S,ules for taking Stereoscopic Photographs. Einocular pictures of a large size, for the reflecting Ste- reoscope, may be taken with an ordinary View lens of about 15 inches focus. The ground glass of the Camera having been ruled with cross lines in the manner described Iodide of Potassium Bromide of Potassium 4 grains. 1 grain. SECTION IV. THE COLLODION PROCESS. 233 at page 231, tlie position of some prominent object is marked upon one of the lines with a pencil, and the first view is taken. The stand is then moved laterally to the proper distance, and the Camera adjusted to its sepond position by shifting it until the marked object occupies the same place as before. The distance between the two positions should be about one foot when the foreground of the pic- ture is twenty-five feet from the instrument, or four feet when it is at thirty or forty yards. But, as before shown at page 71, this rule is not to be followed implicitly, much depending upon the character of the picture and the effect desired. Photographs for the lenticular Stereoscope are taken with small lenses of about 4^ inches focus. For portraits, a Camera may advantageously be fitted with two double- combination lenses, of If inches diameter, exactly equal in focal length and in rapidity of action. The caps are removed simultaneously, and the pictures impressed at the same instant. The centres of the lenses may be separated by three inches when the Camera is placed at about six feet from the sitter, or four inches w^hen the distance is increased to eight feet. Pictures taken with a binocular Camera of this kind, re- quire to be mounted in a reversed position to that which they occupy on the glass : for since the image of the Camera is inverted, when it is turned round and made erect, the right-hand picture will necessarily stand on the left side, and vice versa. Mr. Latimer Clark has devised an arrangement for tak- ing stereoscopic pictures with a single Camera, which is exceedingly ingenious. Its most important feature is a contrivance for rapidly moving the Camera in a lateral direction without disturbing the position of the image upon the ground glass. This will be understood by a reference to the following woodcut. "A strongly-framed Camera-stand carries a flat table, about 20 inches wide by 16, furnished with the usual ad- 234 MANIPULATIONS OF justments. Upon this are laid two flat bars of wood in the direction of the object, and parallel, and about the width of the Camera asunder. They are 18 inches in length; their front ends carry stout pins, which descend into the table and form centres upon which they turn. Their op- posite ends also carry similar pins, but these are directed upwards, and fit into two corresponding holes in the tail- board of the Camera. " JN^ow when the Camera is placed upon these pins, and moved to and fro laterally, the whole system exactly re- sembles the common parallel ruler. The two bars form the guides, and the Camera, although capable of free late- ral motion, always maintains a parallel position. In this condition of things it is only suited to take stereoscopic pictures of an object at an infinite distance ; but to make it move in an arc, converging on an object at any nearer distance, it is only necessary to make the two guide-bars approximate at their nearer end so as to converge slightly towards the object; and by a few trials some degree of convergence will be readily found at which the image will remain as it were fixed on the focussing glass while the Camera is moved to and fro. To admit of this adjustment, one of the pins descends through a Slot in the table and THE COLLODION PROCESS. 235 carries a clamping-screw, by means of which it is readily- fixed in any required position. " In order however to render the motion of the Camera smoother, it is advisable not to place it directly upon the two guides, but to interpose two thin slips of wood, lying across them at right angles, beneath the front and back of the Camera respectively (and which may be fixed to the Camera if preferred), and to dust the surfaces with pow- dered soap-stone or French chalk.'* In addition to this arrangement for moving the Camera laterally, the slide for holding the sensitive plates must be modified from the common form. It is oblong in shape, and being about ten or eleven inches long, requires some little adaptation to fit it to the end of an ordinary Camera. The glasses are cut to about 6f inches by 3 J ; and when coated with Iodide of Silver, the two images are impressed side by side, the plate being shifted laterally about 2| inches, at the same time and in the same direction as the Camera itself. The operation of taking a portrait is thus performed. The focus having been adjusted for both positions, and the Camera and the slide both drawn to the left-hand, the door is raised and the plate exposed ; the Camera and the slide are then shifted to the right-hand, and the plate in its new position having been again exposed, the door is closed and the operation completed.^ Pictures taken with this instrument do not require to be reversed in mounting, the left picture being purposely formed on the right-hand side of the glass. SECTION Y. On the JPhotographic delineation of Microscopic objects. Many specimens of Micro-photography which have been exhibited are exceedingly elaborate and beautiful j and * See * Photographic Journal/ vol. i. page 59. 236 MANIPULATIONS OF their production is not difficult to one tlioronglily ac- quainted with the use of the Microscope and with the manipulations of the Collodion Process. It is important however to possess a good apparatus, and to have it pro- perly arranged. The object-glass of the ordinary compound Microscope is the only part actually required in Photography, but it is useful to retain the hody for the sake of the adjustments, and the mirrors used in the illumination. The eye-fiece however, which simply magnifies the image formed by the object-glass, is not necessary, since the same elFect of en- largement may be obtained by lengthening out the dark chamber, and throwing the image further off. Arrangement of the Apparatus, — The Microscope is placed with its body in a horizontal position, and the eye- piece being removed, a tube of paper, properly blackened in the interior, or lined with black velvet, is inserted into the instrument, to prevent irregular reflection of light from the sides. A dark chamber of about two feet in length, having at one end an aperture for the insertion of the eye -piece end of the body, and at the other a groove for carrying the slide containing the sensitive plate, is then attached ; care being taken to stop all crevices likely to admit diffused light. An ordinary Camera may be employed as the dark chamber, the lens being removed, and the body lengthened out if required by a conical tube of gutta-percha, made to fasten into the flange of the lens in front. The whole ap- paratus should be placed exactly in a straight line, that the ground glass used in focussing may fall at right angles to the axis of the Microscope. The length of the chamber, measuring from the object- glass, may be from two to three feet, according to the size of image required ; but if extended beyond this, the pencil of light transmitted by the object-glass is diffused over too arge a surface, and a faint and unsatisfactory picture is the result. The object should be illuminated by sunlight THE COLLODION PROCESS. 237 if it can be obtained, but a bright diffused daylight will succeed with low-power glasses, and especially when Posi- tives are taken. Employ the concave mirror for reflecting the light on the object in the latter case ; but in the former the plane mirror is the best, except with powers exceeding a quarter of an inch, and of large angular aperture. The image upon the ground glass should appear bright and distinct, and the field of a circular form and evenly illuminated ; when this is the case, all is ready for inserting the sensitive plate. The time of exposure must be varied according to the in- tensity of the light, the sensibility of the Collodion, and the degree of magnifying power ; a few seconds to a minute will be about the extremes ; but minute directions are not required, as the operator, if a good Photographer, will easily ascertain the proper time for exposing (seepage 224). At this point a difficulty will probably occur from the plane of the chemical focus not corresponding, as a rule, with that of the visual focus. This arises from the fact that the object-glasses of Microscopes are "over-cor- rected" for colour, in order to compensate for a little chro- matic aberration in the eye-piece. The violet rays, in con- sequence of the over- correction, are projected heyond the yellow, and hence the focus of chemical action is further from the glass than the visible image. The allowance may be made by shifting the sensitive plate, or, what amounts to the same thing, by removing the object-glass a little away from the object with the fine adjustment screw ; the latter is the most convenient. The exact distance must be determined by careful experiment for each glass ; but it is greatest with the low powers, and decreases as they ascend. Mr. Shadbolt gives the following as a guide : — " An inch and a half objective of Smith and Beck's make required to be shifted l-50th of an inch, or two turns of their fine adjustment ; a 2-3rds of an inch, l-200th of an inch, or half a turn; and a 4-lOths of an inch, 1-lOOOth of an inch, 238 MANIPULATIONS OF or about two divisions of the adjustment. With the l-4th and higher powers, the difference between the foci was so small as to be practically unimportant." There is also reason to think that the Tcincl of light employed has an influence upon the separation of the foci. Mr. Delves finds that with sunlight the difference between them is very small even with the low powers, and inappreciable with the higher ; whereas in using diffused daylight which has undergone a previous reflection from white clouds, it is considerable. The object-glasses of the same maker, and particularly those of different makers, also vary much ; so that it will be necessary to test each glass separately, and to register the allowance which is required. Having found the chemical focus, the principal difficulty has been overcome, and the remaining steps are the same in every respect as for ordinary Collodion Photographs. To those who cannot devote their time to Photography during the day, Mr. Shadbolt's observations on the use of artificial light may be of service. He employs CamjpMne, which gives a whiter flame than gas, or a moderator lamp ; placing the source of light in the focus of a plano-convex lens of 2| to 3 inches diameter (the flat side towards the lamp), and condensing the parallel rays so obtained on the object, by a second lens of about 1^-inch diameter and 3-inch focus. This mode of illumination, being feeble in chemical rays, is best adapted for object-glasses of low power. The ex- posure required to produce a Negative impression with the one-inch glass may be from three to five minutes. As the sensitive plate would be liable to become dry during that time, it is recommended to coat it with some preservative solution by the modes described in the sixth Chapter. Mr. Crookes having lately shown that the Bromide of Silver is more sensitive than the Iodide to artificial light, a mixture of the two salts may conveniently be used (see pp. 66 and 232). THE COLLODION PROCESS. 239 The development may be conducted in the same manner as that for preserved sensitive plates ; fixing with Cyanide of Potassium before the development is fully complete, if any tendency to fogging is observed (see page 224). The Rev. W. Towler Kingsley has communicated a pro- cess by which very beautiful Microscopic Photographs have been obtained. He illuminates (in the absence of sunlight) with the brilHant light produced by throwing a jet of mixed Oxygen and Hydrogen gases upon a small cone of Lime or Magnesia. Particular stress is laid upon the object-glass of the Microscope being a good one for the purpose ; and indeed all who have given attention to the subject are agreed upon this point — that there is a considerable difference in the Photographic value of ob- jectives, and this independent of the angular aperture of the glass. 240 CHAPTER lY. THE PRACTICAL DETAILS OF PHOTOGRAPHIC PEINTI^^a. This Chapter is divided as follows : — Section I. — The ordinary direct process of positive printing. Section 11. — Positive printing by development. Section III. — The mode of toning Positives by Sel d'or. Section IY. — On printing enlarged or reduced Posi- tives, transparencies, etc. SECTION 1. JPositive Printing hy the direct action of Light, This includes — the preparation of sensitive paper, — of fixing and toning Baths, — and the manipulatory details of the process. Selection of Paper for Photographic Printing. — The ordinary varieties of paper sold in commerce are not well adapted for the production of Positive prints. Papers are manufactured purposely which are more smooth and uni- form in texture. Many samples of even the finest paper are however defective, and hence each sheet should be ex- amined separately by holding it against the light, and if spots or irregularities of texture are seen, it should be re- jected. These spots usually consist of small particles of brass or iron, which, when the paper is rendered sensitive, OF PHOTOGRAPHIC PRINTING. 241 decompose tlie Nitrate of Silver and leave a circular mark very noticeable after fixing. The foreign papers, French and German, are different from the English. They are porous and sized with starch, the English being sized with gelatinous animal matter. In all cases there is a difference in smoothness between the two sides of the paper, which may be detected by holding each sheet in such a manner that the light strikes it at an angle ; the wrong side is that on which dark wavy bands, of an inch to an inch and a half in breadth, are seen, caused by the strips of felt on which the paper was dried. With most qualities of paper no difficulty whatever will be ex- perienced in detecting the broad and regular bands above referred to ; but when they cannot be seen, the wrong side of the sheet may be known by wire markings crossing each other, or if the paper be wetted at the corner, one side may appear evidently smoother than the other. PREPARATION OF SENSITIVE PAPER. There are three principal varieties of sensitive paper in common use, viz. the Albuminized, the plain, and the Am- monio-Nitrate paper. Eormula I. Preparation of Albuminized Paper. — This includes the salting and albuminizing, and the sensitizing with Nitrate of Silver. The Salting and Alhumirdzing. — Take of Chloride of Ammonium, or Pure Chloride of Sodium .... 200 grains. Water 10 fluid ounces. Albumen 10 fluid ounces. If distilled water cannot be procured, rain water or even common spring water^ will answer the purpose. To obtain the Albumen, use new-laid eggs, and be careful * If the water contained much Sulphate of Lime, it is likely that the sensitiveness of the paper would be impaired (?). R 242 THE PEACTICAL DETAILS that in opening the shell the yolk is not broken ; each egg will yield about one fluid ounce of Albumen. When the ingredients are mixed, take a bundle of quills or a fork, and beat the whole into a perfect froth. As the froth forms, it is to be skimmed off and placed in a flat dish to subside. The success of the operation depends entirely upon the manner in which this part of the pro- cess is conducted ; — if the Albumen be not thoroughly beaten, flakes of animal membrane will be left in the liquid, and will cause streaks upon the paper. When the froth has partially subsided, transfer it to a tall and nar- row jar, and allow to stand for several hours, that the membranous shreds may settle to the bottom. Then pour off the upper clear portion, which is fit for use. Albu- minous liquids are too glutinous to run well through a paper filter, and are better cleared by subsidence. A more simple plan than the above, and one equally efficacious, is to fill a bottle to about three parts with the salted mixture of Albumen and water, and to shake it well for ten minutes or a quarter of an hour until it loses its glutinosity and can be poured out smoothly from the neck of the bottle. It is then to be transferred to an open jar, and allowed to settle as before. The solution prepared by the above directions will con- tain exactly ten grains of salt to the ounce, dissolved in an equal bulk of Albumen and water. Some operators employ the Albumen alone without an addition of water ; but this commonly gives a highly varnished appearance, which is thought by most to be objectionable. Much how- ever will depend upon the kind of paper which is employ ed, certain varieties taking more gloss than others ; Papier E;ive, for instance, often requires the Albumen to be nearly or quite undiluted. The principal difficulty in Albuminizing paper, is to avoid the occurrence of streaky lines, which, when the paper is rendered sensitive, bronze strongly under the influ- ence of the light. To avoid them, use the eggs quite fresh, OF PHOTOGRAPHIC PPvINTING. 2*3 aud lower tlie paper on to the liquid by one steady move- ment ; if a pause be made, a line will probably be formed. Some papers are not readily wetted b}" the Albumen, and when such is the case, a few drops of spirituous solution of bile, or a fragment of the prepared Ox-Gall sold by the artists'- colourmen, will be found a useful adjunct. Care must be taken however not to add an excess, or the Al- bumen will be rendered too fluid, and will sink into the paper, leaving no gloss. In salting and albuminizing Photographic paper by the formula above given, it is found that each quarter-sheet, measuring eleven inches by nine inches, removes one fluid drachm and a half from the bath, equivalent to about one grain and three-quarters of salt (including droppings). In salting plain paper, each quarter- sheet takes up only one drachm ; so that the glutinous nature of the Albumen causes a third part more of salt to be retained by the paper. English papers are not good for albuminizing ; they do not take the Albumen properly, and curl up when laid upon the liquid : the process of toning the prints is also slow and tedious. The thin negative paper of Canson, the Papier Rive, and Papier Saxe, have succeeded with the writer better than Canson's Positive paper, which is often recommended; they have a finer texture, and give more smoothness of grain. To apply the Albumen, pour a portion of the solution into a flat dish to the depth of half an inch. Then, having previously cut the paper to the proper size, take a sheet by the two corners, bend it into a curved form, convexity downwards, and lay it upon the Albumen, the centre part first touching the liquid, and the corners being lowered gradually. In this way all bubbles of air wall be pushed forwards and excluded. One side only of the paper is wetted: the other remains dry. Allow the sheet to rest upon the solution for one minute and a half, and then raise it ofi*, and pin it up by two corners. If any circular 244 THE PEACTICAL DETAILS spots, free from Albumen, are seen, caused by bubbles of air, replace the sheet for the saine length of time as at The paper must not be allowed to remain upon the salt- ing Bath much longer than the time specified, because the solution of Albumen being alkaline (as is shown by the strong smell of Ammonia evolved on the addition of the Chloride of Ammonium) tends to remove the size from the paper and to sink in too deeply ; thus losing its surface gloss. Albuminized paper will keep a long time in a dry place. Some have recommended to press it w^ith a heated iron, in order to coagulate the layer of Albumen upon the sur- face ; but this precaution is unnecessary, since the coagu- lation is perfectly effected by the Nitrate of Silver used in the sensitizing ; and it is doubtful whether a layer of dry Albumen would admit of coagulation by the simple appli- cation of a heated iron. To render the paper sensitive. — This operation must be conducted by the light of a candle, or by yellow light. Prepare a sufficient quantity of this solution, and lay the sheet upon it in the same manner as before. Three mi- nutes' contact will be sufiicient with the thin IN egati ve paper, but if the Canson Positive paper be used, four or five mi- nutes must be allowed for the decomposition. The papers are raised from the solution by a pair of bone forceps or common tweezers tipped with sealing-wax ; or a pin may be used to lift up the corner, which is then taken by the finger and thumb and allowed to drain a little before again putting in the pin, otherwise a white mark will be pro- duced upon the paper, from decomposition of the Nitrate of Silver. When the sheet is hung up, a small strip of first. Take of Pused Nitrate of Silver Glacial Acetic Acid . . Distilled Water . . , . 60 grains. . \ minim. . 1 ounce. OF PHOTOGRAPHIC PRINTING. 246 blotting-paper suspended from the lower edge of the paper will serve to drain off the last drop of liquid. A Bath prepared by the above formula is stronger than is really necessary. Forty grains of Nitrate to the ounce of water is abundantly sufficient if the sample be pure ; but it must be borne in mind that the strength of the Bath diminishes rapidly by use, and hence, when the prints be- gin to be wanting in vigour, with pale shadows and perhaps a spotted appearance, an addition of Nitrate of Silver must be made. Fused Nitrate of Silver is recommended in pre- ference to the crystallized Nitrate, on account of the latter being occasionally contaminated with an impurity alluded to at page 101. This when present will be likely to redden the pictures and to interfere with the rapidity of bronzing. The solution of Nitrate of Silver becomes after a time discoloured by the Albumen, but may be used for sen- sitizing until it is nearly black. The colour can be re- moved by Animal Charcoal,^ but a better plan is to use the "kaolin," or pure white china clay. This substance often contains Carbonate of Lime, and effervesces with acids : it must in such a case be purified by washing in vinegar, or the Bath will become alkaline, and dissolve off the Albumen. It has been stated that an addition of Al- cohol to the Nitrate Bath prevents it discolouring with Albumen. Sensitive albuminized paper will usually keep for several days, if protected from the light, but afterwards turns yellow from partial decomposition. Formula II. Preparation of plain paper. — Take of Chloride of Ammonium or Sodium . 160 grains. Purified Gelatine 20 grains. Iceland Mossf 60 grains. Water 20 ounces. * Common Animal Charcoal contains Carbonate and Phospliate of Lime, tlie former of which renders the Nitrate of Silver alkaline ; purified Animal Charcoal is usually acid from Hydrochloric Acid. t Iceland Moss is recommended because the writer finds that Positives 246 THE PEACTICAL DETAILS Pour boiling water upon tlie Moss and Gelatine and stir until the latter is dissolved, then cover the vessel and set aside until cold ; add the salt, and strain. Use Papier Saxe or Towgood's paper,=^ floated upon the salting Bath in the same manner as directed for Albumen at p. 243. Kender sensitive by floating for two or three minutes upon a solution of Nitrate of Silver, 40 grains to the ounce. Thirty grains to the ounce, or less, will be sufficient if the sample be pure ; but in that case occasional additions of fresh Nitrate of Silver must be made, as the Bath loses strength. A second Formula for plain pajoer. — Take of Chloride of Ammonium 200 grains. Citrate of Sodaf 200 „ Gelatine 20 „ Water 20 fluid ounces. If Towgood's or any English paper be used, the Citric Acid, Carbonate of Soda, and Gelatine may be omitted. With a foreign paper the Citrate tends to give a purple tone to the Positive, when toned by Sel d'or, but the gold toning Bath must be in active order, or the prints will be too red. The Citric Acid also should not be in excess over the alkaline Carbonate. Hender sensitive by floating for three minutes upon a Nitrate Bath of sixty grains to the ounce of water. Formula III. Ammonio- Nitrate Pajper. — This is always prepared without Albumen, which is dissolved by Am- monio-Nitrate of Silver. Take of so printed stand the action of destructive tests better than prints on plain paper, and equal to prints upon Ammonio- Nitrate paper. * The writer does not recommend the Positive paper of De Canson, having noticed that prints upon that paper do not withstand the action of sulphu- retting agents so well as others (?). t This salt may be obtained at the operative chemists ; or it may be pre- pared extemporaneously by neutralizing 112 grains of pure Citric Acid, free from Tartaric Acid, with 133 grains of the dried Bicarbonate or " Sesquicar- bonate" of Soda, used for effervescing draughts. OF PHOTOGRAPHIC PRINTING. 247 Chloride of Ammonium Citrate of Soda . . Gelatine Water 200 „ 20 20 fluid ounces. 100 grains. Dissolve the G-elatine by the aid of heat ; add the other ingredients, and filter. The solution cannot be kept longer than two or three weeks without becoming mouldy. The Saxony paper, or Towgood's English paper, may be em- ployed ; the Grelatine and Citrate being retained or omit- ted, according to the taste of the operator and the mode of toning which is adopted. Render sensitive by a solution of Ammonio-Nitrate of Silver, 60 grains to the ounce of water, which is prepared as follows : — Dissolve the Nitrate of Silver in one-half of the total quantity of water. Then take a pure solution of Am- monia and drop it in carefully, stirring meanwhile with a glass rod. A brown precipitate of Oxide of Silver first forms, but on the addition of more Ammonia it is re- dissolved.^ When the liquid appears to be clearing up, add the Ammonia very cautiously, so as not to incur an excess. In order still further to secure the absence of free Ammonia, it is usual to direct, that when the Hquid becomes perfectly clear, a drop or two of solution of Ni- trate of Silver should be added until a slight turbidity is again produced. Lastly, dilute with water to the proper bulk. If the crystals of Nitrate of Silver employed con- tain a large excess of free Nitric Acid, no precipitate will be formed on the first addition of Ammonia. The free Nitric Acid, producing Nitrate of Am^nonia with the al- kali, keeps the Oxide of Silver in solution. This cause of error however is not likely to happen frequently, since the amount of Nitrate of Ammonia required to prevent all pre- cipitation would be considerable. From the same reason, * If the excess of Ammonia does not readilj dissolve it, probably the Ni- trate of SUver is impure. 248 THE PBACTICAL DETAILS viz. the presence of Nitrate of Ammonia, it is often useless to attempt to convert an old Nitrate Bath already used for sensitizing, into Ammonio-Nitrate. Ammonio-Nitrate of Silver should be bept in a dark place, being more prone to reduction than the Nitrate of Silver. Sensitizing paper with Ammonio-Nitrate. — It is not usual to float the paper when, the Ammonio-Nitrate of Silver is used. If a bath of this liquid were employed, it would not only become quickly discoloured by the action of organic matter dissolved out of the papers, but would soon contain abundance of free Ammonia (see the Vocabu- lary, Part III., art. Ammonio-Nitrate") ; and an excess of Ammonia in the liquid produces an injurious effect by dissolving away the sensitive Chloride of Silver. The Ammonio-Nitrate is therefore applied with a glass rod, or by brushing, and in neither case is any of the liquid which has once touched the paper allowed to return into the bottle. Brushes are manufactured purposely for applying Silver solutions, but the hair is soon destroyed unless the brush be kept scrupulously clean. Lay the salted sheet upon blotting-paper, and wet it thoroughly by drawing the brush first lengthways and then across. Allow it to remain flat for a minute or so, in order that a sufficient quantity of the solution may be absorbed (you will see when it is evenly wet by looking along the surface), and then pin up by the corner in the usual way. If, on drying, white lines appear at the points last touched by the brush, it is probable that the Ammonio-Nitrate contains free Ammonia. The employment of a glass rod is a very simple and eco- nomical mode of applying Silver solutions. Procure aflat piece of board somewhat smaller than the sheet to be ope- rated on, and having turned over the edges of the paper, secure them with a pin. Next bring the board near to the corner of the table, and laying the glass rod along the edge of the paper, allow the fluid to drop into the groove so OF PHOTOGRAPHIC PRINTING. 249 formed ; then carry the rod directly across the sheet, when an even wave of fluid will be spread over the sui-face. A pipette made of glass tubing, when dipped into the bottle and the upper end closed with the finger, will withdraw as much of the Ammonio-Nitrate as is required ; and if a scratch be made upon the tube at a point corresponding to 30 or 40 minims, it will be found sufficient for a quarter sheet of the Papier Saxe. Ammonio- Nitrate paper, however prepared, cannot be kept many hours without becoming brown and disco- loured. Use of a solution of Oxide of Silver in Nitrate of Am- monia. — The great objection to the use of Ammonio-Ni- trate of Silver is the decomposition which it sometimes experiences by keeping, metallic Silver separating and Ammonia being set free. To obviate this liberation of Ammonia, the Author employs Nitrate of Ammonia as the solvent for the Oxide of Silver. The solution is prepared as follows : — Dissolve 60 grains of Nitrate of Silver in half an ounce of water, and drop in Ammonia until the preci- pitated Oxide of Silver is exactly re-dissolved. Then divide this solution of Ammonio-Nitrate of Silver into two equal parts, to one of which add Nitric Acid cautiously, until a piece of immersed litmus-paper is reddened by an excess of the acid ; then mix the two together, fill up to one ounce with water, and filter from the milky deposit of Chloride or Carbonate of Silver, if any be formed. This solution of Oxide of Silver in Nitrate of Ammonia appears to possess all the advantages of the Ammonio- Nitrate without the inconvenience of liberating so much free Ammonia upon the surface of the sensitive sheets. Hints in selecting from the above Formnlce, — Albumin- ized paper is the most simple and generally useful ; it is well fitted for small portraits and stereoscopic Photo- graphs. The Ammonio- Nitrate Process requires more ex- perience, but gives excellent results when black tones are required : it may be used for larger portraits, engravings, etc. 250 THE PRACTICAL DETAILS Plain paper rendered sensitive by floating upon a Batli of Nitrate of Silver is easier of manipulation than the Am- monio-Nitrate, and will be found to be better adapted for toning by the Sel d'or Bath (p. 267) than the Albuminized Paper. PREPAEATION OF THE FIXING AND TONING BATH. Dissolve the Hyposulphite of Soda in four ounces of the water, the Chloride of Gold in three ounces, the Nitrate of Silver in the remaining ounce ; then pour the diluted Chloride by degrees into the Hyposulphite, stirring with a glass rod ; and afterwards the Nitrate of Silver in the same way. This order of mixing the solutions is to be strictly observed : if it were reversed, the Hyposulphite of Soda being added to the Chloride of Gold, the result would be the reduction of Metallic Gold ; Hyposulphite of Gold, which is formed, being an unstable substance, and not capable of existing in contact with unaltered Chloride of Gold. If however it be dissolved by Hyposulphite of Soda immediately on its formation, it is rendered more permanent, by conversion into a double salt of Soda and In place of Nitrate of Silver, recommended in the for- mula, Chloride of Silver may be used, but not Iodide of Silver, as the formation of Iodide of Sodium would be ob- jectionable (p. 136). For the same reason it is better not to add any part of the Hyposulphite Bath used for fixing Negatives, to the Positive colouring solution. * The common kind of Hyposulpliite of Soda occurring in yellow and dis- coloured masses, is too impure for use in Photography, and requires re- crystallization. Take of Chloride of Gold . . Nitrate of Silver . . .Hyposulphite of Soda=^ Water 4 grains. 16 grains. 4 ounces. 8 fluid oimces. Gold. OF PHOTOGEAPHIC PEINTING. 251 This toning Bath is not to be employed immediately after mixing, but should be set aside until a portion of Sulphur (produced by free Hydrochloric Acid, and Tetra- thionate of Soda reacting upon the Hyposulphite) has subsided. It will be very active at the expiration of a iew days or a week ; but upon keeping for a longer time, loses much of its efficacy by a process of spontaneous change. The immersion of prints also lessens the quantity of Gold ; and hence, when the Bath begins to work slowly, more of the Chloride must be added, the Sulphur being allowed to deposit as before. Filtration through blotting- paper will not be required. The writer finds that after a certain time, when the Bath has been long used, and organic matters, Albumen, etc., have accumulated in it, it is better, and more eco- nomical, to throw away what remains, and to prepare a new solution. The addition of Chloride of Gold to an old Bath will not always make it work as quickly as one recently mixed. THE MANIPULATORY DETAILS OF PHOTOGEAPHIG PEINTING. These include — the exposure to light, or printing pro- perly so called ; the fixing and toning ; and the washing, drying, and mounting of the proof. The Exposure to Light. — For this purpose reversing frames are sold, which admit of being opened at the back, in order to examine the progress of the darkening by light, without producing any disturbance of position. Simple squares of glass however succeed equally well, w^hen a little experience has been acquired. They may be held together by the wooden clips sold at the American warehouses at one shilHng per dozen. The lower plate should be covered with black cloth or velvet. Supposing the frame to be employed, the shutter at the back is removed, and the Negative laid flat upon the glass, Collodion side uppermost. A sheet of sensitive paper is 252 THE PEACTICAL DETAILS tlien placed upon the Negative, sensitive side downwards, and the whole tightly compressed by replacing and bolting down the shutter. This operation may be conducted in the dark room ; but unless the light be strong, such a precaution will not be required. The time of exposure to light varies much with the density of the JS^egative and the power of the actinic rays, as influenced by the season of the year and other ob- vious considerations. As a general rule, the best Nega- tives print slowly ; whereas Negatives which have been under-exposed and under- developed print more quickly. In the early spring or summer, when the light is power- ful, probably about ten to fifteen minutes will be required ; but from three-quarters of an hour to an hour and a half may be allowed in the winter months, even in the direct rays of the sun. It is always easy to judge of the length of time which will be sufficient, by exposing a small slip of the sensitive paper, unshielded, to the sun's rays, and observing how long it takes to reach the coppery stage of reduction. Whatever that time may be, nearly the same will be occupied in the printing, if the Negative be a good one. When the darkening of the paper appears to have pro- ceeded to a considerable extent, the frame is to be taken in and the picture examined. If squares of plate glass are used to keep the Negative and sensitive paper in contact, some difficulty may be experienced at first in returning it precisely to its former position after the examination is complete, but this will easily be overcome by practice. The finger and thumb should be fixed on the lower corners or edge, and the plate raised evenly and quickly. If the exposure to light has been sufficiently long, the print appears slightly darher than it is intended to remain. The toning Bath dissolves away the lighter shades, and re- duces the intensity, for which allowance is made in the exposure to light. A little experience soon teaches what is the proper point ; but much will depend upon the state OF PHOTOGEAPHIC PRINTING. 253 of the toning Batli ; and albuminized paper will require to be printed somewhat more deeply than plain paper. If, on removal from the printing-frame, a peculiar spot- ted appearance is seen, produced by unequal darkening of the Chloride of Silver, either the Nitrate Bath is too weak, the sheet removed from its surface too speedily, or the paper is of inferior quality. On the other hand, if the general aspect of the print is a rich chocolate-brown in the case of Albumen, a dark slate-blue with Ammonio-Nitrate Paper, or a reddish pur- ple with paper prepared with Chloride and Citrate of Silver, probably the su.bsequent parts of the process will proceed well. If, in the exposure to light, the shadows of the proof become very decidedly coppery before the lights are suffi- ciently printed, the Negative is in fault. Ammonio-Nitrate paper highly salted is particularly liable to this fault of ex- cess of reduction, and especially so if the light be power- ful ; hence it is best, in the summer months, not to print by the direct rays of the Sun. Tliis point is important also, because the excessive heat of the Sun's rays often cracks the glasses by unequal expansion, and glues the Negative firmly down to the sensitive paper. An excep- tion however may be made in the case of Negatives of great intensity; which are printed most successfully upon, a weakly sensitized paper (p. 124) exposed to the full rays of the Sun ; a feeble light not fully penetrating the dark parts. The fixing and toning of the proof, — No injury results from postponing this part of the process for many hours, provided the print be kept in a dark place. The mode often followed is to immerse the Positive in. the Hyposulphite Bath in the state in which it comes from the printing-frame ; moving it about in the liquid in order to displace air-bubbles, which, if allowed to remain, pro- duce spots. But the Author, for reasons given in the first part of the Work (pp. 129 and 165), recommends that the 254 THE PEACTICAL DETAILS print sliould first be washed in common water until tlie soluble Nitrate of Silver has been removed.=^ This is known to be the ease when the liquid flows away clear ; the first milkiness being caused by the soluble Carbonates and Chlorides in the water precipitating the Nitrate of Silver. Greater security is thus afforded that the print will be toned in a really permanent manner, since after removing the Nitrate of Silver from the proof, the Bath does not work quickly unless the supply of Gold be well maintained. Immediately on coming in contact with the Hyposul- phite of Soda in the fixing and toning Bath, the chocolate brown or violet tint of the Positive disappears, and leaves the image of a red tone. Albumen proofs become brick red; Ammonio-Nitrate a sepia or brown-black. If the colour is unusually jpale at this stage, probably the Silver Bath is too weak, or the quantity of Chloride of Ammonium or Sodium insufficient. After the print has been thoroughly reddened, the toning action begins, and must be continued until the desired effect is obtained. This may happen in from ten minutes to a quarter of an hour, if the solution is in good working order and the thermometer at 60^ ; but much depends upon the temperature, and the activity of the Bath. English papers, and especially the same prepared with Albumen, tone more slowly than foreign papers plain salted. The brown and purple tints are an earlier stage of colo- ration than the black tones, and therefore the latter require more time. It must be borne in mind however that pro- longed immersion in the Bath is favourable to sulphuration and yellowness ; tending also to render the image unstable and liable to fade in the halftones. This fading may not be seen decidedly whilst the print is in the Bath, but will show itself in the after-processes of washing and drying. The ultimate colour of the Print will vary much with the density of the Negative and the character of the sub- * This water must be free from Hyposulphite of Soda, or the print will become discoloured. OF PHOTOGRAPHIC PRINTING. 255 ject ; copies of line engravings, having but little lialf-tone^, are easily obtained of a dark shade resembling the original impression. Some advise that on removal from the toning Bath the Print should be soaked in new Hyposulphite for ten mi- nutes, to complete the fixation ; but this precaution is not required with a Bath of the strength given in the formula. An analysis of an old Bath which had been extensively used, indicated only ten grains of Hyposulphite of Silver to the ounce, so that it was far from saturated. The occasional addition of fresh crystals of Hyposul- phite of Soda to keep up the strength of the Bath, is use- ful, the exact quantity added not being material. The washing, drying , and mounting of the Positive Proofs. — It is essential to wash out every trace of Hyposulphite of Soda from the Print if it is to be preserved from fading, and to do this properly requires considerable care. Always wash with running luater when it can be ob- tained, and choose a large shallow vessel exposing a con- siderable surface in preference to one of lesser diameter. A constant dribbling of water must be maintained for four or five hours, and the prints should not lie together too closely, or the water does not find its way between them, (see the remarks at p. 162). When running water cannot be obtained, proceed as follows : — first wash the Prints gently, to remove the greater part of the Hyposulphite solution. Then transfer them to a large shallow pan, in which may be placed as many Prints as it will conveniently hold. Leave them in for about a quarter of an hour, with occasional move- ment, and then pour off the water quite dry. This point is important, viz. to drain ofi* the last portion of liquid completely before adding fresh water. Eepeat the process of changing at least five or six times, or more, according to the bulk of water, number of Prints, and degree of at- tention paid to them. Lastly, proceed to remove the size from the Print by 256 THE PEACTICAL DETAILS immersion in boiling water. ^ This process will give some idea of the permanency of the tints, since, if they become dull and red, and do not darlcen on drying, the Print is probably toned with ut Gold. Ammonio -Nitrate and plain paper Prints prepared on foreign papers by the modes de- scribed in this Work, may be expected to stand the test of boiling water ; Albumen Prints and Positives on English paper are a little reddened, although not to an objection- able degree. The size may also be effectually removed from the Print by the common Carbonate of Soda used in washing, al- though the former process is recommended as the most se- cure. Dissolve about a handful of the Soda in a pint of water, and when the milky deposit, if any occurs, has sub- sided, immerse the washed Positives for twenty minutes or half an hour. The Soda renders the paper quite porous, but produces no alteration of tint. If the process be pro- perly performed, ink will run in attempting to write upon the back of the finished picture. After removal from the Soda Bath a second washing Avill be required, but the time of the first washing may be proportionally shortened. Here a difficulty will occur with many kinds of water ; the Carbonate of Soda precipitating Carbonate of Lime, in the form of a white powder Avhich obscures the picture. To obviate this, use rain water until the greater part of the alkaline salt has been removed, and do not allow a stationary layer of liquid to rest too long upon the Print. The New Kiver water supplied to many parts of London, being comparatively soft, answers perfectly, and pro- duces no white deposit, if the proofs are moved about occa- sionally. When the Prints have been thoroughly washed, blot them off between sheets of porous paper and hang up to * The Print must be well washed in cold water, to remove the Hyposul- phite, before using the hot water ; or the half-tones will be hable to be dark- ened, or changed to incipient yellowness, by sulphuration. This point is important as regards the permanency. or PHOTOGRAPHIC PRINTING. 257 dry. Some press them with a hot iron, which darkens the colour slightly, but does so in an injurious manner when Hyposulphite of Soda is left in the paper. Albumen proofs when dry are sufficiently bright without further treatment ; but in the case of plain paper, salted simply, the effect is improved by laying the Print face downwards upon a square of plate-glass and rubbing the back with an agate burnisher, sold at the artists' colour- men's. This hardens the grain of the paper and brings out the details of the picture. Hot-pressing has a similar effect and is often employed. Mount the proofs with a solution of G-elatine in hot water, freshly made ; the best Scotch glue answers well. Grum water, prepared from the finest commercial gum, and free from acidity, may also be used, but it should be made very thick, that it may not sink into the paper, nor produce an unpleasant " cockling up" of the cardboard, which is caused by the damp and expanded print contract- ing as it dries. Caoutchouc dissolved in mineral JN'aphtha to the consis- tence of thick glue or gold-beaters' size, is employed by many for mounting Photographic Prints ; it may be ob- tained at the varnish shops, and is sold in tin boxes. The mode of using it is as follows : — with a broad brush made of stiff bristles, apply the cement to the back of the pic- ture; then take a strip of glass with a straight edge, and by drawing it across the paper, scrape off as much as possible of the excess. The print will then be found to adhere very readily to the cardboard, without causing ex- pansion or cockling ; and any portion of the cement which oozes out during the pressing may, when dry, be removed with a penknife without leaving a stain. REMARKS UPON THE WANT OP CORRESPONDENCE BETWEEN THE FORMULA OF DIEFERENT OPERATORS. The formulae for Positive printing given in the works s 258 THE PRACTICAL DETAILS on practical Photograpliy exhibit great variety ; and it lias been proposed to attempt to reduce tliem to more uniform proportions. This cannot however easily be done, both on account of the difference in the structure and pre- paration of the various Photographic papers, and also be- cause the mode of applying the solutions is not always the same. Take as an illustration the following process, which has long been recommended for its simplicity, and which is in every respect a good one : — Dissolve 40 grains of Chloride of Ammonium in 20 ounces of Distilled Water, and im- merse about a dozen sheets of Towgood's Positive paper, removing air-bubbles with a camels'-hair brush. When the last sheet has been placed in the liquid, turn the batch over and take them out one by one, so that each sheet, remaining in the liquid at least ten minutes, may be thoroughly saturated. When dry, excite by brushing with a 40 or 60-grain solution of Ammonio-Nitrate of Silver in the usual way. Now this formula contains less than one-fifth of the amount of salt often employed, and if a thick foreign paper sized with starch, such as Canson's Positive, were floated upon such a salting Bath, it would be difficult to obtain a good picture. By immersing however a paper sized with Gelatine like the one recommended, a much larger quan- tity of salt is retained upon the surface, and the film is sufficiently sensitive. There are three modes of applying solutions, viz. by brushing, floating, and immersion. The quantity of solution left on the paper varies with each, and consequently each requires a different formula. Immer- sion in a strong salting Bath tends to give a coarse picture wanting in definition ; whereas the plan of brushing a weak salting solution, produces a paper deficient in sensi- tiveness, and yielding a pale red image without proper depth of shadow. But independent of these differences, the chemical na- ture of the size employed also influences the toning of the OF PHOTOGRAPHIC PRINTING. 259 Print. For instance, in the process above given, if the Positives, after having been fully toned in the Gold Bath, and washed in cold water, be treated with boiling water, the tint immediately changes to a dull red ; but on blot- ting off between sheets of bibulous paper and pressing with a hot iron, the dark tones are restored. This destruction of the tint by boiling water, and its restoration by dry heat, is due in great part to the animal substance employed in sizing the paper ; and it will be found that prints upon a foreign paper, such as the Saxony Positive, salted with a ten-grain solution and sensitized with Ammonio-Nitrate, do not lose their tones in hot water and are not much darkened by ironing. The peculiarity of the sizing of the English Photogra- phic papers must therefore be borne in mind, and allow- ance made for the additional sensitiveness and alteration of colour which it produces. When a formula is given, the paper which is recommended for that particular for- mula should alone be used. SECTION II. Positive Printing hy Development. !N"egative printing processes will be found useful during the dull winter months, and at other times when the light is feeble, or when it is required to produce a large number of impressions from a Negative in a short space of time. The plan of development also enables the operator to ob- tain Positives of greater stability than those yielded by the direct action of light. Three processes may be described, the first of which gives Positives of an agreeable colour, but the second, on Iodide of Silver, the greatest permanency under unfavour- able conditions. 260 THE PRACTICAL DETAILS NEGATIVE PRINTING PROCESSES UPON CHLORIDE OF SILVER. Positives may be obtained by exposing paper prepared with Chloride of Silver to the action of light until a faint image is perceptible, and subsequently developing by Gallic Acid ; but in this process it is difficult to obtain sufficient contrast of light and shade ; the impression, if sufficiently exposed and not too much developed, being feeble, with a want of intensity in the dark parts. By as- sociating with the Chloride an organic salt of Silver, such as the Citrate, this difficulty may be overcome, and the shadows be brought out with great depth and distinctness. The papers are salted with a mixed Chloride and Citrate as in the formula for the Ammonio-Nitrate Process.* They are then rendered sensitive upon a Bath of Nitrate of Silver containing either Citric or Acetic Acid, which are used in Negative processes to preserve the clearness of the white parts under the influence of the developer. The Bath of Aceto-Nitrate is prepared as follows : — Ploat the papers (Papier Saxe or Papier E^ive) upon the Bath for three minutes, and suspend them to dry in a room from which actinic rays are perfectly excluded. The exposure to light, — which is conducted in the ordi- nary printing frame, the Negative and sensitive paper being laid in contact in the usual way, — will seldom be longer than three or four minutes, even upon a dull day. It may be regulated by the colour assumed by the projecting margin of the paper ; but it is quite possible to tell by the appear- ance of the image when it has received a sufficient amount of exposure : — the whole of the picture should be seen, * The formula at p. 246 may be modified with advantage : use double the quantity of Gelatine, and half the amount of Citrate and Chloride. Nitrate of Silver . . Glacial Acetic Acid . Water 30 grains. 30 minims. 1 fluid ounce. OF PHOTOGRAPHIC PRINTING. 261 excepting the lightest shades, and it will be found that very few details can be brought out in the development which were altogether invisible before the Gallic Acid was applied. The developing solution is prepared as follows : — In very cold weather it may be necessary to employ a saturated solution of Gallic Acid, containing about four grains to the ounce ; whereas in warm weather the image will develope too quickly, and Acetic Acid must be added (see the remarks at the end of the process, p. 266). To facilitate the solution of the Gallic Acid, stand the bottle in a warm place near the fire. A lump of Camphor floated in the liquid, or a drop of Oil of Cloves added, will to a great extent prevent it from becoming mouldy by keeping ; but if once mould has formed, the bottle must be well cleansed with Nitric Acid, or the decomposition of the fresh Gallic Acid will be hastened. Pour the solution of Gallic Acid into a flat dish, and im- merse the Prints two or three at a time, moving them about, and using a glass rod to remove air-bubbles. The development is rapid, and will be completed in three or four minutes. If the Print developes slowly, becomes very darh in colour by continuing the action of the Gallic Acid, but shows no half-tones, it has not been exposed sufficiently long to the light. An o^er-exposed proof, on the other hand, developes with unusual rapidity, and it is necessary to remove it speedily from the Bath in order to preserve the clearness of the white parts ; when taken out to the light, it appears pale and red, with no depth of shadow. The extent to which the development should be carried depends upon the kind of Print desired. By pushing the action of the Gallic Acid, a dark picture not much altered by the fixing Bath will be produced. But a better result as regards colour and gradation of tone will be obtained Gallic Acid Water . . 2 grains. 1 fluid ounce. 262 THE PEACTICAL DETAILS by removing tlie Print from the developing solution whilst in the light red stage, and toning it subsequently by means of Gold ; in which case it will correspond both in appear- ance and properties to a Positive obtained by the direct action of light (see the remarks at page 167). When it is intended to follow the latter plan, the action of the developer must be stopped at a point when the proof appears lighter than it is to remain ; since the Sel d'or Bath adds a little to the intensity, and the image becomes somewhat more vigorous on drying. Wash the Prints in cold water in order to extract all the Gallic Acid. Then tone with Sel d'or in the manner described in the next Section, and fix in the usual way. The whites will with care be kept pure ; or with only a faint yellow tinge, which is not objectionable. Upon comparing the developed Prints with others ob- tained by the direct action of light upon the same sensi- tive paper, it is evident that the advantage is slightly on the side of the latter ; but the difference is so small that it would be overlooked in printing large subjects, for which the Negative Process is more especially adapted. The co- lour of both kinds of Positives is the same, or perhaps a shade darker in the developed proofs, which are usually of a violet-purple tone, but sometimes of a dark chocolate- brown. A developing process with Serum of Millc, — The use of whey" as a vehicle for Chloride of Silver has something the same effect as that produced by adding a Citrate. This may be traced to the presence of the Milk Sugar and of a portion of uncoagulated Caseine left in the Serum. The only difficulty in the process is to coagulate the milk in such a way as to separate the greater part but not the whole of the Caseine. Milk which has become sour, or to which an acid has been added, is not considered so good for the purpose as that which has been treated with rennet ; and even when rennet is used it must be of the best quality or its action will be imperfect. The serum OF PHOTOGEAPHIC PRINTING. 263 must filter clear through, blotting-paper ; but it should not run very rapidly, or in all probability the whole of the Caseine has been separated, and the fluid contains little be- sides sugar. The whey which is left after cheese-making, commonly answers the purpose, if clarified by beating it up with the white of an egg and subsequently boiling and filtering. Globules of oil must be separated as far as possible, or they will produce a greasiness of the paper.* Salt the prepared Serum with Chloride of Sodium or Ammonium ; in quantity about eight or ten grains to each fluid ounce, and render sensitive upon the same Bath as that recommended for the Citrate Process. A NEGATIVE PRINTING PROCESS UPON IODIDE OF SILVER. Iodide of Silver is more sensitive to the reception of the invisible image than the other compounds of that metal ; and hence it is usefully employed in printing enlarged Po- sitives from small Negatives, by means of the Camera. The great stability of the proofs upon Iodide of Silver will also be a recommendation of this process when unusual permanency is required. The best paper to use will be either Turner's Calotype, or Whatman's or Hollingworth's Negative ; the foreign papers do not succeed with the above formula (p. 258). Float the paper on the iodizing Bath until it ceases to curl up and lies flat upon the liquid : then pin up to dry in the usual way. Kender sensitive upon a Bath of Aceto-Nitrate of Silver containing 30 grains of Nitrate of Silver with 30 minims of Glacial Acetic Acid to each ounce of water. When the sheet is quite dry, place it in contact with the Take of Iodide of Potassium Water . 160 grains. . 20 fluid ounces. * See the Vocabulary, Part III,, Art. *' Milk," for further particulars. 264 THE PEACTICAL DETAILS Negative in a pressure frame, and expose to a feeble light. About 30 seconds will be an average time upon a dull win- ter's day, on which it would be impossible to print at all in the ordinary way. On removing the ISTegative nothing whatever is seen upon the paper, the image being strictly invisible in this process unless the exposure has been car- ried too far. Develope by immersion in a saturated solution of Gallic Acid, prepared in the manner described at page 261. The image appears slowly, and the process may last from 15 minutes to half an hour. If the exposure has been cor- rectly timed, the Gallic Acid appears at length almost to cease acting ; but when the proof has been over-exposed, the development goes on uninterruptedly, and the image becomes too dark, partaking more of the character of a Negative than a Positive. The usual rule, that under-ex- posed proofs develope slowly but show no half-tones, and that the ot;er-exposed develope with unusual rapidity, is also observed in the process with Iodide of Silver. After the picture is fully brought out, wash in cold, and subsequently in warm water, to remove the Gallic Acid, which, if allowed to remain, would discolour the Hyposul- phite Bath. Then fix the Print in a solution of Hyposul- phite of Soda, one part to two of water, continuing the action until the yellow colour of the Iodide disappears. The fixing Bath ought not to produce much change in the tint. If the Positive loses its dark colour on immersion in the Hyposulphite, and becomes pale and red, it has been insufficiently developed. The theory of this part of the process should be understood : — It is particularly the second stage of the development of a Photograph (see p. 144) on which the fixing Bath produces no efiect ; and therefore a considerable change of colour in the Hyposul- phite indicates that too little Silver has been deposited, and the remedy will be to push the development, adding a little Aceto-Nitrate to the Gallic Acid if the strength of the Bath be found insufficient to yield dark tones. OF PHOTOGEAPHIC PEINTING. 265 The colour of Positives developed upon Iodide of Silver is not agreeable, and they become blue and inky when toned with gold. By fixing the proof in Hyposulphite of Soda which has been long used and has acquired sulphu- retting properties, the tint is much improved ; but the permanency of the Print under unfavourable conditions is lessened by adopting that mode of toning. A NEGATIVE PEINTING PEOCESS UPON BEOMIDE OF SILVEE. By substituting the Bromide for the Iodide of Silver in the above process, the proportions and details of manipu- lation being in other respects the same, a more agreeable colour is obtained. Paper prepared with Bromide of Silver is less sensitive than the Iodide, but an exposure of one minute (in the printing frame) will usually be sufficient even on a dull day. The image is nearly latent, but sometimes a very faint outline of the darkest shadows can be seen. The pro- portion of Bromide used is likely to influence this point ; the sensitiveness being diminished, but the image showing more of the details before development, when the quantity of the Silver Salt is reduced to a minimum. Either English or French papers may be used, but in the latter case the Bromide should be dissolved in Serum of Milk (p. 262), or it will be difficult to obtain a good sur- face picture. The proportion of Bromide may be five grains to the ounce of Serum. These proofs, even when simply fixed in plain Hyposul- phite of Soda, are superior in colour to the Positives printed by the last formula upon Iodide of Silver ; and the per- manency is very great if the development be sufficiently pushed. The use of the Serum of Milk gives an advan- tage in resisting the oxidizing influences to which Positives are liable to be exposed (p. 150). GENEEAL EEMAEKS ON NEGATIVE PEINTING. Printing by development should not be attempted until 266 THE PEACTICAL DETAILS the manipulation of tlie ordinary process by direct expo- sure to light has been acquired. Perfect cleanliness is essential. The salting or iodizing solution and the Aceto-Nitrate Bath must be filtered clear, as the effect of small suspended particles in producing spots is more seen when the image is brought out by a developer. It will be necessary to be far more careful in excluding white light than in the ordinary process ; and when Io- dide of Silver is used, all the precautions required in the case of Collodion Negatives must be taken. Observe particularly that the dishes are kept clean, or the G-allo-Nitrate of Silver will be rapidly discoloured (read the remarks at page 179). Stereoscopic Negatives and small portraits are not suc- cessfully printed by development ; since it is difficult to obtain the most elaborate definition, and there is a slight tendency to yellowness in th^ white parts. Positives may be developed upon Albumen paper, but the Grallic Acid is apt to discolour the lights. In printing by development upon Chloride of Silver, the theory of the subject must be particularly studied. When the weather is cold and the light bad, the development of the image proceeds slowly, the Gallic Acid Bath remains clear, and good half-tones are obtained ; but under oppo- site conditions, the developer may become turbid and the shadows be lost by excessive deposit of Silver. This over- development will be remedied by printing the Negative in a more feeble light (near to the open window of a room), and by adding Acetic Acid to the developer, about 5 or 10 minims to the ounce, so as to bring out the image more slowly. The intensity of action is thus lessened, and if the picture be not under-exposed, the half-tones will be good. Observe also when preparing papers with Citrate, that if too much Carbonate of Soda be added in neutralizing the Citric Acid, Carbonate of Silver will be deposited in OF PHOTOGRAPHIC PRINTING. 267 the paper, the effect of which is to remove by degrees the acidity of the Nitrate Bath, and to produce over-develop- ment and excessive sensibility to light. The colour of the proofs when taken from the Gallic Acid should be light red ; the gradation of tone not being usually so perfect when the development is carried into the second or black stage. It is not recommended to prepare too large a stock of the salted papers, as they will probably be liable to mouldiness and decomposition unless kept perfectly dry. SECTIOIN' III. The Sel d'or Process for toning Positives. This process is somewhat more troublesome than the plan of fixing and toning in one solution, but possesses ad- vantages which will presently be enumerated. The de- scription may be divided into the preparation of the toning Bath, and the manipulatory details. THE PREPARATION OF THE TONING BATH. Take of Chloride of Gold 1 grain. Pure Hyposulphite of Soda . 3 grains. Hydrochloric Acid .... 4 minims. Water, distilled or common . 4 fluid ounces. Dissolve the Gold and Hyposulphite of Soda each in two ounces of the water ; then mix quickly by pouring the former solution into the latter, and add the Hydrochloric Acid. If the Chloride of Gold be neutral, the liquid will have a red tinge, but if acid, then the solution may be colourless. The commercial Chloride of Gold, containing usually much free Hydrochloric Acid, will not require any addition of that substance. (See the Vocabulary, Part III.) 268 THE PEACTICAL DETAILS In place of making an extemporaneous Hyposulpliite of Gold by mixing the Chloride with Hyposulphite of Soda, the Crystallized Sel d'or may be used, adding about half a grain to the ounce of water, acidified as before ; but the objection to the employment of this salt is its expense, and also the difficulty of obtaining it in a pure form ; some samples containing less than five per cent, of Gold. It will be found very convenient to keep the two solu- tions on hand ready for mixing, viz. the Chloride of Gold dissolved in water in the proportion of a grain to the drachm, and the Hyposulphite of Soda, three grains to the drachm. When required for use, measure out a fluid drachm of each, dilute with water to two ounces, and mix. It is possible that the three-grain solution of Hyposul- phite of Soda may by long keeping become decomposed, with precipitation of Sulphur. The efiect of this would be to produce a turbidity and deposit of Gold on mixing the ingredients for the Bath, the Chloride of Gold being in excess over the Hyposulphite of Soda (see p. 250). The Bath of Sel d'or is always most active when recently mixed, but it will keep good for some days if contact with free Nitrate of Silver be avoided. The addition of this substance produces a red deposit in the Bath, containing Gold, and the solution then becomes useless. DETAILS OF MANIPULATION. The paper may be prepared by either of the formulae given in the first Section of this Chapter, according to the tint desired. The pure black tones are obtained most easily with the Ammonio-Nitrate paper, and the purple tints, without gloss, on paper prepared with plain Chloride and Citrate of Soda. The printing is not carried quite to the usual intensity, as the half-tones are very little dissolved in this process. On being taken from the frame, the prints are washed thoroughly in common water until it ceases to become OF PHOTOGEAPHIC PRINTING. 269 milky ; tliat is, until the greater part of the Nitrate of Silver has been removed. The washing must be conducted in a dark place, but it is not necessary to hasten it ; the proofs may be thrown into a pan of 'water covered with a cloth, and allowed to remain until required for tinting. A trace of free Nitrate of Silver usually escapes the washing ; this would cause a yellow deposit on the Print, and also in the toning Bath. It must therefore be re- moved, either by adding a little common salt to the water during the last washings, or by means of a dilute solution of Ammonia. For plain paper Prints the former plan will be found the least troublesome ; but with Albumen proofs'^ the Ammonia is required, in order to dissolve away a portion of the Albuminate of Silver which has escaped the action of light, before submitting the print to the gold ; other- wise the dark tones would nearly disappear in the fixing Bath, the Hyposulphite carrying away the Gold with this superficial layer of silver salt. To prepare the Ammonia Bath, take of Liquor Ammonise .... 1 drachm. Common Water .... 1 pint. The exact quantity is not material ; if the liquid smells faintly of Ammonia, it will be sufficient. Place the washed Prints in this Bath, two or three at a time, and allow them to remain until the purple tint gives place to a red tone. The action must be watched, because if the Am- monia Bath be strong, the proof becomes unusually ^ale and red, and when this is the case a little brilliancy is lost in the after-tinting. As the Print is comparatively insensitive to light when the excess of Nitrate has been washed away, it is not ne- cessary to darken the room ; but a bright light proceeding from an open door or window should be avoided. * The amateur is recommended not to use Albuminized paper in this process until he has become accustomed to the manipulations j the plain paper prints being toned with more ease and certainty. 270 THE PRACTICAL DETAILS After using the salt or the Ammoma, soak the Prints again for a minute or so in common water. Then place them in the toning Bath of Gold and acid ; do not put in too many at once, and move them about occasionally, to prevent spots of imperfect action at the point where the sheets touch each other. The foreign papers, plain salted, colour rapidly in two or three minutes. English papers require five to ten minutes ; Albuminized, ten minutes to a quarter of an hour. The tendency of the Grold Bath is to give a blue tone to the image ; hence proofs which are light red after using the salt or Ammonia, become, first red-purple, and then violet- purple in the Sel d'or. Albumen Prints assume some shade of brown, or of purple if not too strongly Albuminized. Ammonio-Nitrate papers highly salted, and prepared with- out Citrate, become first dark purple, and then blue and inky ; the Citrate is intended to obviate this inky tint. When the darkest tones are reached, the Bath produces no further effect, but eventually (more especially if the solution be not shielded from light [?]) there is a little decomposition, producing a cream-coloured deposit upon the lights. The toning being completed, the Prints are again washed for an instant in water, to remove the excess of gold solu- tion. This washing must not be continued longer than two or three minutes, or there will be danger of yellowness of the whites ; this however ought not to happen with proper precautions. Lastly, the proofs are fixed in a solution of Hyposulphite of Soda, one part to four of water ; which may be used many times successively. This Bath alters the tone very little if the deposit of Gold be well fixed on the Print ; but the writer has often observed in the case of Albumen paper and paper prepared with Citrate (Formula II.) that if re- moved too quickly from the Sel d'or, the purple tones change by immersion in the Hyposulphite to a chocolate- brown. Ammonio-Nitrate Prints are less liable to alter in this way. OF PHOTOGRAPHIC PEINTING. 271 In order that the fixing may be properly performed, the time of immersion should not be less than ten minutes with a porous paper, plain salted ; or fifteen minutes in the case of an English or albuminized paper. Ammonia may be used for fixing plain paper Prints ; about one part of the Liquor Ammonise, to four of water. Ten minutes' immersion will usually be sufficient, and the tone is very little affected. This process is a good one, but the pungent smell of the Ammonia is an objection, and the Bath discolours by use. Some care too is required in order to ensure a proper fixing of the prints (see the remarks at page 131). For directions to wash and mount the proofs, see page 255. It will sometimes happen in the Sel d'or process, from the toning Bath having but little solvent action on the light shades, that the Prints, after being washed and dried, ap- pear too dark ; this may be remedied by laying them for a few minutes in a very dilute solution of Chloride of Gold (five or six drops of the yellow solution of the Chloride to a few ounces of water) and washing for an additional quarter of an hour. Or an over-printed Positive may be saved by toning it with Chloride of Gold instead of Sel d'or. In that case, after proper removal of the free Nitrate of Silver, a few drops of a lemon-yellow solution of Chloride of Gold (with a fragment of Carbonate of Soda added to remove acidity, p. 132), should be poured over the Print, which is to be subsequently fixed in the usual way. Advantages of toning hy Sel d'or. — This process will be found especially useful by those who print large Positives, The solutions may be mixed in a few minutes, and, being very dilute, are economical. It is not even necessary to employ a Bath for toning, but if the Sel d'or solution be prepared of about twice or three times the strength given in the formula, it will be sufficient to pour a few drachms upon the surface of the print. As the Gold solution is al- ways used soon after mixing, a uniform and permanent 272 THE PEACTICAL DETAILS tint can be obtained ; whereas the single fixing and toning Bath of Gold and Hyposulphite loses much of its efficacy by keeping, and over-printing of the proof is required in proportion as the Bath becomes older. SECTION IV. On a mode of Printing enlarged and reduced Positives, Transparencies, etc., from Collodion Negatives. To explain the manner in which a Photograph may be enlarged or reduced in the process of printing, it will be necessary to refer to the remarks made at page 52, on the conjugate foci of lenses. If a Collodion JSTegative be placed at a certain distance in front of a Camera, and (by using a tube of black cloth) the light be admitted into the dark chamber only through the Negative, a reduced image will be formed upon the ground glass ; but if the Negative be advanced nearer, the image will increase in size, until it becomes first equal to, and then larger than, the original Negative ; the focus be- coming more and more distant from the lens, or receding, as the Negative is brought nearer. Again, if a Negative portrait be placed in the Camera slide, and the instrument being carried into a dark room, a hole be cut in the window-shutter so as to admit light through the Negative, the luminous rays, after refraction by the lens, will form an image of the exact size of life upon a white screen placed in the position originally occu- pied by the sitter. These two planes, in fact, that of the object and of the image, are strictly conjugate foci, and, as regards the result, it is immaterial from which of the two, anterior or posterior, the rays of light proceed. Therefore in order to obtain a reduced or enlarged copy of a Negative, it is necessary only to form an image of the size required, and to project the image upon a sensitive surface either of Collodion or paper. OF PHOTOGRAPHIC PRINTING. 273 A good arrangement for tliis purpose may be made by taking an ordinary Portrait Camera, and prolonging it in front by a deal box blackened inside and with a double body, to' admit of being lengthened out as required ; or, more simply, by adding a framework of wood covered ia with black cloth. A groove in front carries the Negative, or receives the slide containing the sensitive layer, as the case may be. In reducing Photographs, the Negative is placed in front of the lens, in the position ordinarily occupied by the ob- ject ; but in making an enlarged copy, it must be fixed behind the lens, or, which is equivalent, the lens must be turned round, so that the rays of light transmitted by the Negative enter the back glass of the combination, and pass out at the front. This point should be attended to in order to avoid indistinctness of image from spherical aberration. A Portrait combination of lenses of 2| or 3J inches diameter is the best form to use, and the actinic and lumi- nous foci should accurately correspond, as any difference between them would be increased by enlarging. A stop of an inch or an inch and a half aperture placed heUoeen the lenses obviates to some extent the loss of sharp out- line usually following enlargement of the image. The light may be admitted through the Negative by pointing the Camera towards the sky ; or direct sunlight may be used, thrown upon the Negative by a plane reflec- tor. A common swing looking-glass, if clear and free from specks, does very well ; it should be so placed that the centre on which it turns is on a level with the axis of the lens. The best Negatives for printing enlarged Positives are those which are distinct and clear ; and it is important to use a small Negative, which strains the lens less and gives a better result than one of larger size. In printing by a 2J lens for instance, prepare the Negative upon a plate about two inches square, and afterwards enlarge it four diameters. Paper containing Chloride of Silver is not sufficiently T 274 PBACTICAL DETAILS sensitive to receive the image, and the Print should be formed upon Collodion, or on iodized paper developed by Gallic Acid (see p. 263). The exposure required will vary not only with the in- tensity of the light and the sensibility of the surface used, but also with the degree of reduction or enlargement of the image. In printing upon Collodion the resulting picture is Positive by transmitted light ; it should be backed up with white varnish, and then becomes Positive by reflected light. The tone of the blacks is improved by treating the plate first with Bichloride of Mercury, and then with Am- monia, in the manner described at pages 113 and 207). Mr. Wenham, who has written a paper on the mode of obtaining Positives of the life size, operates in the follow- ing way : — he places the Camera, with the slide containing the Negative, in a dark room, and reflects the sunlight in through a hole in the shutter, so as to pass first through the Negative and then through the lens ; the image is re- ceived upon iodized paper, and developed by Gallic Acid, in the mode described in the second Section of this Chap- ter (p. 263). On printing Collodion transparencies for the Stereoscope. ' — This may be done by using the Camera to form an image of the Negative in the mode described in the last page ; but more simply by the following process : — Coat the glass, upon ^A hich the Print is to be formed, with Collodio-Iodide of Silver in the usual way ; then lay it upon a piece of black cloth, Collodion side uppermost, and place two strips of paper of about the thickness of cardboard and one-fourth of an inch broad, along the two opposite edges, to prevent the Negative being soiled by contact with the film. Both glasses must \)Q perfectly flat, and even then it may happen that the Negative is unavoidably wetted ; if so, wash it immediately with water, and if it be properly varnished, no harm will result. A little ingenuity will suggest a simple framework of OF PHOTOGRAPHIC PRINTING. 275 wood, on which the Negative and sensitive plate are re- tained, separated only by the thickness of a sheet of paper ; and the use of this will be better than holding the combi- nation in the hand. The printing is conducted by the light of gas, or of a camphine or moderator lamp ; diffused daylight would be too powerful. The employment of a concave reflector, which may be purchased for a few shillings, ensures parallelism of rays, and is a great improvement. The lamp is placed in the focus of the mirror, which may at once be ascertained by moving it backwards and forwards until an evenly illumi- nated circle is thrown upon a white screen held in front. This in fact is one of the disadvantages of printing by a naked flame — that the light falls most powerfully upon the central part, and less so upon the edges, of the Negative. The picture must be exposed for a longer or shorter time (about ten seconds will be an average) according to its behaviour during development (see p, 224) ; this process, as well as the fixing, is conducted in the same manner as for Collodion pictures generally. Some adopt the plan of whitening by Corrosive Subli- mate, and again blackening by dilute Ammonia, as an im- provement to the colour of the dark shadows (see p. 113). If this mode of printing upon Collodion be conducted with care, the Negative being separated from the film by the smallest interval only, the loss of distinctness in out- line will scarcely be perceived. Stereoscopic transparencies may also be printed by the dry Collodion process described in Chapter VI., or by the Collodio- Albumen process. Mr. Llewellyn recommends the employment of a solution of Oxymel, so dilute that the plate becomes nearly dry, and may be laid in contact with the Negative without fear of injury (see the foot- note at page 302). 276 CHAPTER V. CLASSIFICATION OF CAUSES OF FAILURE IN THE COLLODION PROCESS. Section I. — Imperfections in Collodion Photograplis. Section II. — Imperfections in Paper Positives. SECTION I. Imperfections in Negative and Positive Collodion JPhotographs. TKe following may be mentioned: — fogging — spots — markings, etc. CAUSES OF FOGGING OF COLLODION PLATES. 1. Over-exposure of the JPlate. — This is likely to happen when using the full aperture of a double combination lens for distant objects brightly illuminated, the Collo- dion being highly sensitive. Also from the film being very blue and transparent, with too little Iodide of Silver (p. 114). 2. Diffused Light. — a. In the developing room. This is a frequent cause of fogging, and especially so when the common yellow calico is employed, which is apt to fade. Use a treble thickness, or procure the waterproof material, in which the pores are stopped with gutta-percha. — b. In the Camera. The slide may not fit accurately, or CAUSES OF FAILURE. 277 tlie door does not shut close. Throw a black cloth over the Camera during the exposure of the plate. — c. From direct rays of the sun or the light of the sky falling upon the lens. With the full aperture of a double combination Lens, a portion of sky included in the field (as for instance to form the background of a portrait) is apt to cause fog- ging. The portrait will probably be more brilliant if a funnel-shaped canvas bag, or a curtain with an oblong aperture admitting only the rays proceeding from the sit- ter, be placed in front of the Camera. 3. Alkalinity of the Bath. — This condition, explained at page 88, may be due to one of the following causes : — a. The use of Nitrate of Silver which has been too strongly fused (p. 13). — h. Constant employment of a Collodion con- taining free Ammonia or Carbonate of Ammonia (p. 89). — c. Addition of Potash, Ammonia, or Carbonate of Soda to the Nitrate Bath, in order to remove free Nitric Acid (p. 89). — d. Use of rain-water or hard water for making the Nitrate Bath (rain-water usually contains traces of Am- monia; hard water often abounds with Carbonate of Lime). In either case the alkalinity may easily be removed by the addition of Acetic Acid, one drop to four ounces of the solution. The proper mode of testing for alkalinity is de- scribed at p. 89. 4. Decomposition of the Nitrate Bath, — a. By constant exposure to light (the injurious effects of this will be mostly seen when Positives are taken). — h. By organic matter: this is sometimes present in Nitrate of Silver which has been prepared from the residues of old Baths ; or it may be introduced by floating papers for the printing process upon the Bath, or by dissolving the crystals of Nitrate of Silver in putrid rain-water, or in impure distilled water collected from the condensed water of steam-boilers and contami- nated with oily matter. — c. Decomposition of the Bath by contact with metallic iron or copper, or with a fixing agent, or a developing agent (p. 90). 6. Faults of the developing solution. — a. Brown and de- 278 CAUSES OF FAILtTEE composed solution of Pyrogallic Acid ; this may sometimes be used with impunity, but it tends, as a rule, to facilitate irregular reduction of Silver. — h. Impure Acetic Acid hav- ing a smell of Garlic and which probably contains Sulphur in organic combination. — c. Omission of the Acetic Acid in the developer : this will produce a universal blackness. 6. Sundry other causes of fogging. — a. Yapour of Am- monia or Hydrosulphate of Ammonia, or the products of the combustion of coal-gas, escaping into the developing room. — h. Development of the image by immersion in so- lution of Sulphate of Iron : this is a safe plan when the films are formed in an acid Nitrate Bath ; but with pale films formed in a chemically neutral Bath it is better to pour the fluid over the plate, and not to use the same por- tion twice. — c. Hedipping the plate in the Bath before de- velopment : this is apt to give a foggy picture when using an old Bath, and is not recommended. Systematic plan of proceeding to detect the cause of the fogging. — If the amateur has had but little experience in the Collodion process, and is using Collodion of moderate sensitiveness and a new Bath, the probability is that the fogging is caused by over-exposure. Having obviated this, proceed to test the Bath; if it is made from pure materials, and does not restore the blue colour of a piece of litmus- paper previously reddened by holding it over the mouth of a glacial Acetic Acid bottle, it may be considered in work- ing order. Next prepare a sensitive plate, and after draining it for two or three minutes in a dark place, pour on the deve- loper : wash, fix, and bring out to the light ; if any misti- ness is perceptible, either the developing room is not suffi- ciently dark, or the Bath was prepared with a bad sample of Nitrate of Silver, or with impure Alcohol, or impure water. On the other hand, if the plate remains absolutely clear under these circumstances, the cause of error may be in the Camera; — therefore prepare another sensitive film, place IN THE COLLODION PROCESS. 279 it in the Camera, and proceed exactly as if taking a pic- ture, with the exception of not removing the brass cap of the lens : allow to remain for two or three minutes, and then remove and develope as usual. If no indication of the cause of the fogging is obtained in either of these ways, there is every reason to suppose that it is due to diffused Light gaining entrance through the lens. This cause of error may often be detected by looking into the Camera from the front, when an irregular reflection will be seen upon the glass. SPOTS UPON COLLODION PLATES. Spots are of two kinds : spots of opacity, which appear black by transmitted light, and white by reflected light ; and spots of transparency, the reverse of the others, being white when seen upon Negatives, and black on Positives. Opaque Spots are referable to an excess of develop- ment at the point where the spot is seen ; they may be caused by — 1. The use of Collodion holding small particles in sus- pension. — Each particle becomes a centre of chemical ac- tion, and produces a speck, or a speck with a tail to it. The Collodion should be placed aside to settle for several hours, after which the upper portion may be poured off. 2. Turbidity of the Nitrate solution. — a. From flakes of Iodide of Silver having fallen away into the solution, by use of an over-iodized Collodion. — b. From a deposit formed by degrees upon the sides of the gutta-percha trough. — c. From the inside of the trough being dusty at the time of pouring in the solution. In order to obviate these inconveniences, it is well to make at least half as much again of the Nitrate solution as is necessary, and to keep it in a stock-bottle, from which the upper part may be poured off when required. The frequent filtration of Silver Baths is unadvisable, since the paper employed may be contaminated with impurities. 280 CAUSES OF FAILIJEE 3. Dust upon the surface of the glass at the time of pouring on the Collodion. — Perfectly clean glasses, if set aside for a few minutes, acquire small particles of dust ; each plate should therefore be gently wiped with a silk handkerchief immediately before being used. 4. Faults of the Slide. — Sometimes a small hole exists, which admits a pencil of hght, and produces a spot, known by its being always in the same part of the plate ; occasion- ally the door works too tightl}^ so that small particles of wood, etc., are scraped off, and projected against the plate when it is raised. Or perhaps the operator, after the ex- posure is finished, shuts down the door with a jerk, and so causes a splash in the liquid which has drained down and accumulated in the groove below ; this cause, although not a common one, may sometimes occur. 5. hisoluble particles in the Pyrogallic Acid. — The so- lution of Pyrogallic Acid will not usually require filtering, but if specks of Mctagallic Acid are present, the deve- loper should be passed through blotting paper before use. Spots of Teansparency may generally be traced to some cause which renders the Iodide of Silver insensible to light at particular points J so that on the application of the developer no reduction takes place. 1. Concentration of the Nitrate of Silver on the surface of the film by evaporation. — When the film becomes too dry after removal from the Bath, the solvent power of the Nitrate increases so much that it eats away the Iodide and produces spots. 2. Small particles of undissolved Iodide of Potassium in the Collodion. — These are likely to occur when Anhydrous Ether and Alcohol are employed. They produce transpa- rent specks at every part of the plate. Allow the Collodion to settle, or add a drop of water, which will dissolve the Iodide. 3. Alcohol or Ether containing too much water. — This causes a reticulated appearance of the film, which is rotten and fuU of holes. IN THE COLLODION PEOCESS. 281 4. Use of glasses improperly cleaned. — This cause is per- haps the most frequent of all, when the film of Pyroxyline is very thin and the Bath neutral. After glasses have been long used it is often difficult to clean them so thoroughly that the breath lies smoothly ; but the use of Potash gives the best chance. MARKINGS OF VAEIOUS KINDS ON COLLODION PLATES. 1. A reticulated appearance on the film after developing, — When this is universal, it often depends upon the em- ployment of Collodion containing water. Or, if not due to this cause, the plate may ha^e been immersed too quickly in the Bath, and the soluble Pyroxyline partially precipi- tated. 2. Oily spots or lines. — a. From raising the plate out of the Nitrate Bath before it has been immersed sufficiently long to have become thoroughly wetted. — h. Removal of the plate from the Bath before the Ether upon the surface has been washed away. —c. Redipping the plate in the Ni- trate Bath after exposure to light, and pouring on the de- veloper immediately ; if a few minutes be not allowed to drain off the excess of Nitrate, the Pyrogallic Acid will not amalgamate readily with the surface of the film. — d. From the Nitrate Bath being covered with an oily scum, which is carried down by the plate. Draw a slip of blot- ting-paper gently along the surface of the liquid before using it. 3. Straight lines traversing the film horizontally. — From a check having been made in immersing the plate in the Bath. 4. Curved lines of over-development. — By employing the developer too concentrated ; or by not pouring it on suffi- ciently quickly to cover the surface before the action be- gins ; or by using too little Acetic Acid, and omitting the Alcohol. The addition of Alcohol to the developer will not be required as a rule when the Bath is newly made ; but when much Ether has accumulated in it, the developer 282 CAUSES OF FAILUEE lias a tendency to run into oily lines, unless containing Alcohol. 5. Stains from too small a quantity of fluid having been employed to develope the image. — In this case, the whole plate not being thoroughly covered during the develop- ment, the action does not always proceed with regularity. 6. Irregular strice. — From fragments of dried Collodion accumulating in the neck of the bottle, and being washed on the film; to avoid this, the finger should be passed gently round the inside of the neck before use. 7. Markings like those represented in the woodcut. — They are caused by using an in- Aft A A A ferior sample of Pyroxyline made from too 1(111^11/1 A hot acids, and are most seen when using an old Bath. 8. Stains on the upper part of the plate ^ from using a dirty slide. — To avoid these, place, if necessary, strips of blotting-paper between the supports and the glass. 9. Wavy marks at the lower parts of the plate. — a. If the Collodion is becoming thick and glutinous from con- stant use, dilute it with a little Ether containing an eighth part of Alcohol. — h. From reversing the direction of the plate after its removal from the Bath, so that the Nitrate of Silver flows back again over the surface and causes a stain on the application of Pyrogaliic Acid. — c. Impurities on the woodwork of the frame ascending the film by capil- lary attraction. This is a frequent source of stains. 10. Marks from the develofer not running up to the edge of the film (p. 212). Eemedy this as far as possible by allowing the Collodion to set a little more firmly before dipping the plate in the Bath. IMPERFECTIONS IN COLLODION NEGATIVES. 1. A want of Intensity. — a. From the development not having been sufficiently pushed (p. 224). — h. From the Collodion film being too blue and transparent for JNTe- IN THE COLLODION PROCESS. 283 gatives. — c. The Collodion newly made from pure ma- terials (p. 114). — d. The plate kept too long between ex- citing and development (p. 100). — e. The Bath newl}- pre- pared from commercial crystallized Nitrate of Silver (p. 10\).—f. The light too feeble, as on very dark wintry days, or in copying interiors, etc. 2. Inferior half-tones, with great intensity of the high Lights. — a. From the plate being insufficiently exposed. — b. The Collodion of inferior quality, either too strongly tinted with Iodine or made from impure materials. — c. The Nitrate Bath old and partially decomposed. — d. The light reflected too strongly from the object. When the light is unusually bright, a feeble Collodion and a newly mixed Nitrate Bath will be found to give better definition in the high lights than an intense Collodion, which may produce chalky Negatives. 3. The image pale and misty. — The plate is over-exposed (if so, the image will probably be a reddish-brown colour by transmitted light), or there is diffused light in the Camera or developing room. The presence of Bromides or Chlorides in the Collodion may occasionally produce the same effect. 4. The high lights of the image are solarized. — A change of colour to a light brown or red tint by transmitted light, with a dark shade by reflected light, is favoured by over- exposure of the plate, by organic decomposition of the Collodion, and by Acetate of Silver and other organic bodies in the Bath. 5. The image dissolves off on applying the Cyanide of Potassium. — The Collodion is probably over-iodized. The same thing may also happen in the Honey preservative process, when the plates have been long kept and the in- durated layer of syrup not properly removed before ap- plying the developer. 6. The developer does not run up to the edge of the film. — This is likely to occur when using Collodion nearly anhy- drous ; and partictdarly so with a new Bath not contain- 284 CAUSES OF FAILURE ing miicli Alcohol. The film will be less repellent, if a longer time be allowed before dipping in the Bath. 7. The film does not stick to the glass. — Clean the plates very carefully, and make the Collodion a little thinner if required. Allow a longer time before dipping in the Bath. A very efiectual plan is to roughen the surface of the plates, about an eighth of an inch round the edges. IMPERFECTIONS IN COLLODION POSITIVES. The principal difficulty in the production of Negatives is to ascertain the right time of exposure to light and the proper point to which to carry the development of the image. A minor amount of fogging, stains, etc., is of less consequence, and will scarcely be noticed in the printing. With direct Positives however the case is dijfferent. The beauty of these pictures depends entirely upon their being clean and brilliant, without fogging, specks, or im- perfections of any kind. On the other hand, the exposure and development of Positives is comparatively simple and easily ascertained. 1. The shadows dark and heavy. — The plate has not received sufficient exposure in the Camera ; — or the film being very transparent and the Silver solution weak, Ni- tric Acid is present in the Bath, or the Collodion is brown from free Iodine ; in the latter case make the Collodion a little thicker, and develope with Sulphate of Iron in pre- ference to Pyrogallic Acid. 2. The shadows good, hut the lights overdone. — The de- veloping fluid may have been kept on too long ; or the ob- ject is not properly illuminated (p. 220) ; or the Collodion is not adapted for Positives. 3. The high lights pale and flatt the shadows misty. — The plate is over-exposed. Indistinctness of outline caused by over-exposure is distinguished from that produced by fogging by holding the plate up to the light ; in the former case the image shows as a Negative. If the Collodion is colourless, clearer shadows will pro- IN THE COLLODION PROCESS. 285 bably be obtained by dropping in Tincture of Iodine until a yellow colour is produced. 4. The picture developes slowly ; spangles of metallic Silver are formed. — Too much Nitric Acid is present in proportion to the strength of the Bath, to the amount of Iodide in the film, and to the quantity of Protosalt of Iron in the developer (p. 112). 5. Circular spots of a black colour after hacking up with the varnish. — These are often caused by lifting the plate too quickly out of the Bath ; or by pouring on the deve- loper at one spot, so as to wash away the Nitrate of Sil- ver ; or by the use of glasses imperfectly cleaned. 6. The image becomes metallic on drying. — If Sulphate of Iron is employed, the solution is too weak, or free Nitric Acid has been added in excess. If Pyrogallic Acid is used to develope, the proportion of Nitric Acid is too great. 7. A green or blue tint in certain parts of the image. — This is caused by the deposit of Silver being too scanty, which may happen from over- action of the light, or from the film of Pyroxyline being very thin ; — if the Collodion is diluted down beyond a certain point, the same quantity of free Nitrate of Silver is not retained upon the surface of the film. Add a few drops of the Bath to the developer before pouring it on the plate. 8. Vertical lines, and mistiness, on the image. — If the Bath has been much used, add to it a third part of a simple solution of Nitrate of Silver in water, without any Alcohol or Iodide. Also prepare the developer with addition of Alcohol, to make it flow^ more readily (p. 211). SECTION II. Imperfections in Paper Positives. 1. The Print marbled and spotty. — The quality of the paper is often inferior, which causes it to imbibe liquids 286 CAUSES OF FAILUEE unevenly at different points ; or tlie amount of Silver in tKe Nitrate Bath is insufficient. In this case the spots are often absent at the lower and most depending part of the sheet, where the excess of liquid drains off. 2. The Print clean on the surface, hut spotted lohen held up to the light. — In this case the spots are probably due to imperfect fixation (see p. 129). 3. The Print becomes pale in the Hyposulphite Bath., and has a cold and faded appearance when finished. — The Chlo- ride of Silver in the paper may have been in excess with regard to the free Nitrate of Silver ; which is especially likely if no bronzing could be obtained by prolonged action of the light, or if a weak solution of Nitrate of Silver was laid on with a brush, or by a glass rod. Prints formed on paper which has been kept too long after sensitizing pre- sent the same appearance, the free Nitrate of Silver having entered into combination with the organic matter. 4. Yellowness of the light parts of the proof. — The fol- lowing causes are likely to produce yellowness : — acidity of the fixing and toning Bath (p. 139), — its action conti- nued for too long a time, — the first washings of the proof not performed quickly, — the toning Bath laid aside until it had become decomposed and nearly useless, — the paper kept for several days after sensitizing. A creamy yellowness is also common in Prints toned by Sel d'or, when the Hydrochloric Acid has been omitted from the formula ; the proof exposed to light during the toning and fixing process ; or too long a time allowed to elapse between the toning and fixing. It is also more fre- quently met with on albuminized paper. 5. Intense bronzing of the deep shadows. — In this case the Negative is in fault ; remedy the evil as far as possible by printing on paper containing but little salt. 6. The definition of the Print imperfect, the Negative being a good one. — Much will depend upon the quality of the paper. Towgood's Positive gives good definition. The use of Albumen will be a great advantage. Citrate of Soda (p. 246) will also improve the definition on plain paper. IN THE COLLODION PROCESS. 287 7. Marhings of a yellow tint in the darJc portions of the Positive. — These are common on Prints toned without Gold ; care should be taken not to handle the paper too much, either before or after sensitizing ; to wash the prints in a clean vessel ; and not to lay them down whilst wet on a wooden table or in contact with anything likely to communicate impurities. 8. Small specks and spots of different hinds. — These, when not corresponding to similar marks upon the Nega- tive, are usually due to metallic specks in the paper ; or to insoluble particles floating in the bath. 9. Markings of the brush in Ammonio-Nitrate pictures. — In this case there is probably an excess of Ammonia, which dissolves the Chloride of Silver. Add a little fresh Nitrate of Silver, or use the Oxide of Silver dissolved in JSTitrate of Ammonia (p. 249). 10. Marbled stains on the surface of the Print. — Draw a strip of blotting-paper gently over the surface of the Nitrate Bath before sensitizing the paper ; and see that the sheet does not touch the bottom of the dish. 11. Streaks on Albuminized paper. — Apply the Albumen more rapidly and evenly to the paper. If this does not succeed, add a little Ox Gall (p. 243). 12. Removal of the Albumen from the paper during sensitizifig. — The Nitrate Bath is probably alkaline (see page 89). 288 CHAPTER VI. LANDSCAPE PHOTOaEAPHT ON PRESERYED COLLODION AND COLLODIO-ALBUMEN. The Collodion process may be applied with success to landscape Photography; but as the plates become dry and lose their sensitiveness shortly after their removal from the Bath, the operator will require to provide himself with a yellow tent or some portable vehicle in which the operations of sensitizing and developing can be conducted. As it is a point of great importance in the Collodion pro- cess that the plate should receive exactly the right amount of exposure in the Camera, — a few seconds more or less sufficing to affect the character of the picture, — many will submit to much trouble and inconvenience in order to have the apparatus complete upon the spot at which the view is taken. The object of the "Collodion Preservative Processes'* is to maintain the sensitiveness of the film for a certain length of time after it has been excited in the Bath. There is some difficulty in doing this, because if the plate be allowed to dry spontaneously, the solution of free Nitrate of Silver upon the surface, becoming concentrated by evaporation, eats away the Iodide of Silver, and produces transparent spots. Some operators have attempted to use a second plate of glass in such a way as to enclose the sensitive film with an ON LANDSCAPE PHOTOGEAPHY. 289 intervening stratum of liquid. The difficulty however of separating tke glasses again without tearing the film, is considerable. In the process of Messrs. Spiller and Crookes, the pro- perty possessed by certain saline substances of remaining for a long time in a moist condition was turned to account. Such salts are termed " deliquescent," and many of them have so great an attraction for water that they absorb it eagerly from the air : the solution having been formed, the water cannot entirely be driven off except by the ap- plication of a considerable heat. More recently, Honey has been employed by Mr. Shad- bolt.* This substance can scarcely be termed deliques- cent, but it possesses, like other uncrystallizable sugars, the property of remaining moist and sticky for a long time. Honey is, according to the Author's views, supe- rior to inorganic deliquescent salts as a preservative agent, from its possessing an affinity for Oxides of Silver, and thus acting chemically in communicating organic intensity to the image. — Collodion plates when kept long in a moist and sensitive state often give a pale and blue image, even although the Nitrate of Silver be left upon the film ; and neither Nitrate of Magnesia nor Glycerine appears capa- ble of supplying the deficient element, both being nearly or quite indifferent to the Salts of Silver. THE HONEY AND OXYMEL KEEPING PROCESSES. When the weather is cool, Collodion plates may be pre- served with tolerable certainty for a few hours, by simply applying Honey to them in the state in which they are taken from the Nitrate of Silver Bath. The best pure Virgin Honey should be obtained by * A claim has lately been advanced by Mr. Maxwell Lyte to be considered as tlie discoverer of the Honey Process. This gentleman appears to have worked simultaneously with Mr. Shadbolt, and to have anticipated him in publishing ; but the object of Mr. Lyte's process was rather to increase the sensibility of the plates than to confer upon them keeping qualities. U 290 ON LANDSCAPE PHOTOGEAPHY AND dripping it immediately from tlie comb. This point is of importance, since if tlie sample of lioney be of inferior quality, or adulterated, the process may not succeed. The quantity of water to be added will vary with the consis- tence of the honey, from about an equal bulk to two parts : it should be sufficient to make the preservative solution pass slowly through filtering-paper. After the plate is removed from the JN'itrate Bath, it is to be drained and wiped on the back in the usual way. The Honey is then poured along the edge in such a manner as to form a broad wave which forces the Nitrate of Silver solution before it and covers the film. Next drain the plate into a measure and pour on a second portion of Ho- ney as before. This second dose may be used again for the first application to the succeeding plate. Lastly, stand the glass on blotting-paper in a dark place for about a quarter of an hour or twenty minutes, and wipe the lower edge before putting it into the plate box. The exposure required will probably be about four or five times as long as that for new and sensitive Collodion, or twice as long as the exposure required for old and brown Collodion. Before applying the developer, immerse the plate in a Bath of rain-water for five minutes, moving it about occasionally to soften the honey. This will probably be sufficient for plates which have not been kept longer than four hours, and beyond that time the process is not consi- dered certain, since the Honey exercises a slow reducing action upon the Nitrate of Silver. The solution of Pyrogallic Acid may be used of the ordi- nary strength, with a full dose of Acetic Acid. Only a faint image comes out at first, but on pouring over the plate a fresh portion of the developer with two or three drops of the Nitrate Bath added to each fluid drachm, it may be intensified to any extent. Pix with Hyposulphite of Soda, and wash in the usual way. THE PEESERVATION OF COLLODION PLATES. 291 When the process fails, from heat of the weather or other causes, the image will probably be feeble and red by transmitted light, and the shadows defective and misty. This is especially likely to happen when the Nitrate Bath is very old and contains much Acetate of Silver ; or when the same portion of Honey is used more than once, and has undergone partial decomposition by the action of the Nitrate of Silver. The use of j^ure Honey, free from moul- diness and fermentation, will, in cool weather, almost cer- tainly ensure success. A modification of the 'process when the plates are to he Tcept over four hours. — In this case the whole, or the greater part of the Nitrate of Silver must be removed before ap- plying the preservative agent. Wash the sensitive plate in water in the manner described for the Oxymel process in the next page. Then apply the syrup as before, using it as thick as possible. Honeyed plates, free from Nitrate of Silver, may commonly be kept for five or six days ; often much longer. Dr. Mansell, who has employed this process with great success, speaks of temperature as a point to be attended to. In hot weather the same length of keeping properties will not be attained. Use of Oxymel for preserving Collodion plates. — The principal difficulty in the employment of Honey in Photo- graphy, is its disposition to ferment, or to become mouldy. Fermentation occurs most readily in a dilute solution, and will be obviated by using the syrup as thick and free from water as possible. Mr. Llewellyn employs "Oxymel," which is a mixture of Honey and Vinegar, as a preservative agent. This substance will keep even in dilute solution for a long time without decomposition ; and, being very readily removed from the plates, does not interfere with the deve- lopment of the image. The preparation of Oxymel is de- scribed in the Vocabulary, Part III. ; it must be diluted with three or four parts of water, and filtered. Certain facts to which attention has been lately drawn by Dr. Norris and Mr. Barnes in working with dry CoUo- 292 ON LANDSCAPE PHOTOGEAPHT AND dion, may be advantageously borne in mind when using Oxymel ; the preservative solution of which is employed in so dilute a state that the process resembles to a great ex- tent a dry Collodion process. The observations above re- ferred to relate to the quality of the Collodion best adapted for the purpose, and will be found at page 298, to which the reader is referred. The manipulation of the Oxymel process is very simple. Two flat gutta-percha dishes are provided, the one contain- ing common water and the other diluted and filtered Oxy- mel. The Collodion plate, on its removal from the Bath, is placed in the first dish, which is gently tilted up and down, to wash away the free Nitrate of Silver. In a few seconds, when the liquid is rendered milky, it is poured away, and fresh water being introduced, the process is re- peated until the oily lines disappear, and the surface of the film becomes smooth and glassy. The plate is then, after a slight draining, removed to the second tray, and the Oxymel waved backwards and forwards for about half a minute, after which the glass is lifted out and placed ver- tically on blotting-paper, which must be renewed when it becomes wet and saturated. The plates may be used any time within a fortnight from the date of their preparation, and it is not necessary to develope immediately after the exposure. The sensitive- ness will be considerably less than that of fresh Collodion : from two to five minutes may be allowed with a Stereo- scopic view lens having a quarter-inch diaphragm. Before developing, the film should be gently washed for a few seconds with common water. Solution of Pyro- gallic Acid, of the ordinary strength, but previously mixed with a portion of the Nitrate Bath solution, one or two drops to each drachm, may then be poured on in the ordi- nary way. Use less Nitrate of Silver and more Acetic Acid in hot weather. When discoloration of the developer occurs, mix a fresh portion and proceed as before. THE PEESEEVATION OF COLLODION PLATES. 293 PRECAUTIONS TO BE OBSERVED IN KEEPING PROCESSES. The plates must be roughened at the edges, and also upon the surface, to make the film adhere. It is advisable to use a tolerably thick Collodion, giving a yellow film ; the pale opalescent films being more easily afiected by markings on the glass, and not retaining so much of syrup or Nitrate of Silver upon the surface. The room in which the plates are prepared must be carefully guarded from scattered pencils of white light ; the films are exposed to injury from this cause during the whole of the time occupied in applying the preservative syrup ; and hence anything short of absolute chemical darkness will be likely to cause fogging ; especially so when free Nitrate of Silver is left upon the film. The water used for washing away the free Nitrate of Silver before applying the preserving liquid, need not be distilled. Common hard water containing Carbonates and Chlorides, and producing milkiness with Nitrate of Silver, will often suffice. The water of the New Hiver and of the E-iver Thames, with which many parts of Lon- don are supplied, may certainly be used ; but in the case of a very hard water, containing much Sulphate of Lime, it might perhaps be advisable to substitute clean rain-water, free from brown organic discoloration. The preservative Oxymel must be carefully filtered, and kept covered, in order to protect it from dust. It will also be necessary occasionally, before using it, to run it through a piece of white cambric, to stop back suspended particles, which, if allowed to remain, would be a source of spots. If it becomes mouldy, or discoloured by Silver, or ferments and evolves gas, throw it away. After the syrup is applied and the plates are drained, stow them in a grooved box perfectly protected from light ; or place them in slides, which must be kept scrupulously clean, since any trace of impurity would be likely to pro- 294 OJS" LANDSCAPE PHOTOGEAPHY AND duce a stain wlien the plate was left a long time in the slide. If the preserved plates are kept in a cupboard or box, see that no volatile matter, siich as Ammonia, coal- gas, etc., can find entrance. In changing the plates after the exposure in the Camera, use a large bag made of several thicknesses of black calico, with a square of yellow calico let in at the top ; an elastic band securing it round the waist. This process, the theory of which has been briefly ex- plained at page 181, is more sensitive than the one last described, and has the additional advantage of giving dry plates, which do not attract dust, and are less liable to injury. The details of manipulation are complex, but this inconvenience is not so much felt when preparing a large number of plates. Cleaning the Glasses. — Success will greatly depend upon the mode in which this part of the process is performed. The layer of Albumen which is applied to the Collodion film tends to swell and to raise the latter in blisters ; the most efiectual mode of obviating which will be to clean the glass ^ so that the film adheres with unusual tenacity. The Liquor Potassse of the Druggists, diluted with three or four parts of water, and rubbed on the glass by a roll of flannel (page 214), is very effectual. A mixture of Tri- poli-water and Nitric Acid may however, if desired, be sub- stituted : — Lay the glass flat on a cloth, and rub the surface carefully with a tuft of cotton- wool dipped in the Tripoli ; then, be- fore the cream dries, wipe it off* with a second tuft, and polish with a third. Lastly, breathe upon the glass, and having ascertained that it is chemically clean, apply the Collodion. THE COLLODIO-ALBUMEN PROCESS. Tripoli . . Nitric Acid . Water . . 1 drachm. 30 minims. 1 ounce. THE PRESERVATION OF COLLODION PLATES. 295 Coatmg with Collodion. — Choose a rather thin Collodion which adheres tightly to the glass. A preparation which has been kept a long time after iodizing will usually answer the purpose very well, and, as a rule, a non-con- tractile, structureless Collodion is better than one which is glutinous and wavy. The degree of sensibility of the Collodion is not thought to have much influence upon the result. Coating the Plate. — Apply the Collodion in the usual manner, and allow it full time to set perfectly, before dip- ping in the Bath, in order to favour its adherence to the glass. With Cpllodion prepared from anhydrous spirits, about half a minute may be given in cool weather. TJie Nitrate Bath.— Take of Fused Mtrate of Silver ... 40 grains. Glacial Acetic Acid 30 minims. Alcohol 20 minims. Water 1 fluid ounce. Saturate with Iodide of Silver as described at page 204, and filter. An immersion of one minute will be sufficient; after which, give the plate an up-and-down movement, and wash it in plain water, in the manner advised for the Oxymel preservative process, at page 292. Then stand it on blot- ting-paper, to drain for a minute or two, wipe the back of the glass, and pour on the Albumen. This Bath may become discoloured after a time; conti- nue to use it until it is of a dark sherry-colour, and then treat it with "Kaolin," in the manner and with the pre- cautions advised at pages 91 and 245. The Iodized Alhumeyi. — Procure eggs, fresh laid, or not more than two or three days old. Separate the whites in the same way as for Albuminized paper (p. 241), and mix by the following Formula : — Albumen Water . 9 fluid ounces. 3 fluid ounces. 296 ON LANDSCAPE PHOTOGEAPHY AND Liquor Ammonise 2 fluid drachms. Iodide of Potassium .... 48 grains. Bromide of Potassium .... 12 grains. The Iodide and Bromide should be free from Carbonate of Potash, which is said to cause pin-holes in the Negatives. To ensure the absence of this salt, dissolve the total quan- tity of both Iodide and Bromide in the three ounces of water advised in the formula ; then, previously to adding the Ammonia and Albumen, introduce an excessively mi- nuie 'particle of Iodine, enough barely to colour the liquid. The Iodine decomposes the Carbonate of Potash, but it must not be used in excess, since free Iodine possesses the property of coagulating Albumen. Iodide of Cadmium also coagulates Albumen, so that the Iodides of Potassium and Ammonium are the best. Having mixed the ingredients in the order above given, introduce them into a bottle, and shake it violently until they have thoroughly amalgamated. Then transfer to a tall narrow jar ; allow to settle for twenty-four hours, and draw off the upper clear portion for use. Particulars of this part of the process have already been given under the head of Albuminized Paper, to which the reader is re- ferred (p. 241). The ammoniacal solution of Albumen may be kept for some time in a stoppered bottle without much decompo- sition. If mucous threads form in it, filter through fine linen. Mode of applying the Albumen. — Cover the moist film with the Albumen in the same way as advised for Collodion (p. 216), pouring on at once a sufficient quantity to cause it to spread in an even and undivided sheet ; otherwise a veined appearance may be produced, which will show in the development. Beturn the excess of Albumen into the bottle, and pour it once again upon the plate : the film will remain clear and transparent, if the whole of the Ni- trate of Silver has been properly washed away from the THE PRESERVATION OF COLLODION PLATES. 297 Collodion. Lastly, stand the plate nearly vertically on blotting-paper to dry. This will occupy five or six hours ; but the process may be hastened by artificial heat. After the Albumen solution has been used to coat a num- ber of plates successively, it becomes diluted with water ; the result of which is, that unequal intensity of image is produced at the upper and lower edge of the film. The iodized Albumen plates are at this stage of the pro- cess nearly or quite insensitive to light, and may be pre- served unchanged for many weeks. Sensitizing the Albumen film. — When the plate has be- come thoroughly dry, it is again introduced into the Bath of Aceto-Nitrate of Silver, and allowed to remain for one minute: then washed with water in the same manner as before, but with even greater care, in order to obviate cloud- ing in the development. If blisters should form on drying, it will be found useful to hasten the process by holding the plates to the fire — or a hot iron may be placed in the centre of a covered box and the glasses reared up round the sides. They will thus dry quickly, and there will not be time for the Albumen to swell much by imbibition. Exposure in the Camera. — This may be performed at any period within a few weeks from the date of preparation of the plates. For a landscape view with a small Stereo- scopic single lens, allow about three minutes in the winter, or one minute and a half in the summer. Develojpment of the image. — This can be deferred as long as fourteen days after the exposure, with successful results. Pour water over the plate until the film is thoroughly wetted ; then cover it with a solution of Pyrogallic Acid containing one grain of the acid to the ounce of water, and twenty minims of Glacial Acetic Acid. Two drops of a neutral solution of I^itrate of Silver made with forty grains of Nitrate to the ounce of water must be pre- viously added to each fluid drachm of the Pyrogallic. The development, in the case of a landscape view taken with sun-hght, commences almost immediately, and may be com- 298 ON LANDSCAPE PHOTOGEAPHY AND pleted in abont ten minutes, but the time occupied in de- veloping will vary greatly with the length of exposure, the quantity of Nitrate of Silver, and the nature of the subject copied — a badly lighted interior, for instance, often taking an hour or longer to appear in all its details. If the de- veloper should discolour before the proper intensity has been obtained, pour it off and mix a fresh quantity. JFixinq the image. — Hyposulphite of Soda (one ounce to four of water) will be found preferable to the Cyanide of Potassium, as the latter has a solvent effect upon the Albumen. An unusually long time will be required, as the ffxing agent must penetrate the Albumen, to reach the Collodion beneath. Careful washing in water for five or ten minutes re- moves the excess of Hyposulphite, and the plate may then be varnished in the usual way. THE DKY COLLODION PEOCESS. The earlier attempts to employ sensitive Collodion plates in a desiccated condition were unsuccessful. The film of Pyroxyline shrinks on drying, and becomes almost imper- vious to moisture : hence, the developing solution not pene- trating properly, density cannot easily be obtained. We are indebted to Dr. Hill N orris, of Birmingham, for esta- blishing the theory of the subject upon a more correct basis. He has pointed out the importance of distinguishing two different conditions of the Collodion surface,^ viz. the contractile, common in newly -mixed Collodion, — and the short or powdery, in Collodion which has been iodized with the alkaline Iodides, and kept until much Iodine has been set free. The latter is the most suitable condition for the dry process ; and the practical mode of distinguishing be- tween them is by sensitizing a plate and passing the finger across it ; if it can be easily pushed away in a firm and connected skin, it will be unfit for the purpose required. In order still further to preserve the film in a condition * See these states of the film more fully described at page 83. THE PEESEEVATION OF COLLODION PLATES. 299 permeable by tbe developer, it is recommended to coat it whilst moist with a solution of Gelatine. The dry Collodion process, although less sensitive, is more simple than that on Collodio-Albumen, and possesses many of its advantages ; but it is less universally applica- ble, since it depends entirely for success upon the peculiar state of the Collodion, resembling in this respect the Oxy- mel process already described. Mode of jpreparing the plates. — The glasses are coated with the Collodion in the usual way. Blistering during development being liable to happen in this process as in the last, every care must be taken to make the films adhere with the greatest possible tenacity, both by cleaning the glasses with extra care (see p. 294), and also by allowing the Collodion to set firmly before dipping in the Bath. The plate may be held from twenty to thirty seconds previous to immersion, or even longer, provided the film, when lifted out of the Bath, appears of uniform thickness throughout (see page 218). The sensitizing having been completed, wash the plates with plain water, exactly in the same way as for Oxymel (p. 292). If Nitrate of Silver be left, clouding will take place in the process of development. After washing, drain for a few seconds, and immerse in the solution of Gelatine. To prepare this Bath, take of Nelson's patent Gelatine . . . 128 grains. Distilled water ...... 14 ounces. Alcohol 2 J ounces. Put the Gelatine in the cold water, and allow it a quarter of an hour to soften and swell ; it will then readily dissolve on applying a gentle heat. This may be done in a glazed saucepan or a pipkin of earthenware, taking care not to scorch the bottom part by too strong a heat. Next clarify the solution by adding to it, whilst barely warm, a tea- spoonful of white of egg (previously beaten up with a silver fork), and afterwards heating nearly to the boiling 300 ON LANDSCAPE PHOTOGEAPHY AND point. Tlie Alcohol must now be added, to facilitate tlie coagulation of the Albumen. When this takes place and the liquid becomes clear, filter through a clean piece of cambric folded three or four times. If a hot water filtering apparatus can be obtained, the solution may be made to pass through J) aper ; but as it tends to gelatinize on cooling, the ordinary mode of filtration commonly fails. The quan- tity of Alcohol in the above formula is greater than is usually recommended, allowance having been made for a partial evaporation of the spirit. The filtered liquid may be poured into a flat porcelain dish, or a vertical trough, but in either case it will be ne- cessary to stand the vessel in warm water, in order to pre- vent gelatinization. The Collodion plate, thoroughly washed, is to be im- mersed in this solution and moved up and down for two or three minutes. It is then removed, drained on blotting- paper, and dried. The use of artificial heat in drying will be found a great advantage ; it prevents the gelatine from set- thng unequally upon the plate. Those who possess an ap- paratus made purposely for drying plates by hot air, will experience no difficulty, but an ordinary deal trunk may be made to answer, with a little management. Cover the bot- tom of the box with blotting-paper, and having heated one or two " flat irons," place them in the centre : then range the glasses side by side, with the coated surface looking in- wards ; in a quarter of an hour, or from that to twenty mi- nutes, the desiccation will be complete. If the Collodion plates are prepared in a room containing a fire, they may be reared up side by side at a distance of two or three feet, and in that way may be safely dried without fear of injury, provided white light be excluded. When dry they can be stowed away in a box ; all the precautions given at page 293 being observed. The sen- sibility remains good for many days, possibly for weeks or months in cold weather. JExjposure in the Camera. — Allow from four to eight times THE PRESERVATION OF COLLODION PLATES. 301 the exposure of the most sensitive moist Collodion. On a clear summer's day, a sun-lit view may require one minute or a minute and a half, with a short focus Stereoscopic lens, having a diaphragm of a quarter of an inch diame- ter. The average time however with the same lens would be about twice as much, viz. three minutes. Development of the Image. — Make a saturated solution of Gallic Acid in water by the directions given at page 261. Then dissolve forty grains of pure Nitrate of Silver in one ounce of distilled water. Pour into a fiat porcelain dish a sufficient quantity of the Gallic Acid solution to flood the plate readily . Then measure it, and to each fluid ounce add ten minims of the solution of Silver, or five mi- nims in hot weather. It is important that no discolora- tion should occur on mixing these liquids together, to ob- viate which, observe the following precautions : — Clean the porcelain vessel very carefully with Nitric Acid or Cyanide before use. Employ a pure solution of Nitrate of Silver ; and mix it with the Gallic Acid, in preference to adding the Gallic Acid to the Silver solution (read the remarks at p. 179). The picture may be expected to appear in five or ten minutes, and in one hour, or from that to four hours (p. 298), the development will be complete. It wiU not be necessary to keep the plates in motion, but simply to lay them side by side in the solution of the Gallic Acid. If in spite of all precautions the developer begins to blacken be- fore the intensity has reached the proper point, it must be poured off* and a fresh mixture prepared. This however will not often happen. Lastly, when a full amount of opacity has been obtained, wash the plate with water, and fix it in a solution of Hypo- sulphite of Soda, or dilute solution of Cyanide of Potas- sium. Failures in the process. — Stains in the development may arise from using dirty dishes, or glasses which have been left in Gallo-Nitrate of Silver and improperly cleaned. It 302 PEESERVATION OF COLLODION PLATES. must be borne in mind that these impurities are not visible to the eye, although they produce the effect of discolouring the developer. A thorough cleansing with strong Nitric Acid or Potash will prove a remedy. BHsters, unless of large size, may often be disregarded, as they disappear on drying. General cloudiness may de- pend upon the film having been imperfectly washed. Irre- gular reduction at certain parts may be due to the Gela- tine setting before the plate has become dry, or to stains produced by the finger applied to the upper edge of the plate.'* * Since the above was written, Mr. Maxwell Lyte has communicated to the * Photographic Journal ' (vol. iii.) a dry process in which a modified Gela- tine is used. The change is produced by boiling a solution of gelatine with dilute Sulphuric Acid, which is afterwards neutralized and removed by means of chalk. The result is to destroy the gelatinizing property of the animal substance ; the solution retains its fluidity on cooling, and the ne- cessity of employing artificial heat in drying the plates is avoided. PART III. OUTLINES or GENERAL CHEMISTRY. 305 OUTLINES OF GENERAL CHEMISTRY. CHAPTER L THE CHEMICAL ELEMENTS AND THEIR COMBINATIONS. The limits of the present Work allow only of a simple sketch of the subjects which it is proposed to treat in this Chapter. Our attention therefore must be confined to an explanation of certain points which are alluded to in the First Part of the Work, and without a proper under- standing of which it will be impossible for the reader to make progress. The following division may be adopted: — The more important Elementary Bodies, with their symbols and atomic weights ; the Compounds formed by their union ; the class of Salts ; illustrations of the nature of Chemical Affinity ; Chemical JSTomenclature ; Symbolic Notation ; the laws of Combination ; the Atomic Theory ; the Chemistry of Organic Bodies. THE CHEMICAL ELEMENTS, WITH THEIR SYMBOLS AND ATOMIC WEIGHTS. The class of elementary bodies embraces all those sub- stances which cannot, in the present state of our know- ledge, be resolved into simpler forms of matter. The chemical elements are divided into "metallic" and X 306 THE CHEMICAL ELEMENTS ** non-metallic," according to the possession of certain ge- neral characters. The following are some of the principal non-metallic elements, with the symbols employed to designate them, and their atomic weights — Gases. Solids. < Symbol. Atomic Wt. 0 8 H 1 . IN" 14 . CI 36 , I 126 . c 6 . s 16 p 32 , Br 78 F 19 Liquid. Bromine .... Unknown. Fluorine .... The metallic elements are more numerous. The follow- ing list includes only those which are commonly known: — Symbol. Atomic Wt. Metals of the / Potassium K 40 AlkaHes. \ Sodium JSTa 24 Metals of the f Barium Ba 69 ) Calcium Ca 20 [ Magnesium Mg 12 'Iron Pe 28 Zinc Zn 32 Cadmium Cd 56 Copper Cu 32 Lead Pb 104 Tin Sn 59 Arsenic As '75 [Antimony Sb 129 r Mercury Hg 202 J SHver Ag 108 \ Gold Au 197 [Platinum Pt 99 * The atomic weights, with the exception of that of Gold, are taken from the last edition of Brande's ' Manual of Chemistry.' Alkaline Earths. Metals Proper. Noble Metals. AND THEIE COMBINATIONS. 307 ON THE BINARY COMPOUNDS OF THE ELEMENTS. Many of the elementary bodies exhibit a strong ten- dency to combine with each other, and to form compounds, which differ in properties from either of their constituent elements. This attraction, which is termed " Chemical Affinity," is exerted principally between bodies which are opposed to each other in their general characters. Thus, taking for example the elements Chlorine and Iodine — they are analogous in their reactions, and therefore there is but little attraction between them, whereas either of the two combines eagerly with Silver, which is an element of a different class. So, again. Sulphur unites with the metals, but two metallic elements are comparatively indif- ferent to each other. Oxygen is by far the most important in the list of che- mical elements. It combines with all the others, with the single exception, perhaps, of Fluorine. The attraction, or chemical affinity, however, which is exerted, varies much, in different cases. The metals, as a class, are easily oxi- dized ; whilst many of the non-metallic elements, such as Chlorine, Iodine, Bromine, etc., exhibit but little affinity for Oxygen. Nitrogen is also a peculiarly negative ele- ment, showing little or no tendency to unite with the others. Classification of binary compounds containing Oxygen. — When one simple element unites with another, the product is termed a "binary" compound. There are three distinct classes of binary compounds of Oxygen : — Neutral Oxides, basic Oxides, and acid Oxides. Neutral and basic Oxides. — Take as examples — the Oxide of Hydrogen, or Water, a neutral Oxide; the Oxide of Potassium, or Potash, a basic Oxide. Water is termed a neutral oxide, because its affinities are low, and it is comparatively indifferent to other bodies. Potash and Oxide of Silver are examples of basic oxides ; but there is a great difference between the two in chemi- 308 THE CHEMICAL ELEMENTS cal energy,tlie former belonging to a superior class of bases, viz. the alkaline. By studying the properties of an alkali (such as Pot- ash or Soda) which are familiar to all, we gain a correct notion of the whole class of basic oxides. An alkali is a substance readily soluble in water, and yielding a solu- tion which has a slimy feel from its solvent action upon the skin. It immediately restores the blue colour of reddened litmus, and changes the blue infusion of cab- bage to green. Lastly, it is neutralized and loses all its characteristic properties upon the addition of an acid. The weaker bases are, as a rule, sparingly or not at all soluble in water, neither have they the same caustic and solvent action upon the skin ; but they restore the colour of reddened litmus, and neutralize acids in the same man- ner as the more powerful bases, or alkalies. The Acid Oxides. — This class, taking the stronger acids as the type, may be described as follows : — very soluble in water, the solution possessing an intensely sour taste, and a corroding rather than a solvent action upon the skin ; changes the blue colour of litmus and other vegetable substances to red, and neutralizes the alkalies and basic oxides generally. Observe however that these properties are possessed in very various degrees by dijQTerent acids. Prussic Acid and Carbonic Acid, for instance, are not sour to the taste, and being feeble in their reactions, redden litmus scarcely or not at all. All acids however, without any exception, tend to combine with bases and to neutralize themselves ; so that this may be said to be the most characteristic pro- perty of the class. Chemical composition of Acid and Basic Oxides con- trasted. — It is a law commonly observed, although with many exceptions, that bases are formed by the union of Oxygen with metals ; and acids, by Oxygen uniting with non-metallic elements. Thus, Sulphuric Acid is a com- pound of Sulphur and Oxygen ; Nitric Acid, of Nitrogen AND THEIR COMBINATIONS. 309 and Oxygen. But the alkali, Potasli, is an oxide of the metal Potassium ; and the oxides of Iron, Silver, Zinc, etc. are bases, and not acids. Again, the composition of acids and bases is different in another respect ; the former invariably contain more Oxygen in proportion to the other element than the latter. Taking the same examples as before, the two classes may be represented thus : — Acids f^^^ of Vitriol, Sulphur 1 atom, Oxygen 3 atoms. lAqua-fortis, Nitrogen 1 ,, Oxygen 5 ,, -g^^^^r Oxide of Silver, Silver 1 atom. Oxygen 1 atom. lOxide of Iron, Iron 1 „ Oxygen I „ The class of Hydrogen Acids. — Oxygen is so essen- tially the element v^^hich forms the acidifying principle of acids, that its very name is derived from that fact {o^v9y acid, and yez/mo), to generate). Still there are exceptions to this rule, and in some acids Hydrogen appears to play the same part ; the Hydracids, as they are termed, are formed principally b}^ Hydrogen uniting with elements like Chlorine, Bromine, Iodine, Fluorine, etc. Thus, Mu- riatic or Hydrochloric Acid contains Chlorine and Hydro- gen ; Hydriodic Acid contains Iodine and Hydrogen. Observe, however, that the position held by the Hydro- gen in these compounds, is different from that of the Oxy- gen in the " Oxyacids, " as regards the number of atoms usually present ; thus — Aqua-fortis = Nitrogen 1 atom. Oxygen 5 atoms, Muriatic Acid = Chlorine 1 „ Hydrogen 1 atom ; so that the composition of the Hydracids is analogous to the basic oxides, in containing a single atom of each con- stituent. THE TERNARY COMPOUNDS OF THE ELEMENTS. As the various elementary substances unite with each other to form Binary Compounds, so these binary com- pounds again unite and form Ternary Compounds. 310 THE CHEMICAL ELEMENTS Compound bodies lioweyer do not, as a rule, unite with simple elements. In illustration, take the action of Nitric Acid upon Silver, described at page 12. IN'o effect is pro- duced upon the metal until Oxygen is imparted ; then the Oxide of Silver so formed dissolves in the JN^itric Acid. In other words, it is necessary that a binary compound should be first formed, before the solution can take place. The mutual attraction or chemical affinity exhibited by compoimd bodies is, as in the case of elements, most strongly marked when the two substances are opposed to each other in their general properties. Thus, acids do not unite with other acids, but they combine instantly with alkalies; the two mutually neu- tralizing each other and forming " a salt." Salts therefore are ternary compounds produced by the union of acids and bases ; common Salt, formed by neu- tralizing Muriatic Acid with Soda, being taken as the type of the whole class. General characters of the Salts. — An aqueous solution of Chloride of Sodium, or common Salt, possesses those characters which are usually termed saline ; it is neither sour nor corrosive, but, on the other hand, has a cooling agreeable taste. It produces no effect upon litmus and other vegetable colours, and is wanting in those ener- getic reactions which are characteristic of both acids and alkalies ; hence, although formed by the union of two binary compounds, it differs essentially in properties from both. All salts however do not correspond to this description of the properties of Chloride of Sodium. The Carbonate of Potash, for instance, is an acrid and alkaline salt, and the Nitrate of Iron reddens litmus -paper. A perfectly neu- tral salt is formed when a strong acid unites with an ener- getic base ; but if, of the two constituents, one is more powerful than the other, the properties of that one are often seen in the resulting salt. Thus the Carbonate of Potash is alkaline to test-paper, because the Carbonic AND THEIR COMBINATIONS. 311 Acid is feeble in its reactions ; but if Nitric Acid and Potash are brought together, then a Nitrate of Potash is produced, which is neutral in every sense of the term. The Chloride of Sodium and salts of a similar kind are freely soluble in water, but all salts are not so. Some dis- solve only sparingly, and others not at all. The Chloride and Iodide of Silver are examples of the latter class ; they are not bitter and caustic like the Nitrate of Silver, but are perfectly tasteless from being insoluble in the fluids of the mouth. It is seen therefore from these examples, and many others which might be adduced, that the popular notion of a saline body is far from beiug correct, and that, in the lan- guage of strict definition, any substance is a salt which is produced by the union of an acid with an alkali, indepen- dent of the properties it may possess. Thus, Cyanide of Potassium is a true salt, although highly poisonous ; Nitrate of Silver is a salt ; the green Sulphate of Iron is a salt ; so also is Chalk or Carbonate of Lime, which has neither taste, colour, nor smell. On the Hydracid " class of Salts. — The distinction be- tween Oxyacids and Hydracids has already been pointed out (p. 309), the latter having been shown to consist of Hy- drogen united with elements analogous in their reactions to Chlorine, Iodine, Bromine, etc. In a salt formed by an Oxygen Acid, both the basic and acid elements appear. Thus the common Nitre, which is a Nitrate of Potash, is found by analysis to contain Oxide of Potassium as a base, in a state of combination with Nitric Acid. But if a salt be formed by neutralizing an alkali with a Hydrogen Acid, the product in that case does not contain all the elements. This is seen from the fol- lowing example : — Hydrochloric Acid + Soda = Chloride of Sodium -J- Water ; or, stated more at length, — 312 THE CHEMICAL ELEMENTS (Chlorine Hydrogen) + (Oxygen Sodium) = (Chlorine Sodium) + (Oxygen Hydrogen). Observe that the Hydrogen and Oxygen, being present in the correct proportions, unite to form Water, which is an Oxide of Hydrogen. This water passes off when the solution is evaporated, and leaves the dry crystals of salt. On the other hand, with the Oxyacid Salts, the elemen- tary Hydrogen being absent, no water is formed, and the Oxygen remains. It must therefore be borne in mind that salts like the Chlorides, Bromides, Iodides, etc. contain only tico ele- ments ; but that in the Oxyacid Salts, such as Sulphates, JN'itrates, Acetates, three are present. Thus, Nitrate of Silver consists of Nitrogen, Oxygen, and Silver, but Chlo- ride of Silver contains simply Chlorine and metallic Silver united, without Oxygen. The Hydracid salts however, when decomposed, yield products similar to the Oxyacid salts. For instance, if Iodide of Potassium be dissolved in water, and dilute Sul- phuric Acid added, this acid, being powerful in its chemical affinities, tends to appropriate to itself the alkali ; but it does not remove JPotassium and liberate Iodine, but takes the Oxide of Potassium and sets free Hydriodic Acid, In other words, as an atom of water is produced during the formation of a Hydracid Salt, so is an atom destroyed and made to yield up its elements in the decomposition of a Hydracid Salt. The reaction of dilute Sulphuric Acid upon Iodide of Potassium may be stated thus : — Sulphuric Acid plus (Iodine Potassium) jplus (Hydrogen Oxygen) equals (Sulphuric Acid, Oxygen Potassium) or Sulphate of Potash, and (Hydrogen Iodine) or Hydriodic Acid. THE NATURE OF CHEMICAL AFFINITY FURTHER ILLUSTRATED. Illustration from the Non-metallic Elements, — If a stream AND THEIE COMBINATIONS. 313 of Chlorine gas be passed into a solution containing tlie same salt as before mentioned, viz. the Iodide of Potas- sium, the result is to liberate a certain portion of Iodine, which dissolves in the liquid, and tinges it of a brown colour. The element Chlorine, possessing a degree of che- mical energy superior to that of Iodine, prevails over it, and removes the Potassium with which the Iodine was previously combined. Chlorine + Iodide of Potassium = Iodine + Chloride of Potassium. The same Law illustrated hy the Metals. — A strip of Iron dipped in solution of Nitrate of Silver becomes imme- diately coated with metallic Silver ; but a piece of Silver- foil may be left for any length of time in Sulphate of Iron without undergoing change : the difference depends upon the fact, that metallic Iron has a greater attraction for Oxygen than Silver, and hence it displaces it from its so- lution. Iron -f- Nitrate of Silver = Silver -f Nitrate of Iron. Illustrations amongst Binary Comjpounds. — If a few drops of solution of Potash be added to solution of Nitrate of Silver, a brown deposit is formed, which is the Oxide of Silver, sparingly soluble in water. That is to say, as a stronger metal displaces metallic Silver, so does an oxide of the same metal displace Oxide of Silver. Therefore bases like the alkalies, alkaline earths, etc. cannot exist in a free state in solutions of the salts of weaker bases, — a liquid containing Nitrate of Silver could not also contain free Potash or Ammonia. In the list given at page 306, the metallic elements are arranged principally in the order of their chemical affini- ties ; those of Potassium, Sodium, Barium, etc. being the most marked. As the alkalies displace the weaker bases from their 314 THE CHEMICAL ELEMENTS combination with acids, so tlie strong acids displace weak acids from their combination with bases. Thus, as Oxide of Potassium + Acetate of Silver = Oxide of Silver + Acetate of Potash ; So Nitric Acid + Acetate of Silver = Acetic Acid + Nitrate of Silver. In the list of acids. Sulphuric Acid is usually placed first as being the strongest, and Carbonic Acid, which is a gaseous substance, last. The vegetable acids, such as Acetic, Tartaric, etc., are intermediate, being weaker than the mineral acids, but stronger than Carbonic, or Hydro- cyanic Acid. The order of decompositions affected hy the insolubility or the volatility of the ^products which may he formed. — It might be inferred from remarks already made, that on mixing saline solutions, a gradual interchange of elements would take place, until the strongest acids were associated with the strongest bases, and vice versa. There are many causes however which interfere to prevent this ; one of which is volatility.- — The violent efiervescence which takes place on treating a Carbonate of any kind with an acid is due to the gaseous nature of Carbonic Acid and its escape in that form, which greatly facilitates the decomposition. Insolubility is also a cause which exercises a great in- fluence on the result which will follow in mixing solutions. If the formation of an insoluble substance is possible by any interchange of elements, it will take place. A solu- tion of Chloride of Sodium added to Nitrate of Silver invariably produces Chloride of Silver ; the insolubility of Chloride of Silver being the cause which determines its formation. So again, Sulphate of Lead and Protonitrate of Iron are produced by mixing Nitrate of Lead with Sulphate of Iron ; but if Nitrate of Potash be substituted for Nitrate AND THEIR COMBINATIONS. 315 of Lead, tlie result is iincertam, because there are no ele- ments present wbieli can, by interchanging, form an inso- luble salt ; Sulphate of Potash, although sparingly soluble in water, not being insoluble, like the Sulphate of Lead or the Sulphate of Baryta. ON CHEMICAL NOMENCLATURE. The nomenclature of the chemical elements is mostly independent of any rule ; but an attempt has been made to obviate this in the case of those of later discovery. Thus the names of the newly-found metals usually end in um, as Potassium, Sodium, Barium, Calcium, etc. ; and those elements which possess analogous characters have corresponding terminations assigned to them, as Chlorine, Bromine, Iodine, Fluorine, etc. Nomenclature of Binary Compounds. — These are often named by attaching the termination ide to the more im- portant element of the two ; as, the Oxide of Hydrogen, or Water ; the Chloride of Silver ; the Sulphic?^ of Silver. Binary compounds of Sulphur however are sometimes termed Sulphurets, as the Sulphuret or the Sulphide of Silver indifferently. When the same body combines with Oxygen, or the corresponding element, in more than one proportion, the prefix pj^oto is applied to that containing the least Oxy- gen ; sesqui to that with once and a half as much as the proto ; hi or bin to that with twice as much ; and 'per to the one containing the most Oxygen of all. As examples, take the following : — The Protoxide of Iron ; the Sesqui- oxide of Iron : the Protochloride of Mercury ; the Bi- chloride of Mercury. In these examples the Sesquioxide of Iron is also a Peroxide, because no higher simple oxide is known, and the Bichloride of Mercury is a Perchloride for a similar reason. When an inferior compound is discovered, it is often termed sub ; as the Suboxide of Silver, the Subchloride of Silver. These bodies contain the least known quantity 316 THE CHEMICAL ELEMENTS of Oxygen and Chlorine respectively, and are hence enti- tled to the prefix j)roto ; but being of minor importance, they are excepted from the general rule. The combinations of metallic elements Avith each other are termed " alloys or if containing Mercury, " amal- gams." Nomenclature of binary Compounds possessing acid properties, — These are named on a different principle. The termination ic is applied to one element. Thus, taking as an illustration the liquid known as " Oil of Vitriol," it is truly an Oxide of Sulphur, but as it possesses strong acid properties it is termed Sulphur^c Acid. So JN^itric Acid is an Oxide of JSTitrogen ; Carbonic Acid is an Oxide of Carbon, etc. When there are two oxides of the same element, both possessing acid properties, the most impor- tant has the termination ic, and the other ous ; as Sul- phuric Acid, Sulphuroz^5 Acid ; Nitric Acid, Wxi^ous Acid. Nomenclature of the Hydracids. — The Hydrogen Acids are distinguished from Oxyacids by retaining the names of both constituents, the termination ic being annexed as usual. Thus, HydrochioYiG Acid, or the Chloride of Hy- drogen ; HydriodiG Acid, or the Iodide of Hydrogen. Further illustrations of the nomenclature of Binary Compounds. — The Oxides of Nitrogen, and also of Sul- phur, afford an interesting illustration of the principles of nomenclature. The former are as follows : — Nitrogen. Oxygen. Protoxide of Nitrogen ... 1 atom. 1 atom. Binoxide of Nitrogen . . . 1 „ 2 „ Nitrous Acid 1 „ 3 „ Peroxide of Nitrogen . . . 1 „ 4 ,, Nitric Acid 1 „ 5 „ Observe, that two only out of the five possess acid pro- perties, the others being simple oxides. Nitric Acid is, strictly speaking, the " Peroxide," but as it belongs to the class of acids, that term naturally falls to the compound below. AND THEIR COMBINATIONS. 317 The binary compounds of Sulphur with Oxygen all pos- sess acid properties ; they may be represented (in part) as follows : — Sulphur. Oxygen. Hyposulphurous Acid ... 2 atoms. 2 atoms. Sulphurous Acid .... 1 „ 2 „ Hyposulphuric Acid . . . 2 5 „ Sulphuric Acid 1 3 „ In this case the Sulphuric 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 i?j/^osulphuric and Hypo- sulphurous (from vtto, 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 ^vi\^\iates ; Nitric Acid, Wiivates ; but Sulphuroz^^ Acid forms Sulph^7e5, and Nitrous Acid, Witvites. 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 Proz^osulphate of Iron, or Sulphate of the Protoxide ; the Persulphate 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 P^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 in excess with regard to the acid, and which are usually known as "basic salts thus, the red powder which de- 318 THE CHEMICAL ELEMENTS posits from solution of Sulphate of Iron, is a basic Per- sulphate of Iron, or a Sulphate of the Peroxide of Iron with more than the normal proportion of oxide. Nomenclature of the Hydracid Salts. — The composition of these salts being different from those formed by Oxygen Acids, the nomenclature varies also. Thus, in neutrahzing 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 306. — 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 Chlorine, Cd for Cadmium, and Cu for Copper. The chemical symbol however 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 succeeding pages, when speak- ing of the Laws of Combination. FormulcB 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 NO^, not NO^AgO. The juxtaposition of symbols expresses combination; thus, PeO is a compound of one proportion of Iron with one of Oxygen, or the Protoxide of Iron," If more than AND THEIR COMBINATIONS. 319 one equivalent be present, small figures are placed below the symbols : thus, FejOg represents two equivalents of Iron united with three of Oxygen, or the "Peroxide of Iron SO,, one equivalent of Sulphur with three of Oxy- gen, or Sulphuric Acid. Larger figures placed before and in the same line with the symbols, affect the whole compound which the symbols express : thus, 2 SO3 means two equivalents of Sulphuric Acid; 3 JNOj, 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 two 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 follow, 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 addUion, no combination of any kind being in- tended. Thus the decomposition which follows on mixing 320 THE CHEMICAL ELEMENTS Chloride of Sodium with Nitrate of Silver may be written as follows : — NaCl+AgO NO5 r= AgCl+jN"aO NO5 ; that is, — Chloride of Sodium added to Nitrate of Silver. = Chloride of Silver and Nitrate of Soda. ON EQUIVALENT PEOPOBTIONS. 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 the Hydriodic 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 every one of them a figure can be assigned which repre- AND THEIR COMBINATIONS. 321 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 " O " indicates 8 parts by weight of Oxygen ; "CI" one equi- valent, 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 to 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 NO^, is 54. The same rule applies with regard to salts. Take for instance the Nitrate of Silver : it contains Equivalent. Nitrogen 14 6 Oxygen 48 Silver 108 Total of equivalents, or equiva- ") 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 bodies both unite with and replace each other in equiva- Y 322 THE CHEMICAL ELEMENTS 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 Chloride of Silver, the weight of Chlo- ride of Sodium which will be necessary is deduced thus : — one equivalent, or 170 parts, of Nitrate of Silver, is de- composed by an equivalent, or 60 parts, of Chloride of Sodium. Therefore as 170 : 60 : : 100 : 35 2; that is, 35'2 grains of Salt will precipitate, in the state of Chloride, the whole of the Silver contained in 100 grains of Nitrate. So again, in order to form the Iodide of Silver, the pro- portions in which the two salts should be mixed is thus shown. The equivalent of Iodide of Potassium is 166, and that of Nitrate of Silver is 170. These numbers so nearly correspond, that it is common to direct that equal weights of the two salts should be taken. One more illustration will suffice. Supposing it be re- quired to form 20 grains of Iodide of Silver — how much 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 yellow preci- pitate, when washed and dried, will weigh precisely 20 grains. ON THE ATOMIC THEORY. The atomic theory, originally proposed by Dalton, so much facilitates the comprehension of chemical reactions generally, that it may be useful to give a short sketch of it. AND THEIE COMBINATIONS. 323 It is supposed that all matter is made up of an infinite number of minute atoms, wliicli 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 uni- 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. Fig. 2. Fig. 3. 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 NO5. TJie term '^atomic weight'' substituted for equivalent proportion. — If we suppose that the simple atoms of dif- ferent kinds of matter differ in weight, and that this difierence 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, 324 THE CHEMICAL ELEMENTS in the reaction between Chloride 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 OEGANIC SUBSTANCES. By "organic" substances are meant those which have possessed life, with 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, ma}^ 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. AND THEIE COMBINATIONS. 325 Composition of organic and inorganic bodies contrasted. — There are more than fifty elementary substances found in the inorganic kingdom, but on\j four, 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 : — Starch C24H20O20 Lignine C24H20O20 Cane Sugar .... C24H22O22 Grape Sugar . . . C24H2SO28 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 different, — 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 w^hich contains the three elements united as C24H20O20. Mode of distinguishing between Organic and Inorganic matter. — A simple means of doing this is as follows: — 326 THE CHEMICAL ELEMENTS. place the suspected substance upon a piece of Platinum- foil, and lieat it to redness with a spirit-lamp : if it first hlacl<:ens, 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. 327 CHAPTER 11. YOCABTJLABT OP PHOTOGRAPHIC 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 contliminated 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 imder the name- 328 YOCABITLAEY 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 follows : — dissolve a small crystal of JN"itrate 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 light. 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 first, becomes discoloured by the action of light. Glacial Acetic Acid sometimes has a smell of garlic. In this state it probably contains an organic Sulphur Acid, and is unfit for use. Many 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 Phar- macopoeia, containing 30 per cent, real acid. It will be advisable to test it for Sulphuric Acid (see Sulphuric Acid), and other impurities, before use. ACETATE OF SILYEU. See Silvee, Acetate of. ALBUMEN. Albumen is an organic principle found both in the animal and vegetable kingdom. Its properties are best PHOTOGEAPHIC CHEMICALS. 329 studied in the wJiite of egg, which is a very pure form of Albumen. Albumen is capable of existing in two states ; in one of which 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. — A moderately strong solution of Albumen becomes opalescent and coagulates on being heated to about 150° Fahrenheit, but a tempera- ture of 212° is required if the liquid is very dilute. A layer of dried Albumen cannot easily be coagulated by the mere application of heat. 2. By addition of strong acids. — ISTitric Acid coagulates Albumen perfectly without the aid of heat. Acetic Acid however acts difierently, 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 completely. Nitrate of Silver does so ; also the Bichloride of Mercury. Ammo- niacal Oxide of Silver however does not coagulate Albu- men. The white precipitate formed on mixing Albumen with Nitrate of Silver is a chemical 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 an organic compound of a Suboxide of Silver. It is then almost insoluble in Ammonia, but enough dissolves to tinge the liquid wine-red. The red coloration of so- lution of Nitrate of Silver employed in sensitizing the 330 VOCABULARY OF Albuminized pliotograpliic paper is probably produced by the same compound, although, often referred to the pre- sence of Sulphuret of Silver. Albumen also combines with Lime and Baryta. "When Chloride of Barium is used with Albumen, a white precipi- tate of this kind usually forms. Chemical composition of Albumen. — Albumen belongs to the Nitrogenous class of organic substances (see page 325). It also contains small quantities of Sulphur and Phos- phorus. ALCOHOL. Symbol, C4Hg02. 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 a 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 196 of this work ; but in order to render the Alcohol thoroughly anhydrous, it is necessary to employ quick- 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. — Pure anhydrous Alcohol is a limpid liquid, of an agreeable odour and pungent taste ; sp. gr. at 60°, '794. It absorbs vapour of water, and be- comes diluted by exposure to damp air ; boils at 173° Fahr. 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 PHOTOGRAPHIC CHEMICALS. 331 is usually about '840, and it contains 80 to 83 per cent, of absolute Alcohol. AMMONIA. Symbol, NH3 or ^1140. Atomic weight, 17. The liquid known by this name is an aqueous solution of the volatile gas Ammonia. Ammoniacal gas contains one 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 cir- cumstances, 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 308). Ammo- nia however differs from the other alkalies in one impor- 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 Car- bonic Acid rapidly from the air, and is converted into Car- bonate of Ammonia ; it should therefore be preserved in stoppered bottles. Besides Carbonate, commercial Am- monia 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 '936. Ammonia, although forming a large class of salts, ap- pears at first sight to contrast strongly in composition with the alkalies proper, such as Potash and Soda. Mineral bases generally are protoxides of metals, as already shown at page 308, but Ammonia consists simply of Nitrogen and Hydrogen united without Oxygen. The following 332 VOCABULARY OF remarks may perliaps tend somewhat to elucidate the dif- ficulty: — Theory of Ammonium. — This theory supposes the exist- ence of a substance possessing the properties of a metal, but differing from metallic bodies generally in being com- pound in structure : the formula assigned to it is NH^, one atom of Nitrogen united with four of Hydrogen. This hypothetical metal is termed " Ammonium and Ammo- nia, associated with an atom of water, may be viewed as its Oxide, for NHg + HO plainly equals JSTH^O. Thus, as Potash is the Oxide of Potassium, so Ammonia is the Oxide of Ammonium. The composition of the salts of Ammonia is on this view assimilated to those of the alkalies proper. Thus, Sul- phate of Ammonia is a Sulphate of the Oxide of Ammo- nium ; Muriate or Hydrochiorate of Ammonia is a Chlo- ride of Ammonium, etc. AMMONIO-NITEATE OF SILYEE. See SiLVEE, Ammonio-Niteate of. AQUA EEGIA. See Niteo-Hydeochloeic Acid. BAEYTA, JSriTEATE OF. See Niteate of Baeyta. BICHLOEIDE OF MEECUEY. See Meecuey, Bichloeide of. 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 PHOTOGEAPHIC CHEMICALS. 333 a disagreeable odour, and fuming strongly at common tem- peratures ; sparingly soluble in water (1 part in 23, Lowig), 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. BEOMIDE OF SILYEE. See Silver, Bromide of. CAEBONATE OF SODA. Symbol, NaO C02+IO Aq. This salt was formerly obtained from the ashes of sea- weeds, 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. Projperties. — The perfect crystals contain ten atoms of water, which are driven off by the application of heat, leav- ing a white powder — the anhydrous Carbonate. Common 334 VOCABULARY OP Washing Soda is a neutral Carbonate, contaminated to a certain extent with Chloride of Sodium and Sulphate 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 alkali ; 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 alkaline. CARBONATE OF POTASH. See Potash, Carbonate of, CASEINE. See Milk. CHARCOAL, 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. 89). 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, consist§,solely 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 PHOTOGRAPHIC CHEMICALS. 335 of Charcoal, but is in great measure peculiar to those derived from the animal kingdom. CHINA CLAY, OE 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. Commercial Kaolin may contain chalk, in which state it produces alkalinity in solution of Nitrate of Silver. The impurity, detected by its effervescence with acids, is re- moved by washing the Kaolin in diluted vinegar and sub- sequently in water. CHLOEINE. 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. F reparation. — 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 extent in water, the solution possessing the odour and colour of the gas. It is nearly 2| times as heavy as a cor- responding bulk of atmospheric air. 336 VOCABULARY OF Chemical properties. — Clilorine belongs to a small na- tural group of elements which contains also Bromine, Iodine, and Fluorine. They are characterized by having a strong affinity for Hydrogen, and also for the metals ; but are comparatively indifferent to Oxygen. Many me- tallic substances actually undergo combustion when pro- jected into an atmosphere of Chlorine, the union between the two taking place with 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 311). The test by which the presence of Chlorine is detected, either free or in combination with bases, is Nitrate of Silver; it gives a white curdy precipitate of Chloride of Silver, insoluble in Nitric Acid, but soluble in Ammonia. The 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, NH4CI. 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- PHOTOGRAPHIC CHEMICALS. 337 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. CHLOEIDE OF BAEIUM. Symbol, BaCl+2 HO. Atomic weight, 123. Barium is a metallic element very closely allied to Cal- cium, the elementary basis of Lime. The Chloride of Ba- rium is commonly employed as a test for Sulphuric Acid, with 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 124). Properties of Chloride of JBarium. — 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^^, leaving the anhydrous Chloride. CHLOEIDE OF GOLD. See Gold, Chloeide of. CHLOEIDE OF SODIUM. Symbol, NaCl. 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 z 338 YOCABULAEY OF minute quantity in rectified spirit. Soluble in tliree parts of water, both hot and cold. Crystallizes in cubes, which are anhydrous. Impurities of Common Salt. — Table Salt often contains 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 SILYEE. 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 JPotash ; 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. When the Citrate of Silver is 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 OE POTASSIUM. Symbol, K, C^'N, or KCy. Atomic weight, 66. This salt is a compound of Cyanogen gas with the me- PHOTOGRAPHIC CHEMICALS. 339 tal Potassinm. Cyanogen is not an elementary body, like Chlorine 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, and is there- 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 44, to which the reader is re- ferred. ETHEE. Symbol, C4H5O. 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 water, 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 85 and 195. 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 falling 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 are 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 employed 340 VOCABULARY OF 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. 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 85). 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 OF POTASSIUM. Symbol, KF. Atomic weight, 59. JPreparation . — Fluoride of Potassium is formed by satu- rating Hydrofluoric Acid with Potash, and evaporating to dryness in a platinum vessel. Hydrofluoric Acid contains Fluorine combined with Hydrogen ; it is a powerfully acid and corrosive liquid, formed by decomposing Fluor 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 H^^drochloric Acid. Properties. — A deliquescent salt, occurring in small and imperfect crystals. Very soluble in water : the solution acting upon glass in the same manner as Hydrofluoric Acid. FOEMIC ACID. Symbol, CjHOg. Atomic weight, 37. This substance was originally discovered in the red ant (Formica rtifa), but it is prepared on a large scale by distil- ling Starch withBinoxideof Manganese and Sulphuric Acid, PHOTOGRAPHIC CHEMICALS. 341 Trojperties. — The strengtli of commercial Formic Acid is uncertain, but it is always more or less dilute. The strongest acid, as obtained by distilling Formiate of Soda with 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, CyHgOg+HgO. Atomic weight, 94. The chemistry of Gallic Acid is sufficiently described at page 27, to which the reader is referred. GELATINE. Symbol, C13H10O5N2. 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 and cut into slices, glue. Gelatine, as it is sold in the shops, is a pure form of Glue. Isinglass is gelatine prepared, chiefly in Eussia, 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 wiU retain about three grains of Isinglass without gelatinizing ; but much depends upon the tempe- rature, a few degrees greatly affecting the result. When long boiled in water, and especially in presence of an acid, such as the Sulphuric, Gelatine undergoes a peculiar modification, and the Solution loses either partially or entirely its property of solidifying to a jelly. 342 VOCABULAEY OF GLYCEEINE. Fatty bodies are resolved by treatment witli an alkali into an Acid — whicb combines with the alkali, forming a soa]p, — 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. GLYCYERHIZINE. Glycyrrhizine, obtained from the fresh root of Liquorice, is a substance intermediate in properties between a sugar and a resin. Sparingly soluble in water but very soluble in Alcohol. It precipitates strong solution of Nitrate of Silver white, but the deposit becomes reddened by expo- sure to light. Its preparation is described in the larger works on organic chemistry. GOLD, CHLOEIDE OF. Symbol, AUCI3. 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 PHOTOGRAPHIC CHEMICALS. 343 of the Clilorine will be driven off in tlie form of gas. At the expiration of a few hours add fresh Aqua E^egia in quantity the same as at first, which will 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 Copper, which must be allowed several hours to separate thoroughly. The Chloride of Gold is thus freed from Copper and Silver, with which the metallic Gold is alloyed in the standard coin of the realm. The solution so prepared will be alkaline, and conse- quently prone to a reduction of metallic Gold: a slight extra quantity of Hydrochloric acid should therefore be added, sufficient to redden a piece of immersed litmus-paper. The weight of a half-sovereign is about 61 grains, of which 56 grains are pure Gold. This is equivalent to 86 grains of Chloride of Gold, which will be the quantity contained in the solution. The following process for preparing Chloride of Gold 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 distilled water, and add a filtered aqueous solution of common Sulphate of Iron (6 parts to 1 of Gold) ; collect the precipitated Gold, which is now free from copper; redissolve in Aqua Eegia, and evaporate to dr3aiess on a water bath. Avoid using Ammonia to neutralize Chloride of Gold, as it would occasion a deposit of " Fulminating Gold," the properties of which are described in the next page. Properties of CJiloride 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 alkah. Chloride of Sodium, which 344 YOCABULAEY OF unites with tlie 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, FULMUSTATING. This is a yellowish-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. Fulminating Gold is probably an Aurate of Ammonia, containing 2 atoms of Ammonia to 1 atom of Peroxide of Gold. GOLD, HYPOSULPHITE OF. Symbol, AuO SgOj. Atomic weight, 253. Hyposulphite of Gold is produced by the reaction of Chloride of Gold upon Hyposulphite of Soda (see page 133). 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 resulting 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). Fro^erties. — Hyposulphite of Gold is unstable and can- PHOTOGEAPHIC CHEMICALS. 345 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 stroDg Nitric Acid (free from Chlorine), diluting with water, and afterwards collecting and igniting the yel- low powder, which is metallic Gold. GEAPE SUGAE. Symbol, C24H23O23. Atomic weight, 396. This modification of Sugar, often termed Granular Su- gar, or Glucose, exists abundantly in the juice of grapes and in many other varieties of fruit. It forms the saccha- rine 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 soluble in water (1 part dissolves in 1^ of cold water). Grape Sugar tends to absorb Oxygen, and hence it possesses the property of decomposing the salts of the noble metals, and reducing them by degrees to the metal- lic state, even without the aid of light. Cane Sugar does not possess these properties to an equal extent, and hence it is readily distinguished from the other variety. The product of the action of Grape Sugar upon Nitrate of Silver appears to be a very low form of Oxide of Silver combined with organic matter. 346 VOCABULAEY OF HONEY. This substance contains two distinct kinds of Sugar, Grape Sugar, and an uncr3^stallizable substance analogous to, or identical with, the Treacle found associated with common Sugar in the cane-juice. The agreeable taste of Honey probably 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 crystaUine portion. Much of the commercial article is adulterated, and, for Photographic use, the Virgin Honey should be obtained direct from the comb. HYDEOCHLOmC ACID. Symbol, HCl. Atomic weight, 37. Hydrochloric Acid is a volatile gas, which may be libe- rated from most of the salts termed Chlorides by the action of Sulphuric Acid. The acid, by its superior affinities, removes the base ; thus, — JSTaCl + HO 80.,=: NaO SO3 + HCl. 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 of organic matter ; commercial Muriatic Acid also often contains a portion of free Chlorine and of Sulphuric Acid. PHOTOGRAPHIC CHEMICALS. 347 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 Sulphuric Acid, the Hydriodic Acid first formed is subsequently decomposed into Iodine and Hydrogen. An aqueous so- lution 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, HSh-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 li- beration 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- logous in composition to the Hydrochloric and Hydriodic Acid. It is usually prepared by the 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. JProjperties. — Cold water absorbs three times its bulk of Hydrosulphuric Acid, and acquires the peculiar 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 decom- posed by JN^itric Acid, and also by Chlorine and Iodine. 348 VOCABULAEY OF It precipitates Silver from its solutions in tlie form of black Sulpkuret of Silver ; also Copper, Mercury, Lead, etc. ; but Iron and other metals of that class are not af- fected, 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 on 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 until 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. Hydrosulphate of Ammonia is employed in Photography to darken the Negative image, and also in the preparation of Iodide of Ammonium, the separation of Silver from Hyposulphite solutions, etc. HYPOSULPHITE OF SODA. Symbol, NaO SgOj + 5 HO. Atomic weight, 125. The chemistry of Hyposulphurous Acid and the Hypo- sulphite of Soda has been sufficiently described at pages 43, 129, and 137 of the present Work. The crystallized salt includes five atoms of water of crystallization. HYPOSULPHITE OF GOLD. See Gold, Hyposul- phite OF. PHOTOGEAPHIC CHEMICALS. 349 HYPOSULPHITE OF SILVEE. See Silver, Hypo- sulphite 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 helj), which is the fused ash obtained on burning seaweeds. The wa- ters 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 ©f kelp is eva- porated to dryness and distilled with Sulphuric 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, like 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. 350 VOCABULARY OF Chemical Troperties. — Iodine belongs to tKe Clilorine group of elements, characterized bj 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 Acids aturated 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 198, to which the reader is referred. IODIDE OF CADMIUM. Symbol, Cdl. Atomic weight, 182. See page 199, for the preparation and properties of this 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 in Alcohol. 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 Iodine may recombine. By very careful evaporation, hydrated crystals of Proto-iodide may PHOTOGEAPHIC CHEMICALS. 351 be obtained, but the composition of the solid salt usually sold under that name cannot be depended on. The Periodide of Iron, corresponding to the Perchlo- ride, has not been examined, 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 IO5) 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. Projoerties. — It forms cubic and prismatic crystals, which should be hard, and very slightly or not at all deli- quescent. Soluble in less than an equal weight of water at 6O0 ; 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 : — 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 Potassium is instantly coloured brown by free Chlorine ; also very rapidly by Peroxide of JSTitrogen (page 86) ; ordinary acids however act less quickly, Hydriodic Acid being first formed, and subsequentl}^ decomposing spon- taneously. The impurities of commercial Iodide of Potassium, with the means to be adopted for their removal, are fully given at page 197. IODIDE OF SILYEE. See Silver, Iodide of. lODOFOEM. The composition of this substance is analogous to that 352 YOCABULARY OF of Chloroform, Iodine being substituted for Chlorine. It is obtained on boiling together Iodine, Carbonate of Potash, and Alcohol. Iodoform occurs in yellow nacrous crystals, which have a saffron-like odour. It is insoluble in water, but soluble in spirit. lEOJSr, PEOTOSULPHATE OF. Symbol, EeO SO3+7HO. 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 Bin- 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, PEOTONITEATE OF. Symbol, FeO NO^+T HO. Atomic weight, 153. This salt, by careful evaporation in vacuo over Sul- phuric 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 air. The preparation of solution of Protonitrate of Iron for developing Collodion Positives, is given at page 206. lEON, PEECHLOEIDE OF. Symbol, FcgClg. 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- PHOTOGRAPHIC CHEMICALS. 353 ing a green solution, which precipitates a dirty white Prot- oxide 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. MEECUEY, BICHLOEIDE OF. Symbol, HgClg. 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 subliming a mixture of Persulphate of Mercury and Chloride of Sodium. 2 a 354 VOCABULARY OF 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 liot water ; more abundantly so in Alcobol, and also in Ether. The solubility in water may be increased by the addition of free Hydrochloric Acid, or of Chloride of Am- monium. The Protochloride of Mercury is an insoluble white powder, commonly known under the name of Calomel. METHYLIC ALCOHOL. This liquid, known also by the names of wood naphtha and pyroxylic spirit, is one of the volatile products of the destructive distillation of wood. It is very volatile and limpid, with a pungent odour. By a recent excise regulation, ordinary Spirit mixed with ten per cent, of wood naphtha is sold free of duty, under the name of " Methylated Spirit." MILK. The Milk 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 shimmed milk 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 PHOTOGRAPHIC CHEMICALS. 355 tlie calf, possesses the property of coagulating Caseine, 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 portion of the Caseine usually re- mains 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 toJiey until crystallization begins to take place. It is hard and gritty, and only slightly sweet ; slowly soluble, without forming a syrup, in about two and a half parts of boiling, 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 d.ecomjposing 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, NOg. 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. 356 VOCABULAEY OF Properties. — Anhydrous Nitric Acid is a solid substance, 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 15; 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 Pyroxyline. The yellow Nitrous Acid, so called, is a strong Nitric Acid partially saturated with the brown vapours of Peroxide of Nitrogen ; it has a higli 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 compound being formed which has been termed a Sidpliate 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 Gold 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 toater. 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 Acid. — 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 PHOTOGRAPHIC CHEMICALS. 357 equal bulk of distilled water, and adding a few drops of Nitrate of Silver, — a milkiness, which, is Chloride of Silver in suspension, indicates the presence of Chlorine. 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 will be formed. JSTITEOUS ACID. See Silver, Nitrite of. NITEATE OF 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 in what are called Nitre-beds. The properties of Nitrate of Potash are described as far as necessary at page 190. JSITEATE OF BAEYTA. Symbol, BaO NO5. 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. Tt 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°. 358 VOCABULAET OF Nitrate of Lead forms with Sulphuric 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 SILYEE, See Silyee, Nitrate or. NITEO-GLUCOSE. When 3 fluid ounces of cold JSTitro- Sulphuric Acid, con- sisting of 2 ounces of Oil of Vitriol and 1 ounce of highly concentrated Nitric Acid, are mixed with 1 ounce of finely powdered Cane Sugar, there is form.ed at first a thin, trans- parent, pasty mass. If it is stirred with a glass rod for a few minutes without interruption, the paste coagulates as it were, and separates from the liquid as a thick tena- cious mass, aggregating into lumps, which can easily be removed from the acid mixture. This substance has a very acid and intensely bitter taste. Kneaded in warm water until the latter no longer reddens litmus-paper, it acquires a silver colour and a beautiful silky lustre. It may be used in Photography to confer intensity upon newly mixed Collodion ; but is in- ferior to Glycyrrhizine employed for the same purpose. 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 w^ater and liberating Chlo- rine, thus : — NO- + HClr^NO^ + HO + CI. The presence of free Chlorine confers on the mixture the power of dissolving Gold and Platinum, which neither of PHOTOGEAPHIC CHEMICALS. 359 tlie 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 application of a gentle heat assists the solution of the metal ; but if the temperature rises to the boiling point, a violent effervescence and es- cape of Chlorine takes place. NITEO-SULPHIIRIC ACID. For the chemistry of this acid liquid, see page 77. OXYGEN. Symbol, O. 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 the Nitrate, 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 chemical affinity however is not well seen when the ele- mentary body is exposed to the action of Oxygen in the gaseous form. It is the nascent Oxygen which acts most powerfully 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 liquid 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 atmospheric air, as compared with the violent action of 360 yocABULARY or 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 will be fit for use. POTASH. Symbol, KG + HO. Atomic weight, 57. Potash is obtained by separating the Carbonic Acid from Carbonate of Potash by means of Caustic Lime. Lime is a more feeble base than Potash, 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 314). Properties. — Usually 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. 308), 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. PHOTOGRAPHIC CHEMICALS. 361 in order to prevent them from becoming immovably fixed by the solvent action of the Potash upon the Silica of the glass. The Liquor Potassee 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 Carbonate 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, Silica, etc. POTASH, CAEBONATE OF. Symbol, KO COg. 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 form of salts, like the 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 page ]96). PYEOGALLIC ACID. Symbol, C8H4O4 (Stenhouse). Atomic weight, 84. The chemistry of Pyrogallic Acid has been described at page 28. 362 VOCABULARY OF SEL D'OE. See Gold, Hyposulphite of. SILVER. Symbol, Ag. Atomic weiglit, 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 Sulphuretted Hydrogen, it is slowly tarnished from formation of Sulphuret of Silver. It dissolves in Sulphuric Acid, but the best solvent is JN'itric Acid. The standard coin of the realm is an alloy of Silver and Copper, containing about one-eleventh of the latter metal. To prepare pure Nitrate of Silver from it, dissolve in Nitric Acid and evaporate until crystals are obtained. Then wash the crystals with a little dilute Nitric Acid, redis- solve them in water, and crystallize by evaporation a second time. Lastly, fuse the product at a moderate heat, in order to expel the last traces of Nitric and Nitrous Acids. SILVER, AMMONIO-NITRATE OF. Crystallized Nitrate of Silver absorbs Ammoniacal gas rapidly, with production of heat sufficient 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 4he 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, PHOTOGRAPHIC CHEMICALS. 363 converts it into a black powder, termed Fulminating Silver, which possesses the most dangerous explosive properties. Its composition is uncertain. In preparing Ammonio- JN^itrate 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, free Ammonia is necessarily formed. Thus — Chloride of Ammonium -f 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 L, page 17. SILVEE, CHLOEIDE OF. 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. SILYEE, CITEATE OF. See Citric Acid. SILYEE, IODIDE OF. Symbol, Agl. Atomic weight, 234. See Part I. page 16. 364 YOCABULAEY OF SILVEE, FLUOEIDE OF. Symbol, AgF. Atomic weight, 127. This compound differs from those last described in being soluble in water. The dry salt fuses on being heated, and is reduced by a higher temperature, or by exposure to light. SILYEE, 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 Silver is insoluble in water, and nearly so in those substances which dissolve the Chloride, Bromide, and Iodide, such as Ammonia, Hyposulphites, Cyanides, 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 146.) SILYEE, NITEATE OF. Symbol, AgO NO5. Atomic weight, 170. The preparation and properties of this salt have been explained at pages 12 and 362. SILYEE, NITEITE OF. Symbol, AgO NOg. Atomic weight, 154. Nitrite of Silver is a compound of Nitrous Acid, or NO3, with Oxide of Silver. It is formed by heating Nitrate of Silver, so as to drive off a portion of its Oxygen, or more PHOTOGEAPHIC CHEMICALS. 365 conveniently, by mixing JN'itrate of Silver and Nitrite of Potash in equal parts, fusing strongly, and dissolving in a small quantity of boiling water : on cooling, the Nitrite crystallizes out, and may be purified by pressing 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 until effervescence has ceased. The Nitrite of Soda thus formed is afterwards added to Nitrate of Silver in the usual way. Properties. — Nitrite of Silver is soluble in 120 parts of cold water ; easily soluble in boiling water, and crystallizes, 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 Nitrous Acid. — This substance possesses very feeble acid properties, its salts being decomposed even by Acetic Acid. It is an unstable body, and splits up, in contact with water, into Binoxide of Nitrogen and Nitric Acid. The Peroxide of Nitrogen, NO4, is also de- composed by water, and yields the same products. SILVER, ACETATE OF. Symbol, AgO (C4H3O3). Atomic weight, 167. This is a difficultly 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 Silver 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 page 89. 366 VOCABULAEY OF SILVEE, HYPOSULPHITE OF. Symbol, AgO S2O2. Atomic weight, 164. Tliis salt is fully described in Part I. page 129. Eor tbe properties of the soluble double salt of Hyposulphite of Silver and Hyposulphite of Soda, see page 43. SlIGAE OF MILK. See Milk. SULPHURETTED HYDEOGEN. See Hydeosul- PHURic 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- ingly ingenious and beautiful, but it involves reactions which are too complicated to admit of a superficial expla- nation. The Sulphur is first burnt into gaseous Sulphu- rous Acid (SO2), and then by the agency of Binoxide of Nitrogen gas, an additional atom of Oxygen is imparted from the atmosphere, so as to convert the SO2 into SO3, or Sulphuric Acid. Properties. — Anhydrous Sulphuric Acid is a white crys- talline solid. 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 ; boils at 620°, and distils without decom- position. It is not volatile at common temperatures, and therefore does not fume in the same manner as Nitric or Hydrochloric Acid. The concentrated acid maybe cooled PHOTOGRAPHIC CHEMICALS. 367 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 know a as Nitro- Sulphuric Acid. Sulphuric Acid possesses intense chemical powers, and displaces the greater number of ordinary acids from their salts. It cliars 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 be calculated, nearly, from its specific gravity, and a Table is given by Dr. Ure for that purpose. (See Appendix.) Impurities of Commercial Sulphuric Acid. — The liquid acid sold as Oil of Vitriol is tolerably constant in compo- sition, and seems to be as well adapted for Photographic use as the pure Sulphuric Acid, which is far more ex- pensive. The specific gravity should be about 1'836 at 60°. If 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 186). Test for Sulphuric 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 solubility of the Chloride of Barium itself in acid solutions. 368 VOCABULARY OF SULPHUEOUS ACID. Symbol, SOo. Atomic weight, 32. This is a gaseous compound, formed by burning Sulphur in atmospheric air or Oxygen gas : also by heating Oil of Vitriol in contact with metallic Copper, or with Charcoal. "When 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- ng 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. TETEATHIOlSriC ACID. Symbol, S4O5. Atomic weight, 104. The chemistry of the Polythionic Acids and their salts will be found described in the Pirst Part of this Work, page 157. AYATEE. Symbol, HO. Atomic weight, 9. Water is an Oxide of Hydrogen, containing single atoms of each of the gases. Distilled tvater 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. JPure distilled water leaves no re- sidue on evaporation, and should remain perfectly clear on the addition of Nitrate of Silver, even tv/ien cxjposed to the light ; it should also be neutral to test-paper. PHOTOGEAPHIC CHEMICALS. 369 The condensed water of steam-boilers sold as distilled water is apt to be contaminated with oily and empyreumatic matter, T^ hich discolours Nitrate of Silver, and is therefore injurious. Itain-ioater, 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, tingeing 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, 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 Chlorides, 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 JPhoto- 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 Silver. 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 2b 370 PHOTOGRAPHIC CHEMICALS. manner as Carbonate of Soda. (See page 89.) Sulpliate 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 JRiver 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 JNitrate of Silver solution may be added, to sepa- rate the Chlorine, taking care not to use a large excess. 371 APPENDIX. QUANTITATIVE TESTING OF SOLUTIONS OF NITEATE OF SILVER. 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 the 'pure crystallized Chloride of Sodium, and either dry it strongly or fuse it at 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 J grains to 6 fluid ounces. In this way, a staudard 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 accurately 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 v/hich is to be tested ; shake the con- tents of the bottle briskly, until the white curds have 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 milJdness, 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 .crains 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. EECOVEEY OF SILVEE FEOM WASTE SOLUTIONS, — FEOM 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. Sej)aration 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 b. Separation of Silver from solutions containing alkaline Hypo- sulphites^ Cyanides^ or Iodides. — Iq 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 separat€d. 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 Oil 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 Chloride 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 with 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. EEDUCTION OF CHLOEIDE OF 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 off 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 four 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 fused into a button, to burn off organic matter if present, and subsequently converted into Nitrate of Silver by boiling with Nitric Acid diluted with two pas'ts of water. In reducing Chloride of Silver precipitated from old Nitrate Baths containing Iodide of Silver, the grey metallic powder is sometimes contaminated with unreduced Iodide of Silver, which 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 Hypo- sulphite of Soda, and then again with water. MODE OF TAKING THE SPECIFIC GRAVITY 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 distilled water, and with each is sold a brass weight, which counterbalances it when filled vvith pure water. In taking the specific gravity of a liquid, fill the bottle quite full 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 w^eight, — then 1" minus '250, or '750, is the specific gravity; but in the case of Oil of Vitriol the bottle, when fidl, will 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 w^ater, 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 olf the corners with a pair of scissors, and open out the filter into a conical form, when it will be 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 may 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-PAPEKS. The nature of the colouring matter which is employed in the preparation of litmus -paper has already been described at page 353. 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-^di^tv. 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 Ammonite, 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- siinally 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 redy 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 OF SILVEE STAINS FEOM 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. 151). A TABLE SHOWING THE QUANTITY OF ANHYDROUS ACID IN DILUTE SULPHUEIC ACID OF DIFFERENT SPECIFIC GRAVITIES. (URE.) Specific Gravity. Eeal Acid in 100 parts of the Liquid. Specific Gravity. Real 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 ANHYDROUS ACID IN THE LIQUID NITRIC ACID OF DIFFERENT SPECIFIC GRAVITIES. (URE.) Specific Gravity. Real Acid , in 100 parts of the Liquid. Specific Gravity. Real Acid in 100 parts of tht Liquid. Specific Gravity. Real Acid 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 66155 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 51068 1-4700 70-933 1-4228 60-572 1-3630 50-211 1-4670 70-136 1-4189 59-775 1-3579 49-414 APPENDIX. 379 WEIGHTS AND MEASURES. Troy^ or Jpothecaries' Weight. 1 Pound = 12 Ounces. 1 Ounce = 8 Drachms. 1 Draclim = 3 Scruples. 1 Scru^^le = 20 Grains. (1 Ounce Troy = 480 Grains, or 1 Ounce Avoirdupois plus 42*5 grains.) Avoir dwpois 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 24 Troy Ounces plus 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 70,000 Grains. An Imperial Pint of water weighs \\ Pound Avoir- dupois. A fluid Ounce of water iveighs 1 Ounce Avoirdupois, or 437*5 Grains. A Drachm of water iveighs 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 35i English fluid Ounces. French Pleasures of Volume. 1 Litre = 13 Decilitres = 100 Centilitres = 1000 Millilitres = 35 ^ English fluid Ounces. 1 Litre = 1 Cubic Decimetre = 1000 Cubic Centimetres. 1 Cubic Centimetre = 17 English Minims. A Litre of water iveighs a Kilogramme, or something less than 2^ Pounds Avoirdupois. A Cubic Centimetre of water weighs a Gramme. 381 INDEX.* Aberration, chromatic, 54 ; sphe- rical, 56. Accelerating agents, their mode of action in Collodion explained, 95 . Acetate of Silver, its preparation and formula, 365 ; its formation in Nitrate Bath explained, 89 ; ensures absence of free Nitric Acid, 116 ; tends slightly to fa- vour fogging and spots, 104 ; contra-indicated for glass Posi- tives, 111. Acetic Acid, properties and mode of testing purity of, 327 ; use- ful in preventing fogging, 104 ; in rendering the development slow and even, 99 ; does not co- agulate Albumen, 329 ; a good commercial form of acid, 212 ; Acetic Acid essential in Calo- type, waxed paper, and Albu- men processes, 177 ; also in printing paper Positives by de- velopment, 260. Aceto-Nitrate of Silver, term ex- plained, 177. Achromatic Lenses, their construc- tion explained, 55; the visual and chemical foci often coinci- dent in, 60. Acids, nature of, 308. Actinism, explained, 61 ; impor- tance of distinguishing Actinic from visual rays, 62 ; mode of finding Actinic focus, 229. Affinity, chemical, 312. Albumen, its chemistry, 328; forms a compound with oxide of Silver, 20 ; used in Positive printing to produce a fine surface layer, 122 ; to increase sensitiveness, 125 ; affects the colour of the prints, 127 ; protects the image from oxidation, 150 ; putrifies when exposed to moisture, 155 ; discolours the Nitrate Bath, 245. Albumen negative process, its in- vention, 10; theory of, 180; Collodio-albnmen process of M. Taupenot, 294. Albuminized paper, formula for, 241 ; slow infixing, 131 ; not well adapted for toning by Sel d'or, 269 ; good for stereoscopic sub- jects and small portraits, 249 ; cannot be sensitized with Am- monio-Nitrate of Silver, 246. Alcohol, its chemistry, 330 ; some- times too dilute for making Col- lodion, 84 ; mode of rectifying, 196 ; must not contain impuri- ties, 96 ; effects of adding to Col- lodion, 84, 96; to developer, 205. Alkalies, nature of, 308. * The preparation and properties of the Chemicals used in Photography will be found in the Alphabetical List commencing at page 327. 382 INDEX. Alkalinity of Nitrate Bath, ex- plained, 88 ; the evils it pro- duces, 104 ; how to test for it, 377 ; how to remove it, 277. Amber varnish, 226. Ammonia, preparation and proper- ties, 331 ; its use in fixing, 42 ; Mr. Shadbolt's formula for, 271; its action upon Chloride of Gold, 343 ; effect of concentrated Am- monia upon Oxide of Silver, 362. Ammonio-Nitrate of Silver, its chemistry, 862 ; used in Positive printing to increase sensitive- ness, 125 ; to give black tones, 127 ; cannot be used with Al- bumen, 246 ; increases perma- nency of print, 169 ; old Ni- trate Baths not easily convertible into Ammonio - Nitrate, 248 ; 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, 258. Atomic theory explained, 322. Bath for fixing and toning Positives. aS*^^ Fixing and toning Bath. Bichloride of Mercury, whitening action on glass Positives ex- plained, 113 ; solution for, 207; used to intensify Negatives, 118; bleaches paper prints, 151 ; should not be added to paste used in mounting prints, 164 ; removes Silver stains, 377. Binocular vision, phenomena of, explained, 66. Blackening Negatives, 37, 117. Black tones, mode of obtaining, in paper Positives, 168, 246. Bromide of Silver, its preparation and properties, 17 ; its superior sensibility to coloured light, 63 ; less acted on by white light than Chloride, 19 ; less sensitive to invisible image than Iodide, 25 ; employment in Collodion, 101 ; found useful in Photographing by artificial light, 66 ; diagram of chemical spectrum on, 64. Bromo-Iodide of Silver, 173. Brushes, mode of applying Silver solutions by, 248. Calotype process, theory of, 176. Camera, its first invention, 7 ; theory of its construction, 54 ; mode of testing accuracy of, 229 ; cause of the image being in- verted, 53 ; the term flatness of field " explained, 54 ; best position of the Camera for por- traits, 220 ; for architectural subjects, 231 ; a funnel-shaped tube placed in front of the lens, 229 ; stereoscopic Camera, 234 ; microscope Camera, 236. Causes of failure in Collodion pro- cess, 276. Chemical affinity, illustrations of, 312. Chemical elements, 306. Chemical focus, directions for find- ing, 229; shorter than visual in non achromatic lenses, 60; longer than visual in microscopic objectives, 237 ; varies slightly with the nature of the light, 238. Chemical spectrum, 61. Chemicals, Photographic, Vocabu- lary of, 327. Chloride of Silver, its preparation and properties, 14 ; more sensi- tive to white light than Bromide or Iodide, 19 ; less sensitive to invisible image, 24 ; its black- INDEX. 383 ening by light explained, 20, 141 ; 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, 22 ; agents which dissolve it, 42; mode of reducing it to metallic state. 374. Chloride of Gold, its preparation and properties, 342 ; action of Ammonia upon it, 343 ; use of an alkaline solution of, for to- ning, 132, 271 ; compounds formed on adding it to Hyposul- phite of Soda, 133 ; mode of preparing the fixing and toning Bath with, 250 ; the Sel d'or Bath with, 267. Chromatic aberration, 54. Citric Acid, forms a red compound with Suboxide of Silver, 21, 338; used in printing to give purple tones, 128 ; formula for prepa- ring paper with, 246. Cleaning glass plates, theory of, 39 ; details of, 213. Collodion, its discovery, 10 ; che- mistry of Pyroxyline, 75 ; phy- sical effect of Ether and Alcohol in, 83 ; of water in, 85 ; gluti- nosity of, 83 ; coloration of io- dized, explained, 85 ; sensitive- ness and intensity affected by the change, 97, 99 ; details of ma- nufacture of Collodion, 185 ; Po- sitive CoUodion, theory of, 108 ; formula for, 201 ; Negative Col- lodion, theory of, 113 ; formula for, 208 ; Collodion for copying engravings, 231 ; for keeping proces.^es, 298 ; for hot climates, 210 ; for working by artificial light, 238 ; to remove the brown colour from CoUodion, 86. Collodion film, the proper time for immersing it in the Bath, 219 ; a thin film often good for direct Positives, 109 ; a thicker film for Negatives, 113 ; cause of the film falling away from the glass, 83, 293 ; spots and mark- ings on, 281 ; conditions which affect its sensitiveness to light, 92 ; causes influencing its be- haviour with the developer, 98 ; mode of preserving sensitiveness of film, 289. Collodio-Albumen process, theory of, 181 ; practical details of, 294. Colours, their nature explained, 47 ; their chemical action on sensitive film, 64 ; their photo- graphic action assisted by re- flection of white light, 66. Combination, laws of, 307. Conjugate foci, explained, 52, 272. Crookes, Mr., remarks upon che- mical spectrum, 63 ; upon waxed paper process, 180 ; preserva- tive process for Collodion films, 289. Curvature of luminous image form- ed by lens, explained, 53. Cyanide of Potassium, its fixing ac- tion explained, 44 ; preparation of solution of, 207 ; used to re- move stains, 377. Daguerreotype, its invention, 8 ; theory of the process, 171. Development of invisible image, explanation of, 34-40; second, or intensifying stage explained, 37 ; details of developing glass Positives and Negatives, 221- 223 ; development of paper Po- sitives, 259 ; conditions which increase or diminish rapidity of development, 98 ; irregularities of development, 103. 384 INDEX. Developers, their preparation and properties, 26 ; comparative strength of, 98 ; theory of, for Positives, 111; for Negatives, 117 ; formulae for Positive de- velopers, 205 ; for Negative, 211. . Diagrams, mode of copying, 232. Diaphragms for lenses. See Stops. Double decomposition, illustrated, 14 ; explained, 314. Dry Collodion proce^ss, 298. Elementary bodies, table of, 306 ; combination of, 307. Engravings, mode of copying, 231 ; often vield dark-coloured prints, 255. ^ Equivalent proportions, 320. Ether, properties of, 339 ; purifi- cation of, for Photography, 195 ; must be kept in a dark place, 196 ; should not be distilled from residues of old Collodion, 96. Experiments, illustratinsj action of Light upon Chloride of Silver, 21 ; illustrating formation and development of invisible images, 25 ; illustrating photographic action of coloured light, 62. Exposure in the Camera, rules for Positives, 221 ; for Negatives, 225 ; for preserved Collodion plates, 292; for microscopic pho- tographs, 238 ; effects of under and over-exposing, 35 ; exposure required in Calotype process, 177 ; in waxed paper, 180 ; in Albumen negative process, 181 ; in dry Collodion process, 301 ; in Taupenot's process, 297. Fading of Positives, explained at length, 160 ; Author's researches on, 153. Film, sensitive. Collodion film. Filters, mode of cutting, 376. Fixing, theory of, 41 ; of paper prints explained, 128 ; solution for fixing glass Positives and Negatives, 212; manipulatory de- tails of fixing, 225 ; fixing paper Positives vs^ith Ammonia, 271. Fixing and toning Bath, its prepa- ration, 250 ; conditions which favour or retard its action, 135 ; certain states of the Bath inju- rious to the proofs, 136 ; im- portance of keeping it in an active condition, 168 ; must not be employed immediately after mixing, 251 ; must not be al- lowed to become acid by constant use, 168 : theory of the gradual change of properties it under- goes, 156. Foci, actinic and luminous, 60 ; ac- tinic, mode of finding, 229 ; va- riation between them in micro- scopic objectives, 237. Focussing the object, 220. Fogging, theory of, ] 03 ; mode of detecting causes of, 276. Formulae for solutions required in Collodion process, 201 ; for pa- pers used in Positive printing, 241 ; want of correspondence between, 257. Gallic Acid, its preparation and properties, 27 ; used in paper processes, 178; becomes mouldy by keeping, 261 ; formula for developing paper Positives with, 261. Gallo-Nitrate of Silver, 177 ; dis- colours rapidly when developing dishes are not clean, 179. Gelatine, its properties, 341 ; forms a compound with an Oxide of Silver, 21 ; employed in dry INDEX. 385 Collodion process, 299 ; modi- fied form of, 302; affects the colouriii printing processes, 128; used in Positive printing to form an even surface layer, 126 ; as a cement to mount Photographs, 257. Glass plates, rules for cleaning, 39 ; details of cleaning, 213 ; mode of coating with CoUodion, 215 ; with Albumen, 180. Glutinous Collodion, explained, 83. Glycyrrhizine, its nature, 342 ; its action in Collodion, 114 ; for- mula for solution of, 209. Gold, Chloride of. See Chloride of Gold. Gold salts, their use in Photogra- phic printing explained, 131 ; in the Daugerreotype process, 175. Gradation of tone, in CoUodion Photographs, affected by the density of the film, 109, 113 ; by use of Glycyrrhizine, 115. Gradation of tone, in paper Posi- tives, conditions affecting it in prints obtained by direct expo- sure, 123 ; in Positives printed by development, 266. Hadow, Mr., researches on Collo- dion, 77; formula for making Pyroxyline, 187. Heliography, invented by M. Niepce, 7- Historical sketch of Photography, 6. Honey keeping process, 289. Hunt, Mr., introduces Protosalts of Iron in developing, 111. Hypo Bath. See Fixing and To- ning Bath. Hyposulphite of Silver, its peculiar changes in colour, 129 : the sweet compound which it forms with Hyposulphite of Soda, 44. Hyposulphite of Soda, preparation and properties, 43 ; theory of its fixing action, 43 ; blackens Ni- trate of Silver, 129 ; causes a milkiness with acids, 137 ; its decomposition by constant use in fixing, 138 ; the salts it forms with Chloride of Gold, 133 ; its conversion into sulphuretting Tetrathionate by Iodine and Per- chloride of Iron, 139 ; test for presence of, 169, Iceland moss, its use in Positive printing, 128 ; formula for pre- paring paper with, 245. Imperfections in Collodion Nega- tives, 282; in Positives, 284; in paper Positives, 285. Intensity, explanation of term, 92 ; mode of increasing in Ne- gatives, 99, 114; effect of Ace- tate of Silver upon, 116 ; of Ni- trite of Silver upon, 102 ; mode of diminishing, in glass Positives, 109, 110 : conditions affecting intensity in paper Positives, 123 ; in developed paper Positives, 266. Invisible images, theory of forma- tion of, 34 ; development of, 36; experiments illustrating, 25. lodate, how formed in Collodion film, 94 ; produces insensitive- ness, 198. Iodide of Ammonium, preparation of, 198 ; not fitted for iodizing Collodion required to be kept long, 210. Iodide of Iron, an accelerator to CoUodion, 116. Iodide of Potassium and Silver, properties of, 42 ; mode of iodizing Calotype papers by, 177. Iodide of Potassium, tests of purity of, 1 97 ; extent of solubility in 2 c 386 INDEX. Alcohol, 351; dissolves Iodide ! of Silver, 42. | Iodide of Silver, its preparation and properties, 1 6 ; unaffected by di- rect action of light, 19 ; highly sensitive to invisible image, 24 ; hypothesis of formation of latent ! image on, 34 ; possibility of ! its reduction by Pyrogallic Acid I shown, 33 ; excess of Nitrate of ' Silver essential to its blackening | by developer, 36 ; diagrams of | chemical spectrum on, 61, 64 ; | fixing agents for, 42; its solu- | bility in the Nitrate Bath, 86 : retards the action of Hypo fixing , and toning Bath, 136 ; superior | permanency of developed prints on, 167 ; details of Negative printing process on, 263. Iodine, in Collodion, diminishes sensitiveness, 94 ; forms Nitric Acid and lodate in the Bath, 94 ; often useful in Positive Col- lodion, 110; in Negative Collo- dion, if fogging occurs, 105 ; mode of removing from Collo- dion, 86. Iodized Collodion. See CoUodion. Kaolin, properties of, 335 ; used to decolorize Nitrate Baths, 91 ; importance of purifying it be- fore use, 245. Landscape Photography, with pre- served Collodion plates, 288. Latent image. See Invisible image. Laws of substitution explained, 78. Le Grcj'^, M,, his toning process with Chloride of Gold, 132 ; his waxed paper Negative process, 178. Lenses, various forms of, 51 ; foci of, 52 J formation of images by, 53 ; use of stops, 58 ; portrait, 59 ; chemical foci of, 60 j chro - matic aberration of, 54 ; spheri- cal aberration of, 56 ; simple di- rections for using lenses, 227 ; for finding chemical focus, 229. Light; its action upon Silver Salts, 19 ; experiments illustrating, 21 ; formation of invisible images by, 24 ; its alternating action upon Daguerreotype plate, 39 ; its compound nature, 46 ; pho- tographic action of coloured light, 60 ; refraction of light, 49. Llewellyn, Mr., his Oxymel pro- cess, 291. Manipulations of Collodion pro- cess, 213 ; of Photographic print- ing, 251 ; of toning bv Sel d'or, 267. Manuscripts, mode of copying, 231. Markings on Collodion Pictures, 281. Measures and Weights, 379. Microscopic Photography, 235. Moser, M. Ludwig, his researches on the development of invisible images, 37- Mounting Positive Prints, sub- stances which should be avoided in, 155, 164; details of, 257. Negative processes for printing Po- sitives, 259, 263. Negatives, definition of, 106 ; Col- lodion Negatives, theory of pro- duction of, 113 ; Calotype, 176 ; waxed paper, 178 ; Albumen, 180 ; CoUodio- Albumen (Taupe- not), 181 ; mode of developing Collodion Negatives, 37, 117, 223; of converting Positives into, 117 ; formula for solutions for Negatives, 208 ; the Collo- dion best adapted for Negatives, 114 ; spots and markings upon INDEX. 387 Negatives, 282 ; decomposition of Pyroxyliiie a cause of fading of, 166. Nitrate of Silver, preparation and properties of, 12 ; preparation of from standard coin of realm, 362 ; often contains free Nitric Acid, 13 ; when very strongly fused, contains Nitrite {see Ni- trite of Silver), 14 ; not acted on by light, IS ; its reduction byPy- rogallic Acid explained, 31 ; the melted Nitrate more certain in its action, 13, 101; its presence essential in developing the image, 36, 98 ; increases sensitiveness of CoUodion plate, 92 ; dissolves Iodide of Silver, 86; discoloured by Albumen, 245, 329 ; forms a compound vrith Honey, 289 ; with various other organic bodies, 21 ; very little acted on by Glycerine, 342 ; mode of re- covering the Silver from, 372. Nitrate Bath, mode of preparing for glass Positives, 110, 203^; for Collodion Negatives, 116, 211 ; its power of dissolving Iodide of Silver, its occasional acidity and alkalinity explained, 86 ; the mode in which Acetate of Silver may be formed in it, 89 ; a list of the substances by which it is decomposed, 90 ; changes by use, 91 ; effect of these changes on sensitiveness, 97 ; on intensity, 102 ; care re- quired to prevent it from yield- ing foggy pictures, 104 ; a cau- tion against the too frequent ad- dition of alkali, 204 ; quantita- tive testing of the Bath, 371. Nitrite of Silver, adds to rapidity of development, 102 ; tends slightly to produce fogging, 104 ; solarizes the high lights, 111. Nitric Acid, its preparation and properties, 355 ; its oxidizing powers, 12; impairs sensitive- ness of Collodion film, 93 ; lessens rapidity of development, 98 ; tends to prevent fogging, 104 ; sometimes usefully em- ployed for glass Positives, 110; contra-indicated for Negatives, 116 ; its accumulation in the Nitrate Bath explained, 94 ; mode of removing it, 90 ; can- not exist in contact with Acetate of Silver, 116 ; produces stains on cloth, 215; mode of determin- ing the strength of Commer- cial Nitric Acid, 186 ; table of I strength of Nitric Acid of dif- i ferent densities, 378. Nitro-Sulphuric Acid, explained, 77 ; process for making by mix- ed acids, 186 ; by Oil of Vitriol and Nitre, 190; should not be I used cold, 83. I Nomenclature, chemical, 315. Norris, Dr., his dry Collodion pro- cess, 298, Notation, chemical, 318. [ Organic bodies, chemistry of, 324. I Oxide of Silver, preparation and I properties, 17 ; dissolves in the j Nitrate Bath, rendering it alka- I line, 88 ; properties of its solu- tion in Ammonia, 362 ; prepara- tion of ditto, 247 ; its solution in Nitrate of Ammonia used in Photography, 249. Oxymel, keeping process, 291 ; prepai-ation of Oxymel, 360. Paper, Photographic, selection of, 240 ; peculiarity of English pa- pers, 241, Paper, sensitive, for printing. See Sensitive Paper, 388 INDEX. Perchloride of Iron, preparation of toning Bath with, 160. Permanence of Positives, mode of testing, 169. Photographic image, chemical com- position of, 140 ; action of de- structive tests on, 145. Photographic properties of Salts of Silver, 18; of Iodide of Silver upon Collodion, 74. Photographic researches by the Author, 140. Photography, historical sketch of, 6 ; the term explained, 61. Portrait lenses, theory of their con- struction, 59 ; rules for their use, 227 ; mode of finding che- mical focus, 229. Portraits, drapery for, 66 ; direc- tions for taking, 220 ; the po- sition of the Camera, and other points of importance, 228 ; the time of exposure, 221. Positive printing, on Albuminized paper, formulae for, 241 ; on plain paper, formulae for, 245 ; on Ammonio-Nitrate paper, for- mula for, 246, 258; by de- velopment, formulae for, 259 ; manipulatory details of printing, fixing, toning, washing, and mounting, 251 ; process of to- ning by Sel d'or, theory of, 134 ; practice of, 267 ; reasons for the want of correspondence between different formulae, 257 ; use of Chloride of Gold in toning, 132, 271 ; theory of the preparation of the sensitive paper for Posi- tives, 122; theory of the pro- cess of fixing, 129; of toning by Gold, 132 ; the Author's re- searches, 140; rationale of the printing process, 120 ; composi- tion of the image, 140 ; fading of Positive prints, 160 ; destruc- tive action of Sulphur on, 145 ; of oxidizing agents on, 148 ; of Chlorine, acids, boiling water, etc., on, 151 ; of combustion of coal-gas on, 153 ; effect of damp air on, 153 ; theory of mode of washing Positives, 162 ; com- parative permanency of prints, 166; mode of testing perma- nency, 169. Positives, definition of, 106 ; Col- lodion Positives, theory of pro- duction of, 108 ; formula3 for solutions for, 201 ; development of. 111, 221 ; Collodion and Nitrate Bath best adapted for, 109 ; mode of whitening by Bi- chloride of Mercury, 112 ; so- lution for whitening, 207 ; mode of backing up, 226 ; spots and markings on, 284 ; mode of printing Positives on Collodion, 272. Positives, enlarged, mode of print- ing, 272. Practice of Collodion process, 183. Preservative processes for Collo- dion plates, 289. Printing, Photographic, theory of, 120 ; practical details of, 240. Prism, refraction of light by, 51 ; diagram of formation of spec- trum by, 47 ; explained, 54. Prismatic spectrum, 47, 61. Protonitrate of Iron, preparation of, 206 ; a feeble developer when free from excess of Sulphate of Iron, 98 ; theory of its mode of action, and rules for its use, 112 ; cannot be prepared in quantity by adding Nitrate of Potash to Sulphate of Iron, 314 ; some- times requires the addition of Nitrate of Silver, 206. Protosulphate of Iron, its prepara- tion and properties, 29 ; its cha- INDEX. 389 racteristics as a developer for Collodion Positives, 111 ; not well adapted for developing Col- lodion Negatives, 117 ; prepara- tion of the solution for Positives, 205 ; mode of applying it to the plate, 221 ; to remove iron stains on glass, 215. Pyrogallic Acid, its preparation and properties, 28 ; solution for developing glass Positives, 205 ; for Negatives, 211 ; cannot be used without Acetic Acid, 105 ; less adapted for developing 'paper pictures, 178 ; requires addition of Nitric Acid when used for Positives, 111 ; superior to Sul- phate of Iron for developing Negatives, 117, 144 ; mode of obviating the brown discolora- tion of developing solutions, 212. Pyroxyline, its nature and proper- ties, 75 ; preparation of, by Mr. Hadow's formula, 186 ; by a rule-of-thumb mixture of the acids, 188 ; by the Oil of Vitriol and Nitre process, 190 ; details of immersing, washing, and dry- ing, 191 ; the glutinous variety produced by cold acids, 83 ; re- capitulation of the effects of varying the strength of the acid mixture, 193 ; spontaneous de- composition of Pyroxyline, 166. Reduction of metallic oxides by developers, theory of, 26 ; of Silver salts by developers, tlie- ory of, 30 ; practical details of reducing Silver compounds to metallic state, 372. Saltts nature of, 310. Salts of Silver, their preparation and properties, 1 2 ; 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, 134; its practical details, 267 ; its advantages, 271 ; gives permanent prints, 167. Sensitiveness, term explained, 92 ; conditions favourable to, 97- Sensitiveness of Collodion film, causes influencing, 92 ; superior sensitiveness partially explained, 74 ; preservation of sensitive film, 288. Sensitive paper, theory of prepara- tion of, 22, 122; its darkening by light described, 123 ; prepara- tion of Albuminized paper, 241 ; of plain paper, 245 ; of Ammo- nio-Nitrate paper, 246, 258 ; of paper for Negative processes, 259 ; causes which afi'ect the sensitiveness of Positive paper, 123 ; w^hich alter the colour of the image, 126 ; spots and mark- ings on, 285 ; a large excess of Nitrate of Silver essential, 124 ; the paper should not be kept too long, 130, 286. Serum of Milk, preparation of, 262, 355 ; used in Negative printing process, 262. Shadbolt, Mr., his Honey keeping process, 289; employs artificial light in Micro-Photography, 237. Silver, properties of, 362 ; estima- tion of, in 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, 47, 61. Soluble paper. See Pyroxyline. Specific gravity of liquids, mode of finding, 375. 390 INDEX. Spherical aberration, 56. Spirits of Wine, preparation and properties, 330 ; not always suf- ficiently strong for Collodion, 84 ; mode of rectifying, 196 ; sometimes contaminated with, fusel oil, 96. Spots on Collodion plates, 279 ; on paper Positives, 285 ; on prints obtained by development, 266 ; on preserved Collodion plates, 293. Stains, Silver, removal of, 377. Stereoscope, invention of, 67 ; the- ory of, 68 ; Wheatstone's, 69 ; Brewster's, 70. Stereoscopic Photographs, rules for taking, 71 ; practical details of, 232. Stops, theory of use of, 57 ; simple mode of making, 228 ; position of the stop often important, 230. Strength of acids, tables of, 378. Subchloride of Silver, its prepara- tion and properties, 15 ; decom- posed by fixing agents, 141. Suboxide of Silver, its properties, 18 ; forms compounds with or- ganic matters. Citric Acid, Al- bumen, etc., 21. Substitution, laws of, explained, 78. Sulphate of Iron. See Protosulphate of Iron. Sulphate of Quinine, absorption of chemical rays by, 65. Sulphuric Acid, table of strength of, 378. Sutton, Mr., theory of Sel d'or toning process, 134 ; practical details of, 267 ; Negative print- ing process, 262 ; preparation of Serum of Milk for, 355. Symbols, use of, 318. Syruped Collodion film, 289. Talbot, Mr., his discoveries, 9 ; the- ory of Calotype process, 176. Taupenot, M., his CoUodio-Albu- men process, 181 ; practical de- tails of, 294. Temperature, its efi'ect upon de- velopment of Collodion film, 102 ; upon fogging, 105 ; upon keeping Collodion, 210 ; upon action of fixing Bath for paper Positives, 130 ; upon Hypo to- ning Bath, 136. Test-papers, use of, 376. Toning Bath for Positives, with Sel d'or, 134, 267 ; with Hyposul- phite and Gold, see Pixing and Toning Bath ; with Chloride of Iron and Hyposulphite, 160. Toning of Positives, term defined, 121 ; may injure the stability of the proof, 154 ; points to be kept in view to avoid fading, 167 ; manipulatory details of, 253 ; by Sel d'or, 267. Transparencies, mode of printing, 273. Varnishes for Collodion Photo- graphs, 226. View Lenses, directions for using, 230. Vocabulary of Photographic che- micals, 327. Washing Positive prints, rules for, 162 ; details of, 255. Waxed paper process, theory of, 178. Weights and Measures, table of, 379. FEINTED BY JOHIf EDWARD TAYLOR, LITTLE QUEEN STREET, LINCOLN'S INN FIELDS. ^ , 5 & ^ \! \J in ^ S P G o 'S W Q fa H > ON ^ "2 HHeEITYCEtO