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diff --git a/old/63710-0.txt b/old/63710-0.txt deleted file mode 100644 index 72701d1..0000000 --- a/old/63710-0.txt +++ /dev/null @@ -1,14577 +0,0 @@ -The Project Gutenberg EBook of A Manual of Photographic Chemistry: -Including the Practice of the Collo, by T. Frederick Hardwich - -This eBook is for the use of anyone anywhere in the United States and most -other parts of the world at no cost and with almost no restrictions -whatsoever. You may copy it, give it away or re-use it under the terms of -the Project Gutenberg License included with this eBook or online at -www.gutenberg.org. If you are not located in the United States, you'll have -to check the laws of the country where you are located before using this ebook. - -Title: A Manual of Photographic Chemistry: Including the Practice of the Collodion Process - -Author: T. Frederick Hardwich - -Release Date: November 12, 2020 [EBook #63710] - -Language: English - -Character set encoding: UTF-8 - -*** START OF THIS PROJECT GUTENBERG EBOOK MANUAL OF PHOTOGRAPHIC CHEMISTRY *** - - - - -Produced by Tom Cosmas produced from files generously -provided on The Internet Archive. All resultant materials -are placed in the Public Domain. - - - - - - - - - - - - -Transcriber Note - -Text emphasis denoted as _Italics_ and =Bold=. Whole and fractional parts -of numbers as 123-4/5 and some fractions as 1/50th. - - - A MANUAL - - OF - - PHOTOGRAPHIC CHEMISTRY. - - - A MANUAL - - OF - - PHOTOGRAPHIC CHEMISTRY, - - INCLUDING THE - - PRACTICE OF THE COLLODION PROCESS. - - - By - - T. FREDERICK HARDWICH, - - LECTURER ON PHOTOGRAPHY IN KING'S COLLEGE, LONDON; - LATE DEMONSTRATOR OF CHEMISTRY IN KING'S COLLEGE. - - Fourth Edition. - - [Illustration] - - LONDON: - - JOHN CHURCHILL, NEW BURLINGTON STREET. - - MDCCCLVII. - - -[_The Author reserves to himself the right of translating this Edition._] - - -PRINTED BY JOHN EDWARD TAYLOR, LITTLE QUEEN STREET, LINCOLN'S INN FIELDS. - - - - -PREFACE TO THE THIRD EDITION. - -[Illustration] - - -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 -afforded of the rapid advance which the Photographic Art is now making in -this country. - -On once more entering upon the task of revision, the Writer has been led -to reflect in what way the utility of the Work may be promoted; and from -numerous inquiries he believes that this result will best be attained -by carefully omitting everything which does not possess _practical_ as -well as scientific interest. The majority of Photographers 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 observation, but simply to distinguish -between the essential and the non-essential in the theory of the subject. - -The present Edition differs in many important particulars from those which -have preceded it. It has undergone a fresh arrangement throughout. In some -parts it is condensed, in others enlarged. The Chapters on Photographic -Printing are entirely re-written, and include the whole of the 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 -particulars. - -Another point which has been kept in view, is to recommend, 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 materials near at -hand; and, if the common impurities of the commercial articles are pointed -out, and directions given for their removal, the 'London Pharmacopœia' -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 present Edition, which -is now so complete that a reference to it will at once point out the most -important facts relating 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. - -[Illustration] - - -The Author has endeavoured to keep pace with the improvements which are -daily being introduced in the science and art of Photography. In the -present Edition alterations have been made in the style and general -arrangement of the work, and additional matter has been inserted. - -Since the publication of the Third Edition, a series of experiments have -been made on the manufacture of Collodion, 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 Collodion" 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 6th, 1857._ - - -ERRATA. - - Page 24, line 5, _for_ conditions _read_ condition. - Page 115, line 32, _for_ Iodide _read_ Iodine. - Page 194, line 15, _for_ p. 88 _read_ p. 188. - -[_Transcriber Note--Corrections have been applied_] - - - - -CONTENTS. - - - -PART I. - -THE SCIENCE OF PHOTOGRAPHY. - -Page - -Introduction 1 - - -CHAPTER I. - -Historical Sketch of Photography 6 - - -CHAPTER II. - -THE SALTS OF SILVER EMPLOYED IN PHOTOGRAPHY. - - Section I.--_The Chemistry of the Salts of Silver._--The preparation - 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 - Chloride, Bromide, and Iodide of Silver.--The blackening of - Chloride of Silver explained.--Simple experiments with sensitive - Photographic Paper 18 - - -CHAPTER III. - -ON THE DEVELOPMENT OF AN INVISIBLE IMAGE. - - Simple experiments illustrating the process 25 - - Section I.--_Chemistry of the Substances, employed as - Developers._--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 Developing - Agents._--Reduction of Oxide of Silver--of Nitrate and Acetate - of Silver.--Varied appearance of Metallic Silver when finely - divided.--The reduction of the Iodide and other Salts of Silver - containing no Oxygen 30 - - Section III.--_Hypothesis on the Formation and Development of - the Latent Image._--Explanation of the terms under- and - over-exposure.--Diagram of Molecular change.--Moser'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 - Fixing Agents.--Ammonia, Alkaline Chlorides, Bromides, and - Iodides.--Hyposulphite of Soda.--Cyanide of Potassium 41 - - -CHAPTER V. - -ON THE NATURE AND PROPERTIES OF LIGHT. - - Section I.--_The compound Nature of Light._--Its decomposition 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.--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 Photographed 60 - - Section V.--_On Binocular Vision and the Stereoscope._--Phenomena - of Binocular Vision.--Theory of the Stereoscope.--Wheatstone's - reflecting Stereoscope.--Brewster's Stereoscope.--Rules for taking - Stereoscopic pictures 66 - - -CHAPTER VI. - -THE PHOTOGRAPHIC PROPERTIES OF IODIDE OF SILVER UPON COLLODION. - - Section I.--_Collodion._--Pyroxyline--its varieties--its chemical - composition.--Means of obtaining Nitro-Sulphuric Acid of the - proper strength.--Solvents for Pyroxyline.--Variation of physical - properties in different samples of Collodion.--The changes which - Iodized Collodion undergoes by keeping 75 - - Section II.--_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 - Process._--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 Nitrate - Bath best suited for Positives.--Peculiarities of Pyrogallic - 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 Nitrate - Bath best adapted for Negatives.--Use of Glycyrrhizine to increase - the intensity.--Developing solutions for Negatives.--Conversion of - finished Positives into Negatives 113 - - -CHAPTER VIII. - -ON THE THEORY OF POSITIVE PRINTING. - - Section I.--_The Preparation of the Sensitive Paper._--Its darkening - by light.--The conditions which affect its sensitiveness 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 of damp air upon Positive - Prints.--The change in properties of the fixing Bath by constant - use 140 - - Section IV.--_The Fading of Photographic Prints._--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 - film.--Development of the Latent Image.--The strengthening 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 II. - -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. - -FORMULÆ FOR SOLUTIONS REQUIRED FOR COLLODION PHOTOGRAPHS. - - Section I.--_Formulæ for direct Positive Solutions._--The - Collodion.--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 OF THE COLLODION PROCESS. - - Section I.--_Manipulations with moist - Collodion._--Cleaning the Plates.--Coating with - Collodion.--Exciting.--Exposing.--Developing. --Fixing 213 - - Section II.--_Simple directions for the use of Photographic - Lenses._--Portrait Lenses.--View Lenses.--Mode of finding the - chemical Focus 227 - - Section III.--_On copying Engravings, Etchings, Diagrams, etc._--Mode - of intensifying the Collodion 231 - - Section IV.--_Rules for taking Stereoscopic Photographs._--Mr. - Latimer Clark's arrangement for working with a single Camera 232 - - Section V.--_The Photographic delineation of Microscopic - Objects.-_-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 by the ordinary process._--Selection - 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 by Development._--A process on - Chloride and Citrate of Silver.--On Iodide of Silver.--On Bromide - of Silver 259 - - Section III.--_The Sel d'Or Process for toning - Positives._--Preparation of the toning Bath.--Manipulatory details - 267 - - Section IV.--_On Printing enlarged Positives, Transparencies, etc., - upon Collodion_ 272 - - -CHAPTER V. - -CLASSIFICATION OF CAUSES OF FAILURE IN THE COLLODION PROCESS. - - Section I.--_Imperfections in Negative and Positive Collodion - Photographs._--Fogging.--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.--Photography on dry - Collodion.--Taupenot's Collodio-Albumen Process 288 - - - - -PART III. - -OUTLINES OF 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.--Symbolic Notation.--The Laws of - Combination.--The Atomic Theory.--The Chemistry of Organic Bodies - 305 - - -CHAPTER II. - -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 Filtration.--The use - of Test-papers.--The removal of Silver stains from the hands, - etc.--Dr. Ure's tables of the strength of Sulphuric and Nitric - Acid of different densities.--Table of Weights and Measures 371 - - - - -A MANUAL - -OF - -PHOTOGRAPHIC CHEMISTRY. - -[Illustration] - - - - -INTRODUCTION. - - -In attempting to impart knowledge on any subject, it is not sufficient -that the writer should 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 fundamental 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 remarked, 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 unexplored, there is great danger -of erroneously attributing effects to their wrong causes! Perhaps none but -he 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 infinitesimally 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 -subject before proceeding to the practice of it. To assist this object, -care will be taken to avoid needless complexity in the formulæ, 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 unacquainted 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-MATTER OF EACH. - -The title given to the Work is "A Manual of Photographic Chemistry," and -it is proposed to include in it a 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 properties 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, with -its application to artistic purposes; all mention of manipulatory details, -and of quantities of ingredients, being, as a rule, omitted. - -In this division of the Work will be found nine Chapters, the contents of -which are as follows:-- - -Chapter I. is a sketch of the history of Photography, intended to convey a -general notion of the origin and progress of the Art, without dwelling on -minute particulars. - -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 -properties 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 impressions, in order to -render them indestructible by diffused light. - -Chapter V. contains a sketch of the _Optics_ of Photography--the -decomposition of white Light into its elementary rays, the Photographic -properties of the different colours, 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 VI. 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 action of the -developing solutions in bringing out the image. - -Chapter VII. continues the same subject, describing the classification of -Collodion Photographs as Positives and Negatives, with the distinctive -peculiarities of each. - -Chapter VIII. 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 precautions 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 Photographic 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," explains itself. Attention however may be -invited to the fifth Chapter, in which a classification is given of the -principal imperfections in Photographs, with short directions for their -removal; and to Chapter VI., which describes the preservation of the -sensitiveness of Collodion plates and the mode of operating upon films of -Albumenized Collodion. - -In Part III. will be found, in addition to a statement of the laws -of chemical combination, etc., a list of Photographic chemicals, -alphabetically arranged, including their preparation and properties as far -as required for their employment 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 prepared in a state of purity. - - - - -CHAPTER I. - -HISTORICAL SKETCH OF PHOTOGRAPHY. - - -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, without 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 before their eyes, do not -appear to have directed their attention 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 undergone 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-quarters of a century ago, viz. in 1777. It was also -remarked by him that some of the coloured rays of Light were peculiarly -active in promoting the change. - -_Earliest application of these facts to purposes of Art._--The first -attempts to render the blackening of Silver Salts by Light available 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 Nitrate 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 appeared 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 diffused Light. The unchanged Silver Salt being -permitted to remain in the white portions of the paper, naturally caused -the proofs to blacken in every part, unless carefully preserved in the -dark. - -_Introduction of the Camera Obscura, and other Improvements in -Photography._--The "Camera Obscura," or darkened 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 affected by the subdued light of -that instrument. - -In the year 1814, however, twelve years subsequent to the publication of -Wedgwood's paper, M. Niépce, of Chalons, having directed his attention to -the subject, succeeded in perfecting a process in which the Camera could -be employed, although the sensibility was still so low that an exposure of -some hours was required to produce the effect. - -In the process of M. Niépce, which was termed "Heliography," or -"sun-drawing," the use of the Silver Salts was discarded, and a resinous -substance, known as "Bitumen of Judæa," 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 properties, that it -became _insoluble_ in certain essential oils. Hence, on subsequent -treatment with the oleaginous solvent, the shadows dissolved away, and -the _lights_ were represented by the unaltered resin remaining on the -plate. - -_The Discoveries of M. Daguerre._--MM. Niépce and Daguerre appear at one -time to have been associated as partners, for the purpose of mutually -prosecuting their researches; but it was not until after the death of the -former, viz. in 1839, that the process named the Daguerreotype was given -to the world. Daguerre was dissatisfied with the slowness of action of the -Bitumen sensitive surface, 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, although far inferior -to those subsequently produced, possessed a beauty which had not been -attained by any Photographs prior to that time. - -The sensitive plates of Daguerre were prepared by exposing 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 feature, viz. the production -of a _latent_ image upon Iodide of Silver, with its subsequent development -by a chemical reagent, 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 in fixing his proofs, by removal of the unaltered -Iodide of Silver from the shadows. The processes employed however were -imperfect, and the matter was not set at rest until the publication of a -paper by Sir John Herschel, on the property possessed by "Hyposulphites" -of dissolving the Salts of Silver insoluble in water. - -_On a means of Multiplying Photographic Impressions, and other Discoveries -of Mr. Fox Talbot._--The first communication made to the Royal Society -by Mr. Fox Talbot, in January, 1839, included only the preparation of a -sensitive paper for copying objects by application. It was directed 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 cardinal point in Mr. Talbot's -invention, and one of great importance. - -The patent issued for the process named Talbotype or Calotype dates from -February, 1841. A sheet of paper is first coated with Iodide of Silver by -soaking it alternately 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 Silver), 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 invisible -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 attributable 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 -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 John Herschel for the first employment 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. Niépce de Saint-Victor, nephew -to the original discoverer 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 transparent layer, resembling -gold-beater'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 countryman, 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 _Pyro_-gallic 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 progress of the Art; but experience has -now abundantly demonstrated, 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. - - - - -CHAPTER II. - -THE SALTS OF SILVER EMPLOYED IN PHOTOGRAPHY. - - -By the term Salt of Silver we understand that the compound in question -contains Silver, but not in its elementary form; the metal is in fact in -a state of chemical union with other elements which disguise its physical -properties, so 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 crystalline body, easily soluble in water, the solution -possessing an intensely sweet taste; chemical tests prove that it contains -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 characters 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 -experiments illustrating the formation of the Photographic image. - - -SECTION I. - -Chemistry of the Salts of Silver. - -The principal Salts of Silver employed in the Photographic processes are -four in number, viz. Nitrate of Silver, Chloride of Silver, Iodide of -Silver, and Bromide of Silver. In addition to these, it will be necessary -to describe 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 corrosive 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 -violent 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 combination with the metal. - -If we closely examine the effects produced by treating Silver with Nitric -Acid, we find them to be of the following 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.[1] - -[1] The preparation of Nitrate of Silver from the standard coin of the -realm is described in Part III., Art. "Silver." - -It is the instability of Nitric Acid therefore--its proneness 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 Nitrate 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° -Fahrenheit. 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 Photography, until rendered neutral by -fusion and a second crystallization. - -Pure Nitrate of Silver occurs in the form of white crystalline 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 decomposed, 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 to test-paper, from -the presence of minute quantities of Nitrite or basic Nitrite of Silver.[2] - -[2] Nitrite of Silver differs from the Nitrate 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. - - -THE CHEMISTRY OF THE CHLORIDES OF SILVER. - -_Preparation of Protochloride 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,[3] -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. - -[3] For the properties of the element "Chlorine," see the third division -of the Work. - -_Preparation of Chloride of Silver by double decomposition._--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 Chlorine leaves the -Sodium with which it was previously combined, and crosses over to the -Silver; the Oxygen and Nitric Acid are released from the Silver, and unite -with the Sodium; thus - - Chloride of Sodium _plus_ 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 preparation of -Chloride of Silver, are simply that the first should contain Chlorine, -the second Silver, and that both should be soluble in water; hence the -Chloride of Potassium 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 resembling common chalk or whiting. It is -tasteless and insoluble in water; unaffected by boiling with the strongest -Nitric Acid, but sparingly dissolved by concentrated Hydrochloric Acid. - -Ammonia dissolves Chloride of Silver freely, as do solutions 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 semi-transparent substance, which has been termed -_horn 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 Subchloride of Silver._--If a plate -of polished Silver be dipped in solution of Perchloride of Iron, or of -Bichloride of Mercury, a _black stain_ is produced, the Iron or Mercury -Salt losing a portion of Chlorine, which passes to the Silver and converts -it superficially into Subchloride of Silver. This compound differs from -the white Chloride of Silver in containing less Chlorine; the composition -of the latter being represented by the formula AgCl, that of the former -may perhaps be written as Ag{2}Cl(?). - -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 insoluble metallic Silver. - - -THE CHEMISTRY OF IODIDE OF SILVER. - -The properties of _Iodine_ are described in the third division 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 Nitrate of Silver. - -When prepared by the latter mode it forms an impalpable 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 water and in -dilute Nitric Acid. It is scarcely dissolved by Ammonia, which serves -to distinguish it from the Chloride 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 PREPARATION AND PROPERTIES 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 insoluble substance, slightly yellow in colour, and -distinguished 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 Potassium. - -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, collects 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 dissolved by Nitric or Acetic Acid, forming a neutral -Nitrate 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 converts it into a black substance, which is -probably a Suboxide. - -_The Suboxide of Silver_ (Ag{2}O?)--This substance was obtained by Faraday -on exposing a solution of the Ammonio-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 Subchloride 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. - - -SECTION II. - -_On the Photographic 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 Citrate of Silver, etc. Some occur in crystals -which are soluble 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 diffused -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 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 phenomena produced in this case are well illustrated by dipping a -pledget of cotton-wool, or a sheet of white paper, in solution of Nitrate -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 absorb Oxygen_; hence pure -vegetable fibre, free from Chlorides, 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 by Light._--Pure -moist Chloride of Silver[4] 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. - -[4] 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. - -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 as a Photographic agent, but a Chloride with excess of -Nitrate is very sensitive. Even Iodide of Silver, ordinarily unaffected, -is blackened by light when moistened with a solution of the Nitrate of -Silver.[5] - -[5] The reader will understand that the Acetate, Sulphate, or any other -soluble Salt of Silver, might be substituted for the Nitrate in this -experiment. - -Organic matter combined with Chloride and Nitrate of Silver gives a still -higher degree of sensibility, and in this way the Photographic papers are -prepared. - -_The blackening of Chloride of Silver by Light explained._--This may -be studied by suspending pure Chloride of Silver in distilled water, -and exposing it to the sun's rays for several days. When the process of -darkening has proceeded to some extent, the supernatant liquid is found to -contain _free Chlorine_, or, in place of it. _Hydrochloric Acid_ (H Cl), -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 portion of Chlorine is separated, and -the white Protochloride is converted into the violet _Sub_chloride 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 influence upon the darkening of Chloride of -Silver, by rendering 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 _deoxidation_, the Protoxide of Silver -losing a portion of its Oxygen, and a Suboxide of Silver, the product -of the reduction, remaining in union with the oxidized Albumen. The red -compound may therefore be loosely designated as an Albuminate of Suboxide -of Silver. - -_Gelatine_ does not precipitate Nitrate of Silver in the same manner as -Albumen: but if a sheet of transparent Gelatine be allowed to imbibe a -solution of the Nitrate, it becomes 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. - -Caseine, the animal principle of milk, is coagulated by Nitrate of Silver, -and the red substance formed on exposing the curds to light may be viewed -as analogous in composition 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 Wöhler's Citrate of Suboxide of -Silver, which he obtained 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.[6] - -[6] 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. - - -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 way, the 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 surface_, in order that -the various particles of which it is composed may each one individually be -brought into relation with the disturbing force. - -Full directions for the preparation of sensitive Photographic 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 Nitrate of Silver, the Silver bath having been purposely made -stronger in proportion than the salting solution. - -_Illustrative Experiment No. I._--Place a square of sensitive 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 probably reached in from three -to five minutes; but the sensibility of the paper, and also the nature of -the tints, will vary much with the character of the organic matter present. - -_Experiment No. II._--Lay a device cut from black paper upon a sheet -of sensitive paper, and compress the two together 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 _white_ figure upon a dark ground. - -_Experiment No. III._--Repeat the last experiment, substituting a piece -of lace or gauze-wire for the paper device. This is intended to show the -minuteness with which objects can be copied, since the smallest filament -will be distinctly represented. - -_Experiment 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 accurately maintained, and a -pleasing gradation of tone produced. - -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 estimated by chemical reagents. This is well shown by the results 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 reference to this amongst other advantages, and secures a better and -more sharply defined print. - - - - -CHAPTER III. - -ON THE DEVELOPMENT OF AN INVISIBLE IMAGE BY MEANS OF A REDUCING AGENT. - - -It has been shown in the previous Chapter that the majority 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 condition of _Sub_salts. - -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, before 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 Photographic 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 effect will be developed upon the -Iodide paper. Iodide of Silver therefore is the salt commonly used when -sensibility 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 the 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. No -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 observed; 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 Gallic Acid to _develope_ or -bring out to view an invisible image, in preference 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 -experiments conducted by M. Claudet in the Daguerreotype process: he found -that with a sensitive layer of Bromo-Iodide 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. - -To increase the sensitiveness of Photographic preparations 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 advancement, -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 reducing 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 Oxygen which the particular -metal under treatment possesses. In this respect there is considerable -difference, as may be shown by a reference to the two well-known metals, -Iron and Gold. 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 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 belong to the class of _noble_ 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 develope 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.[7] - -[7] 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. - -The most important of the developers are as follows:-- Gallic Acid, -Pyrogallic Acid, and the _Proto_salts of Iron. - - -CHEMISTRY OF GALLIC AND PYROGALLIC 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 puncture 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 analogous 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 oxidation_ 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 account 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. The aqueous solution becomes mouldy on keeping, -to obviate 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 element 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 sublimate forms, which condenses in lamellar crystals; this is -Pyrogallic Acid. - -Pyrogallic Acid is very soluble in cold water, and in Alcohol 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 _neutral_; 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 Oxygen; hence this mixture may conveniently be employed for absorbing -the Oxygen contained in atmospheric air. The compounds of Silver and Gold -are reduced by Pyrogallic Acid even more rapidly than by Gallic Acid, the -reducing agent absorbing the Oxygen, and becoming converted 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 substance known as _Metagallic Acid_, -which is formed when the heat is raised above the proper temperature in -the process of manufacture. - - -CHEMISTRY OF THE PROTOSALTS OF IRON. - -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, Fe O. - Peroxide of Iron, Fe{2}O{3}. - -The Proto- and Persalts of Iron do not resemble each other in their -physical and chemical properties. The former 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 Protosulphate of Iron, and, having reduced -it to powder, pour a little Nitric Acid upon it in a test-tube. On the -application 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 _Proto_sulphate entirely into a _Per_sulphate 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. _Protosulphate of Iron._--This salt, often termed _Copperas_ or _Green -Vitriol_, is an abundant substance, and used for a variety of purposes in -the arts. Commercial Sulphate of Iron however, being prepared on a large -scale, requires re-crystallization to render it sufficiently pure for -Photographic purposes. - -Pure Sulphate of Iron occurs in the form of large transparent, prismatic -crystals, of a delicate green colour: by exposure to the air they -gradually absorb Oxygen and become 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_ Persulphate 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 quantity 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 with 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. - -_The Reduction of Salts of Silver by 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. - -_Reduction of Oxide of Silver._--To illustrate this conveniently, 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-Nitrate 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 reducing 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. - -_Reduction 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 affinities. 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 Gallic or Pyrogallic Acid be employed, it is a black powder;[8] whilst -the salts of Iron, and especially the same with free Nitric Acid added, -produce a sparkling precipitate, resembling what is termed _frosted -silver_. Grape Sugar and many of the essential oils, such as the Oil of -Cloves, etc., separate the metal from Ammonio-Nitrate of Silver in the -form of a brilliant mirror film, and are often employed in silvering glass. - -[8] Silver precipitated by Gallic or Pyrogallic Acid does not appear to be -free from organic matter, and probably contains also a small proportion of -Oxygen. - -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 _Hydracid_ 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 -characterized by forming acids with Hydrogen, which acids are hence called -_Hydr_acids. Hydrochloric Acid (HCl) is an example; so also is Hydriodic -Acid (HI). - -The reduction of the Hydracid Salts requires to be discussed separately, -because it is evidently different 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_, composed of Oxygen -and Hydrogen, takes a part in the reaction. 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 -_number_ of the atoms concerned is omitted, may assist the comprehension -of the change. - -[Illustration: - - Compound Atom of Compound Atom Atom of - Chloride of Gold. of Water. Sulphate of Iron. -] - -The symbol Au represents Gold, Cl Chlorine, H Hydrogen, and O Oxygen. -Observe that the molecules H and O separate from each other and pass in -opposite directions: the latter unites with the Sulphate of Iron; the -former meets Cl, 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 associate with the Iodide an atom -of water to furnish the Oxygen. 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 without 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 the compound which blackens on the application of Sulphate of Iron or -Pyrogallic acid is an Iodide with excess of Nitrate of Silver. - -[Illustration: - - Compound Atom of Compound Atom of Atom of - Iodide of Silver. Nitrate of Silver. Sulphate of Iron. -] - -The mode in which a Salt of Silver, such as the Nitrate, 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 Nitrate of Silver contains a molecule of -Oxygen for the developer, one of Silver (Ag) for the separated Iodine, and -an atom of Nitric Acid (NO{5}), 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 -elementary 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.[9] - -[9] 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 _subsalt_ similar to that -produced 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. - - -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 resulted in the separation -of elements like Chlorine and Oxygen 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 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 correct, but -it requires some explanation. The effects produced 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 -shortened 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 -brightest lights 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 artistic 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 particles 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 appearance of the film, or in its -solubility in Hyposulphite of Soda. - -The following diagrams may perhaps be useful in mechanically illustrating -what is meant by a molecular change. - -Fig. 1 represents a compound molecule of Iodide of Silver, the component -atoms of which are closely associated. - -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. - -[Illustration: Fig. 1.] - -[Illustration: Fig. 2.] - -Now the effect produced on this combination by a developer 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 particles 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 developing solution and Nitrate of -Silver, even after the Iodide has been removed by a fixing agent. The -following experiment will serve to illustrate this.-- - -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 which -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 removed. 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 deposit; this deposit however has no affinity for the -unaltered 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 remarkable phenomena of occasional occurrence in -the Collodion 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 polished surface has -been touched in particular parts by anybody, it acquires the property of -precipitating certain vapours 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 minutes: then remove, allow to -cool, and _breathe_ upon the glass, when the outlines of the device will -be distinctly seen. A plate of polished 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 settling 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 receives a very sharp and -perfect impression under the action 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 contact 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 condition of surface which causes a -vapour to settle in a peculiar 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 very delicate layer of Iodized Collodion and -passed through the Nitrate bath, the solution of Pyrogallic acid will -commonly produce a well-defined outline of the figure even before the -plate has been exposed to the light. This experiment, although it does -not invariably succeed, is nevertheless 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 probably so modify the layer of Iodide of Silver, that the -action 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 Daguerreotype 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 beyond a certain time, it -then diminishes this power and at length takes it away altogether." This -is precisely in accordance 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 occasionally met with, -in which on the application of the Pyrogallic 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 appearance 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 case is probably the remains of a -previous impression which, although apparently removed by washing, had -nevertheless modified the surface of the glass so as to affect the layer -of Iodide of Silver; and if the glass were _breathed_ upon before again -coating it with Collodion, there is every reason to suppose that the -outlines of the accidental image would be seen.[10] - -[10] 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 Hydrofluoric 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 upon 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. - - - - -CHAPTER IV. - -ON FIXING THE PHOTOGRAPHIC IMAGE. - - -A sensitive layer of Chloride or Iodide of Silver on which an -image has been formed, either with or without 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 -remove 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 upon 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 employed. In that case the darkened surface, -not being reduced perfectly to the metallic state, remains soluble to a -certain extent in the fixing liquid. - - -CHEMISTRY OF THE VARIOUS FIXING AGENTS. - -The following will be mentioned:--Ammonia--Alkaline Chlorides--Alkaline -Iodides--Alkaline Hyposulphite--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 necessarily 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," presently to be explained: a peculiar and -unpleasant red tint is always caused by Ammonia acting upon the darkened -material of a sun picture as it comes from the printing-frame: but this is -obviated by the employment of the Gold. - - -ALKALINE CHLORIDES, IODIDES, AND BROMIDES. - -The Chlorides of Potassium, Ammonium, and Sodium possess the property of -dissolving a small portion of Chloride 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 allowing the liquid -to evaporate spontaneously. - -The earlier Photographers employed a saturated solution 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 described. - -It is important to remark in the solution of the insoluble Silver Salts -by Alkaline Chlorides, Iodides, etc., that 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 substance -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 produces 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 ("υρο," under). - -The Hyposulphite of Soda commonly employed by Photographers 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 translucent 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 -decomposition_ always takes place; thus:-- - - Hyposulphite of Soda + Chloride of Silver - = Hyposulphite of Silver + Chloride of Sodium. - -The Hyposulphite of Silver with an excess of Hyposulphite of Soda forms -a soluble double salt, which may be crystallized out by evaporating the -solution. It possesses 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 peculiar chemical changes, as will be better shown in -Chapter VIII. - -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 _Potassium_ 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 the air. It is very -soluble in water, but the solution decomposes 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 dissolve -it. - - - - -CHAPTER V. - -ON THE NATURE AND PROPERTIES OF LIGHT. - - -The present Chapter is devoted to a discussion of the more remarkable -properties of Light; the object being to select certain prominent points, -and to state them as clearly as possible, referring, for information of a -more complete kind, 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 refraction 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 I. - -_The 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 -intermingled. - -This fact may be demonstrated by throwing a pencil of Sunlight upon one -angle of a _prism_, and receiving the oblong image, so formed, upon a -white screen. - -The space illuminated and coloured by a pencil of rays 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 the -Solar Spectrum, viz. red, orange, yellow, green, blue, indigo, and violet. -Sir David Brewster has made observations 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 yellow -spaces intermingled constitute _orange_; the yellow and blue spaces, -_green_. - -[Illustration] - -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 employed -cannot possibly be obtained of the proper tints or laid on in the exact -proportions. - -The decomposition of light is effected in other ways besides that already -given:--- - -First, by _reflection_ form 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 termed _white_ reflect all the 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 constituting white light -to pass; but by the addition of certain metallic oxides to it whilst in a -state of fusion, its properties 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 certain of the elementary colours. - -The _yellow_ is decidedly the most luminous ray. On examining the Solar -Spectrum, it is seen that the brightest part is that occupied by the -yellow, and that the light diminishes 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. - -The _heating properties_ of the sunlight reside principally in the red -ray, as is shown by the expansion of a mercurial 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 influence 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 referred. - - - -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 _refracted_, or bent out of the course which it originally pursued. 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. - -[Illustration] - -The dotted line is drawn perpendicularly to the surface, and it is -seen that the 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 medium, -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. - -Notice 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 law of bending _towards_ the perpendicular -on entering, and away from it on leaving the dense medium, is in each case -correctly observed. - -[Illustration: Fig. 1.] - -[Illustration: Fig. 2.] - -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, diverts them further asunder. - -[Illustration: Fig. 3.] - -[Illustration: 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. - -[Illustration: Fig. 1.] - -[Illustration: Fig. 2.] - -[Illustration: Fig. 3.] - -Fig. 1 is a biconvex lens; fig. 2, a biconcave lens; and fig. 3, a -_meniscus_ lens. - -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 together 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 "principal 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." - -[Illustration] - -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. - -Therefore, although the principal focus of a Lens (as determined 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 by 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. - -[Illustration] - -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 proceeding 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. - - -SECTION III. - -The Photographic Camera. - -The Photographic 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. - -[Illustration] - -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 attained 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 is -_focussed_ upon the ground glass, which is then removed, and a slide -containing the sensitive layer inserted in its place. - -The luminous image, as formed upon the ground glass, is termed the -"Field" 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 biconvex 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 coloured 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 occupy 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:-- - -[Illustration] - -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 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 different parts of their surfaces. The following -diagram shows this:-- - -[Illustration] - -Observe that the dotted lines which fall upon the circumference 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 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 curvature 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 perpendicular double line in the -diagram. Hence it is that with most lenses, when the centre of the field -has been focussed, the glass must be shifted forwards a little to define -the outside sharply. - -[Illustration] - -_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 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 reference to the Lens -is a point of importance, and influences the flatness of the field. - -[Illustration] - -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 spherical -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 with -distant objects, or _vice versâ_. - -_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 admitted. 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. - -[Illustration] - -In this combination the front Lens is an Achromatic plano-convex, with, -the convex side turned toward the object; 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 -glasses requiring to be ground with extreme care, in order to avoid -_distortion_ of the image: hence the most rapid Portrait Lenses, having -large aperture and short focus, are often useless unless purchased of a -good maker. - -_The Variation between the Visual and Actinic Foci in Lenses._--The same -causes which produce chromatic aberration in a Lens, tend also to separate -the chemical from the visual focus. - -The violet and indigo rays are more strongly bent in than the yellow, and -still more than the red; consequently the focus for each of those colours -is at a different point. The following diagram shows this. - -[Illustration] - -V represents the focus of the violet ray, Y of the yellow, and E of the -red. - -Hence, as the chemical action corresponds more to the violet, the most -marked actinic effect would be produced at V. The luminous portion of the -spectrum however is _the yellow_, consequently the visual focus is at Y. - -Photographers have long recognized this point; and therefore, with -ordinary Lenses, not corrected for colour, rules are laid down as to the -exact distance which the sensitive plate should be shifted away from the -visual focus in order to obtain the greatest amount of distinctness of -outline in the image impressed by chemical action. - -These rules do not apply to the Achromatic Lenses recently described. The -coloured rays being in that case bent together again and reunited, the two -foci also nearly correspond. By a little further correction to a point -higher in the Spectrum, they are made to do so perfectly. - - -SECTION IV. - -_On the Photographic Action of Coloured Light._ - -It has already been mentioned in the First Section of this Chapter that -certain of the elementary colours of white light, viz. the violet and -indigo, are peculiarly active in decomposing the Photographic Salts of -Silver; but there are some points of importance relating to the same -subject which require a further notice. - -The term "actinism" (Gr. ἁκτἱς, a ray or flash) has been proposed as -convenient to designate the property possessed by light of producing -chemical change; the rays to which the effect is especially due being -known as actinic rays. - -If the pure Solar Spectrum formed by prismatic analysis in the manner -represented at page 47 be allowed to impinge upon a prepared sensitive -surface of Iodide of Silver, the latent image being subsequently developed -by a reducing agent, the effect produced will be something similar to that -represented in the following diagram:-- - -[Illustration: Fig. 1.] - -[Illustration: Fig. 2.] - -Fig. 1 shows the visible spectrum as it appears to the eye; the brightest -part being in the yellow space, and the light gradually shading off -until it ceases to be seen. Fig. 2 represents the chemical effect -produced by throwing the Spectrum upon Iodide of Silver. Observe that -the darkening characteristic of chemical action is most evident in the -upper spaces, where the light is feeble, and is altogether absent at the -point corresponding to the bright yellow spot of the visible spectrum. -The actinic and luminous spectra are therefore totally distinct from each -other, and the word "Photography," which signifies the process of taking -pictures by light, is in reality inaccurate. - -To those who have not the opportunity of working with the Solar Spectrum, -the following experiments will be useful in illustrating the Photographic -value of coloured light. - -_Experiment I._--Take a sheet of sensitive paper prepared with Chloride -of Silver, and lay upon it strips of blue, yellow, and red glass. On -exposure to the sun's rays for a few minutes, the part beneath the blue -glass darkens rapidly, whilst that covered by the red and yellow glass is -perfectly protected. This result is the more striking from the extreme -_transparency_ of the yellow glass, giving the idea that the Chloride -would certainly be blackened first at that point. On the other hand, the -blue glass appears very dark, and effectually conceals the tissue of the -paper from view. - -_Experiment II._--Select a vase of flowers of different shades of scarlet, -blue, and yellow, and make a Photographic copy of them, by development, -upon Iodide of Silver. The blue tints will be found to act most violently -upon the sensitive compound, whilst the reds and yellows are scarcely -visible; were it not that it is difficult to procure in nature pure and -homogeneous tints, free from admixture with other colours, they would make -no impression whatever upon the plate. - -In exemplifying further the importance of distinguishing between visual -and actinic rays of light, we may observe that if the two were in all -respects the same. Photography must cease to exist as an Art. It would -be impossible to make use of the more sensitive chemical preparations -from the difficulties which would attend the previous preparation and -subsequent development of the plates. These operations are now conducted -in what is termed a dark room; but it is dark only in a _Photographic_ -sense, being illuminated by means of yellow light, which, whilst it -enables the operator easily to watch the progress of the work, produces -no injurious effect upon the sensitive surfaces. If the windows of the -room were glazed with _blue_ in place of yellow glass, then it would -be strictly a "dark room," but one altogether unfitted for the purpose -intended. - -Another point connected with the same subject and worthy of note -is--the extent to which the sensibility of the Photographic compounds is -influenced by atmospheric conditions not visibly interfering with the -_brightness_ of the light. It is natural to suppose that those days on -which the sun's rays are the most powerful would be the best for rapid -impression, but such is not by any means the case. If the light is at all -of a yellow cast, however bright it may be, its actinic power will be -small. - -It will also be often observed in working towards the evening, that a -sudden diminution of sensibility in the plates begins to be perceptible at -a time when but little difference can be detected in the brilliancy of the -light; the setting sun has sunk behind a golden cloud, and all chemical -action is soon at an end. - -In the same manner is explained the difficulty of obtaining Photographs -in the glowing light of tropical climates; the superiority Of the early -months of spring over those of the midsummer; of the morning sun to that -of the afternoon, etc. April and May are usually considered the best -months for rapid impression in this country; but the light continues good -until the end of July. In August and September a longer exposure of the -plates will be required. - - -THE SUPERIOR SENSIBILITY OF BROMIDE OF SILVER TO COLOURED LIGHT. - -In copying the Solar Spectrum alternately upon a surface of Iodide and -Bromide of Silver, we notice a difference in the Photographic properties -of these two salts. The latter is affected more extensively, to a point -lower in the spectrum, than the former. In the case of the Iodide of -Silver, the action ceases in the Blue space; but with the Bromide it -reaches to the Green. This is shown in the following diagrams, which are -drawn from the observations of Mr. Crookes ('Photographic Journal,' vol. -i. p. 100):-- - -[Illustration: Fig. 1. Fig. 2. Fig. 3.] - -Fig. 1 represents the chemical spectrum on Bromide of Silver; fig. 2, the -same upon Iodide of Silver; and fig. 3, the visible spectrum. - -It might perhaps be supposed that the superior sensibility of the Bromide -of Silver to green rays of light would render that salt useful to the -Photographer in copying landscape scenery; and indeed it is the opinion of -many that, in the _Calotype_ paper process, the dark colour of foliage is -better rendered by a mixture of Bromide and Iodide of Silver than by the -latter salt alone. This however cannot depend upon the greater sensibility -of the Bromide to coloured light, as may easily be proved.-- - -The diagrams given above are shaded to represent nearly, the relative -intensity of the chemical action exerted by the rays at different points -of the spectrum; and on referring to them it will be seen that the -maximum point of blackness is in the indigo and violet space, the action -being more feeble in the blue space lower down; there are also highly -refrangible rays extending upwards far beyond the visible colours, and -these invisible rays are actively concerned in the formation of the image. - -It is evident therefore that the amount of effect produced by a pure -green, or even a light blue tint, upon a surface of Bromide of Silver is -very small as compared with that of an indigo or violet; and hence, as in -copying natural objects radiations of all kinds are present at the same -time, the green tints have not time to act before the image is impressed -by the more refrangible rays. - -Sir John Herschel proposed to render coloured light more available in -Photography by separating the actinic rays of high refrangibility, and -working only with those which correspond to the blue and green spaces -in the spectrum. This may be done by placing in front of the Camera a -vertical glass trough containing a solution of Sulphate of Quinine. -Professor Stokes has shown that this liquid possesses curious properties. -In transmitting rays of light it _modifies_ them so that they emerge _of -lower refrangibility_, and incapable of producing the same actinic effect. -Sulphate of Quinine is, if we may use the term, _opaque_ to all actinic -rays higher than the blue-coloured space. The proposition of Sir John -Herschel above referred to was therefore to employ a bath of Sulphate of -Quinine, and having eliminated the actinic rays of high refrangibility, to -work upon Bromide of Silver with those corresponding to the lower-coloured -spaces. In this way he conceived that a more natural effect might be -obtained. - -If Photographic compounds should be discovered of greater sensibility -than any we at present possess, the use of the Quinine bath will perhaps -be adopted; but at present we trust to the superior intensity of the -invisible rays for the formation of the image, and hence the employment of -Bromide of Silver is less strongly indicated. - -These remarks apply to Photographs taken by sunlight. Mr. Crookes states -that in working with artificial light, such as gas or camphine, the case -is different. Actinic rays of high refrangibility are comparatively -wanting in gas-light, the great bulk of the Photographic rays beings found -to lie within the limits of the visible spectrum, and consequently acting -more energetically upon Bromide than on Iodide of Silver. - -_Explanation of the mode in which Coloured Objects impress the Sensitive -Film._--The fact of which we have been speaking, viz. that the natural -colours are not always correctly represented in photography, is often -urged in depreciation of the art,--"when lights, are represented by -shadows," it is said, "how can a truthful picture be expected?" The -insensitiveness of Iodide of Silver to the colours occupying the lower -portion of the spectrum would indeed present an insuperable difficulty _if -the tints of Nature were pure and homogeneous:_ such however is not the -case. Even the most sombre colours are accompanied by scattered rays of -white light in quantity amply sufficient to affect the sensitive film. - -This is especially seen when the coloured body _possesses a good -reflecting surface;_ and hence some varieties of foliage, as for instance -the Ivy, with its smooth and polished leaf, are more easily photographed -than others. So again with regard to drapery in the department of -portraiture--it is necessary to attend not only to the colour, but also to -the material of which it is composed. Silks and satins are favourable, as -reflecting much light, whilst velvets and coarse stuffs of all kinds, if -at all dark, produce very little effect upon the sensitive film. - - -SECTION V. - -_On Binocular Vision and the Stereoscope._ - -An object is said to be "stereoscopic" (στρεοϛ solid, and σκοπεω, I see) -when it stands out in relief, and gives to the eye the impression of -solidity. - -This subject was first explained by Professor Wheatstone in a memoir on -binocular vision, published in the 'Philosophical Transactions' for 1838; -in which he shows that solid bodies project different perspective figures -upon each retina, and that the illusion of solidity may be artificially -produced by means of the "Stereoscope." - -The phenomena of binocular vision may be simply sketched as follows:--If a -cube, or a small box of an oblong form, be placed at a short distance in -front of the observer, and viewed attentively with the right and left eye -separately and in succession, it will be found that the figure perceived -in the two cases is different; that each eye sees more of one side of the -box, and less of the other; and that in neither instance is the effect -exactly the same as that given by the two eyes employed conjointly. - -A silver pencil-case, or a pen-holder, may be used to illustrate the same -fact. It should be held at about six or eight inches distant from the root -of the nose, and quite at right angles to the face, so that the length of -the pencil is concealed by the point. Then, whilst it remains fixed in -this position, the left and right eye are to be alternately closed: in -each case a portion of the opposite side of the pencil will be rendered -visible. - -[Illustration: Fig. 1. Fig. 2.] - -The preceding diagrams exhibit the appearance of a bust as seen by each -eye successively. - -Observe that the second figure, which represents the impression received -by the right eye, is more of a full face than fig. 1, which, being viewed -from a point removed a little to the left, partakes of the character of a -profile. - -The human eyes are placed about 2-1/2 inches, or from that to 2-5/8 -inches, asunder; hence it follows that, the points of sight being -separated, a _dissimilar_ image of a solid object is formed by each eye. -We do not however see two images, but a single one, which is stereoscopic. - -In looking at a picture painted on a flat surface the case is different: -the eyes, as before, form two images, but these images are in every -respect similar; consequently the impression of solidity is wanting. A -single picture, therefore, cannot be made to appear stereoscopic. To -convey the illusion _two_ pictures must be employed, the one being a right -and the other a left perspective projection of the object. The pictures -must also be so arranged, that each is presented to its own eye, and that -the two appear to proceed from the same spot. - -The reflecting stereoscope, employed to effect this, forms _luminous -images_ of the binocular pictures, and throws these images together, so -that, on looking into the instrument, only a single image is seen, in -a central position. It should, however, be understood, that no optical -arrangement of any kind is indispensably required, since it is quite -possible, with a little effort, to combine the two images by the unaided -organs of vision. The following diagram will make this obvious:-- - -[Illustration] - -The circles A and B represent two wafers, which are stuck on paper at -a distance of about three inches from each other. They are then viewed -by _squinting_ strongly, or turning the eyes inwards towards the nose, -until the right eye looks at the left wafer, and the left eye at the right -wafer. Each wafer will then appear to become double, four images being -seen, the two central of which will gradually approach each other until -they coalesce. Stereoscopic pictures, properly arranged, may be examined -in the same manner; and it will be found that the resultant solid image is -formed midway, at a point where two lines, drawn across from the eyes to -the pictures, cut one another. The experiment here mentioned is sometimes -a painful one, and cannot easily be made if the eyes are not of equal -strength; but it will serve to show that the essential principle resides -in the binocular representation of the object, and not in the instrument -employed to view it. - -In Mr. Wheatstone's reflecting Stereoscope _mirrors_ are used. The -principle of the instrument is as follows:-- objects placed in front of -a mirror have their reflected images apparently _behind_ the mirror. By -arranging two mirrors at a certain inclination to each other, the images -of the double picture may be made to approach until they coalesce, and the -eye perceives a single one only. The following diagram will explain this. - -[Illustration] - -The rays proceeding from the star on either side pass in the direction of -the arrows, being thrown off from the mirror (represented by the thick -black line) and entering the eyes at R and L. The reflected images appear -behind the mirror, uniting at the point A. - -The reflecting Stereoscope is adapted principally for viewing large -pictures. It is a very perfect instrument, and admits of a variety of -adjustments, by which the apparent size and distance of the Stereoscopic -image may be varied almost at pleasure. - -The "lenticular" Stereoscope of Sir David Brewster is a more portable form -of apparatus. A sectional view is given in the diagram. - -[Illustration] - -The brass tubes to which the eyes of the observer are applied contain -each a semi-lens, formed by dividing a common lens through the centre and -cutting each half into a circular form (fig. 1 in the following page). The -half-lens viewed in section (fig. 2) is therefore of a prismatic shape, -and when placed with its sharp edge as in the diagram above, alters the -direction of the rays of light proceeding from the picture, bending them -outwards or away from the centre, so that in accordance with well-known -optical laws they appear to come in the direction of the dotted lines in -the diagram (in the last page), and the two images coalesce at their point -of junction. In the instrument as it is often sold, one of the lenses is -made movable, and by turning it round with the finger and thumb it will be -seen that the positions of the images may be shifted at pleasure. - -Fig. 1. - -[Illustration] - -Fig. 2. - -_Rules for taking Binocular Photographs._--In viewing very distant objects -with the eyes, the images formed on the retinæ are not sufficiently -dissimilar to produce a very Stereoscopic effect; hence it is often -required, in taking binocular pictures, to separate the Cameras more -widely than the two eyes are separated, in order to give a sufficient -appearance of relief. Mr. Wheatstone's original directions were, to allow -about one foot of separation for each twenty-five feet of distance, but -considerable latitude may be permitted. - -If the Cameras be not placed far enough apart, the dimensions of the -stereoscopic image from before backwards will be too small,--statues -looking like bas-reliefs, and the circular trunks of trees appearing oval, -with the long diameter transverse. On the other hand, when the separation -is too wide, the reverse obtains,--objects for instance which are square, -assuming an oblong shape pointing towards the observer. - -To understand the cause of this, the following law in optics should be -studied:--"The distance of objects is estimated by the extent to which -the axes of the eyes must be converged to view them." If we have to turn -our eyes strongly inwards, we judge the object to be near; but if the eyes -remain nearly parallel, we suppose it to be distant. - -[Illustration] - -The above figures represent six-sided truncated pyramids, each with -its apex towards the observer, the centres of the two smaller interior -hexagons being more widely separated than those of the larger exterior -ones. By converging the eyes upon them so as to unite the central images -in the manner represented in page 68 a greater amount of convergence -will be required to bring together the two summits than the bases, and -hence the summits will appear the nearest to the eye; that is to say, -the resultant central figure will acquire the additional dimension of -_height_, and appear as a solid cone, standing perpendicularly upon its -base: further, the more widely the summits are separated in relation to -the bases, the taller will the cone be, although a greater effort will be -required to coalesce the figures. - -Binocular Photographs taken with too much separation of the Cameras, are -distorted from a similar cause,--so strong a convergence being required to -unite them that certain parts of the picture appear to approach near to -the eye; and the depth of the solid image is increased. - -This effect is most observable when the picture embraces a variety of -objects, situated in different planes. In the case of views which are -quite distant, no near objects being admitted, the Cameras may be placed -with especial reference to them, even as far as twelve feet apart, without -producing distortion. - -It is sometimes observable, in looking at Stereoscopic pictures, that -they convey an erroneous impression of the real size and distance of the -object. For instance, in using the large reflecting Stereoscope, if, -when the adjustments have been made and the images properly united, the -two pictures be moved slowly forward, the eyes remaining fixed upon the -mirrors, the Stereoscopic image will gradually change its character, -the various objects it embraces appearing to become diminished in size, -and approaching near to the observer: whilst if the pictures be pushed -_backwards_, the image will enlarge and recede to a distance. So, again, -if an ordinary slide for the lenticular Stereoscope be divided in the -centre, and, looking into the instrument until the images coalesce, the -two halves be slowly separated from each other, the solid picture will -seem to become larger and to recede from the eye. - -It is easy to understand the cause of this. When the pictures in the -reflecting Stereoscope are moved _forwards_, the convergence of the optic -axes is increased: the image therefore appears _nearer_, in accordance -with the last-mentioned law. But to convey the impression of nearness -is equivalent to an apparent diminution in size, for we judge of the -dimensions of a body very much in relation to its supposed distance. Of -two figures, for instance, appearing of the same height, one known to -be a hundred yards off might be considered colossal, whilst the other, -obviously near at hand, would be viewed as a statuette. - -These facts, with others not mentioned, are of great interest and -importance, but their further consideration does not fall within the -bounds originally prescribed to us. The practical details of Stereoscopic -Photography have been arranged in a distinct Section, and will be found -included in the Second Part of the Work.[11] - -[11] For a more full and detailed explanation of the Stereoscopic -phenomena, see an abstract of Professor Tyndall's lectures in the third -volume of the 'Photographic Journal.' - - - - -CHAPTER VI. - -THE PHOTOGRAPHIC PROPERTIES OF IODIDE OF SILVER UPON COLLODION. - - -In the preceding part of this Work the physical and chemical properties of -Chloride and Iodide of Silver have been described, with the changes which -they experience by the action of Light. Nothing however has been said of -the surface used to support the Iodide of Silver, and to expose it in a -finely divided state to the influence of the actinic radiations. This -omission will now be supplied, and the use of Collodion will engage our -attention. - -The sensibility of Iodide of Silver upon Collodion is greatly superior to -that of the same salt employed in conjunction with any other vehicle at -present known. Hence the Collodio-Iodide film will supersede the paper -and Albumen processes in all cases where objects liable to move are to be -copied. The causes of this superior sensitiveness, as far as ascertained, -may be referred to the state of _loose coagulation_ of a Collodion film -and other particulars presently to be noticed. It must however be allowed -that there are yet some points affecting the sensitiveness of Iodide of -Silver, both mechanical and chemical, of the exact nature of which we are -ignorant. - -The present Chapter may be divided into four Sections:--the nature of -Collodion; the chemistry of the Nitrate Bath; the causes affecting the -formation and development of the Image upon Collodion; the various -irregularities in the development of the Image. - - -SECTION I. - -_Collodion._ - -Collodion (so named from the Greek word κολλἁω, to _stick_) is a -glutinous, transparent fluid, procured, as generally said, by dissolving -Gun-Cotton in Ether. It was originally used for surgical purposes only, -being smeared over wounds and raw surfaces, to preserve them from -contact with the air by the tough film which it leaves on evaporation. -Photographers employ it to support a delicate film of Iodide of Silver -upon the surface of a smooth glass plate. - -Two elements enter into the composition of Collodion: first, the -Gun-Cotton; second, the fluids used to dissolve it. Each of these will be -treated in succession. - - -CHEMISTRY OF PYROXYLINE. - -Gun-Cotton or _Pyroxyline_ is Cotton or Paper which has been altered in -composition and properties by treatment with strong acids. - -Both Cotton and Paper are, chemically, the same. They consist of fibres -which are found on analysis to have a constant composition, containing -three elementary bodies, Carbon, Hydrogen, and Oxygen, united together -in fixed proportions. To this combination the term _Lignine_ or -_Cellulose_[12] has been applied. - -[12] Lignine and Cellulose are not precisely identical substances. The -latter is the material composing the cell-wall; the former, the contained -matter in the cell. - -Cellulose is a definite chemical compound, in the same sense as Starch or -Sugar, and consequently, when treated with various reagents, it exhibits -properties peculiar to itself. It is insoluble in most liquids, such as -Water, Alcohol, Ether, etc., and also in dilute acids; but when acted -upon by Nitric Acid of a certain strength it liquefies and dissolves. - -It has been already shown (p. 12) that when a body dissolves in Nitric -Acid the solution is not usually of the same nature as an aqueous -solution; and so in this case--the Nitric Acid imparts Oxygen first to the -Cotton, and afterwards dissolves it. - -_Preparation of Pyroxyline._--If, instead of treating Cotton with Nitric -Acid, a mixture of Nitric and Sulphuric Acids in certain proportions be -used, the effect is peculiar. The fibres contract slightly, but undergo -no other visible alteration. Hence we are at first disposed to think the -mixed Acids ineffectual. This idea however is not correct, since on making -the experiment the properties of the cotton are found to be changed. Its -weight has increased by more than one-half; it has become soluble in -various liquids, such as Acetic Ether, Ether and Alcohol, etc., and, what -is more remarkable, it no longer burns in the air quietly, but explodes on -the application of flame with greater or less violence. - -This change of properties clearly shows, that although the fibrous -structure of the material is unaffected, it is no longer the same -substance, and consequently chemists have assigned it a different name, -viz. Pyroxyline. - -To produce the peculiar change by which Cotton is converted into -Pyroxyline, both Nitric and Sulphuric Acids are, as a rule, required; but -of the two the former is the most important. On analyzing Pyroxyline, -Nitric Acid, or a body analogous to it, is detected in considerable -quantity, but not Sulphuric Acid. The latter Acid, in fact, serves but -a temporary purpose, viz. to prevent the Nitric Acid from dissolving -the pyroxyline, which it would be liable to do if employed alone. The -Sulphuric Acid prevents the solution by removing water from the Nitric -Acid, and so producing a higher degree of concentration; Pyroxyline, -although soluble in a dilute, is not so in the strong Acid, and hence it -is preserved. - -The property possessed by Oil of Vitriol of removing water from other -bodies, is one with which it is well to be acquainted. A simple experiment -will serve to illustrate it. Let a small vessel of any kind be filled to -about two-thirds with Oil of Vitriol, and set aside for a few days; at the -end of that time, and especially if the atmosphere be damp, it will have -absorbed sufficient moisture to cause it to flow over the edge. - -Now even the strongest reagents employed in chemistry contain, almost -invariably, water in greater or lesser quantity. Pure Anhydrous Nitric -Acid is a white, solid substance; Hydrochloric Acid is a gas: and the -liquids sold under those names are merely solutions. The effect then of -mixing strong Oil of Vitriol with aqueous Nitric Acid is to remove water -in proportion to the amount used, and to produce a liquid containing -Nitric Acid in a high state of concentration, and Sulphuric Acid more or -less diluted. This liquid is the Nitro-Sulphuric Acid employed in the -preparation of Pyroxyline. - -Various forms of Pyroxyline.--Very soon after the first announcement of -the discovery of Pyroxyline, most animated discussions arose amongst -chemists with regard to its solubility and general properties. Some -spoke of a "solution of Gun-Cotton in Ether;" whilst others denied its -solubility in that menstruum; a third class, by following the process -described, obtained a substance which was not explosive, and therefore -could scarcely be termed Gun-Cotton. - -On further investigations some of these anomalies were cleared up, and -it was found that there were varieties of Pyroxyline, depending mainly -upon the degree of strength of the Nitro-Sulphuric Acid employed in the -preparation. Still the subject was obscure until the publication of -researches by Mr. E. A. Hadow. These investigations, conducted in the -Laboratory of King's College, London, were published in the Journal of the -Chemical Society. Constant reference will be made to them in the following -remarks. - -We notice--first, the chemical constitution of Pyroxyline; secondly, its -varieties; and thirdly, the means adopted to procure a Nitro-Sulphuric -Acid of the proper strength. - -a. _Constitution of Pyroxyline._--Pyroxyline has been sometimes spoken -of as a Salt of Nitric Acid, a Nitrate of Lignine. This view however is -erroneous, since it can be shown that the substance present is not Nitric -Acid, although analogous to it. It is the Peroxide of Nitrogen, which is -intermediate in composition between Nitrous Acid (NO{3}) and Nitric Acid -(NO{5}). Peroxide of Nitrogen (NO{4}) is a gaseous body of a dark red -colour; it possesses no acid properties, and is incapable of forming a -class of salts. In order to understand in what state this body is combined -with cotton fibre to form Pyroxyline, it will be necessary to digress for -a short time. - -Law of Substitution.--By the careful study of the action of Chlorine, -and of Nitric Acid, upon various organic substances, a remarkable series -of compounds has been discovered, containing a portion of Chlorine or -of Peroxide of Nitrogen in the place of Hydrogen. The peculiarity of -these substances is, that they strongly resemble the originals in their -physical, and often in their chemical properties. It might have been -supposed that agents of such active chemical affinities as Chlorine and -Oxide of Nitrogen would, by their mere presence in a body, produce a -marked effect; yet it is not so in the case before us. The primitive type -or constitution of the substance modified remains the same, even the -crystalline form being often unaffected. It seems as if the body by which -the Hydrogen had been displaced had stepped in quietly and taken up its -position in the framework of the whole without disturbance. Many compounds -of this kind are known; they are termed by chemists "substitution -compounds." The law invariably observed is, that the substitution takes -place in equal atoms: a single atom of Chlorine, for instance, displaces -one of Hydrogen; two of Chlorine displace two of Hydrogen, and so on, -until, in some cases, the whole of the latter element is separated. - -In illustration of these remarks, take the following instances:--Acetic -Acid contains Carbon, Hydrogen, and Oxygen; by the action of Chlorine -the Hydrogen may be removed in the form of Hydrochloric Acid, and an -equal number of atoms of Chlorine be substituted. In this way a new -compound is formed, termed _Chloracetic Acid_, resembling in many -important particulars the Acetic Acid itself. Notice particularly that -the peculiar properties characteristic of Chlorine are completely masked -in the substitution body, and no indication of its presence is obtained -by the usual tests! A soluble _Chloride_ gives with Nitrate of Silver a -white precipitate of Chloride of Silver, unaffected by Acids, but the -Chloracetic Acid does not; hence it is plain that the Chlorine exists in a -peculiar and Ultimate state of combination different from what is usual. - -The substance we have been previously considering, viz. Pyroxyline, -affords another illustration of the Law of Substitution. Omitting, for -the sake of simplicity, the number of atoms concerned in the change, -the action of concentrated Nitric Acid upon ligneous fibre may be thus -explained:-- - - { Carbon - Cotton _or_ { Hydrogen + Nitric Acid - { Hydrogen - { Oxygen - -equals - - { Carbon - Pyroxyline _or_ { Hydrogen + Water - { Peroxide Nitrogen - { Oxygen - -Or in symbols:-- - - CH{11}O + NO{5} = C (H{n-1}NO{4}) O + HO - -By a reference to the formula, it is seen that the fifth atom of Oxygen -contained in the Nitric Acid takes one of Hydrogen, and forms an atom of -Water; the NO{4} then steps in, to fill the gap which the atom of Hydrogen -has left. All this is done with so little disturbance that even the -fibrous structure of the cotton remains as before. - -b. _Chemical Composition of the varieties of Pyroxyline._--Mr. Hadow has -succeeded in establishing _four_ different substitution compounds, which, -as no distinctive nomenclature has been at present proposed, may be termed -compounds A, B, C, and D. - -_Compound A_ is the most explosive Gun-Cotton, and contains the largest -amount of Peroxide of Nitrogen. It dissolves _only in Acetic Ether_, and -is left on evaporation as a white powder. It is produced by the strongest -Nitro-Sulphuric Acid which can be made. - -_Compounds B and C_, either separate or in a state of mixture, form the -soluble material employed by the Photographer. They both dissolve in -Acetic Ether, and also in a mixture of Ether and Alcohol. The latter, -viz. C, also dissolves in glacial Acetic Acid. They are produced by a -Nitro-Sulphuric Acid slightly weaker than that used for A, and contain a -smaller amount of Peroxide of Nitrogen. - -_Compound D_ resembles what has been termed _Xyloidine_, that is, the -substance produced by acting with Nitric Acid upon Starch. It contains -less Peroxide of Nitrogen than the others, and dissolves in Ether -and Alcohol, and also in Acetic Acid. The ethereal solution leaves, -on evaporation, an opaque film, which is highly combustible, but not -explosive. - -By bearing in mind the properties of these compounds, many of the -anomalies complained of in the manufacture of Gun-Cotton disappear. If the -Nitro-Sulphuric Acid employed is too strong, the product will be insoluble -in Ether; whilst if it is too weak, the fibres are gelatinized by the Acid -and partly dissolved. - -c. _Means adopted to procure a Nitro-Sulphuric Acid of the requisite -strength for preparing Pyroxyline._--This is a point of more difficulty -than would at first appear. It is easy to determine an exact formula for -the mixture, but not so easy to hit upon the proper proportions of the -acids required to produce that formula; and a very slight departure from -them altogether modifies the result. The main difficulty lies in _the -uncertain strength of commercial Nitric Acid_. Oil of Vitriol is more to -be depended upon, and has a tolerably uniform Sp. Gr. of 1·836;[13] but -Nitric Acid is constantly liable to variation; hence it becomes necessary -to make a preliminary determination of its real strength, which is done -either by taking the specific gravity and referring to tables, or, better -still, by a direct analysis. As each atom of Sulphuric Acid removes only a -given quantity of water, it follows that the weaker the Nitric Acid, the -larger the amount of Sulphuric which will be required to bring it up to -the proper degree of concentration. - -[13] The later experience of the writer induces him to believe, that the -specific gravity of Oil of Vitriol cannot always be taken as an indication -of its real strength; which is best ascertained by analysis. - -To avoid the trouble necessarily attendant upon these preliminary -operations, many prefer to use, in place of Nitric Acid itself, one of -the salts formed by the combination of Nitric Acid with an alkaline -base. The composition of these salts, provided they are pure and nicely -crystallized, can be depended on. - -Nitrate of Potash, or _Saltpetre_, contains a single atom of Nitric Acid -united with one of Potash. It is an _anhydrous_ salt, that is, it has -no water of crystallization. When strong Sulphuric Acid is poured upon -Nitrate of Potash in a state of fine powder, in virtue of its superior -chemical affinities it appropriates to itself the Alkali and liberates the -Nitric Acid. If care be taken to add a sufficient excess of the Sulphuric -Acid, a solution is obtained containing Sulphate of Potash dissolved in -Sulphuric Acid, and free Nitric Acid. The presence of the Sulphate of -Potash (or, more strictly speaking, of the _Bi_-Sulphate) does not in any -way interfere with the result, and the effect is the same as if the mixed -acids themselves had been used. - -The reaction may be thus represented:-- - - Nitrate of Potash _plus_ Sulphuric Acid in excess - = Bisulphate Potash _plus_ Nitro-Sulphuric Acid. - - -CHEMISTRY OF THE SOLUTION OF PYROXYLINE IN ETHER AND ALCOHOL, OR -"COLLODION." - -The substitution compounds B and C, already alluded to as forming the -Soluble Cotton of Photographers, are both abundantly soluble in Acetic -Ether. This liquid however is not adapted for the purpose required, -inasmuch as on evaporation it leaves the Pyroxyline in the form of a white -powder, and not as a transparent layer. - -The rectified Ether of commerce has been found to answer better than any -other liquid as a solvent for Pyroxyline. - -If the sp. gravity be about ·750, it contains invariably a small -proportion of _Alcohol_, which appears to be necessary; the solution not -taking place with absolutely pure Ether. The Pyroxyline, if properly -prepared, begins almost immediately to gelatinize by the action of the -Ether, and is soon completely dissolved. In this state it forms a slimy -solution, which, when poured out on a glass plate, dries up into a horny -transparent layer. - -In preparing Collodion for Photographic purposes, we find that its -physical properties are liable to considerable variation. Sometimes -it appears very thin and fluid, flowing on the glass almost like -water, whilst at others it is thick and glutinous. The causes of these -differences will now engage our attention. They may be divided into two -classes: first, those relating to the Pyroxyline; second, to the solvents -employed. - -a. _Variation of Properties in different Samples of soluble -Pyroxyline._--The substitution compounds A, B, C, and D differ, as already -shown, in the percentage amount of Peroxide of Nitrogen present, and the -former are more explosive and insoluble than the latter. But it often -happens in preparing Pyroxyline, that two portions of Nitro-Sulphuric -Acid taken from the same bottle yield products which vary in properties, -although they are necessarily the same in composition. - -Taking _extremes_ in illustration, we notice two principal modifications -of soluble Pyroxyline. - -The first, when treated with the mixture of Ether and Alcohol, sinks down -to a gummy or gelatinous mass, which gradually dissolves on agitation The -solution is very fluid in proportion to the number of grains used, and -when poured out spreads into a beautifully smooth and glassy surface, -which is quite structureless, even when highly magnified. The film adheres -tightly to the glass, and when the finger is drawn across it, separates in -short fragments, and broken pieces. - -The second variety produces a Collodion which is thick and glutinous, -flowing over the glass in a slimy manner, and soon setting into numerous -small waves and cellular spaces. The film lies loose upon the glass, is -apt to contract on drying, and may be pushed off by the finger in the form -of a connected skin. - -This subject is not thoroughly understood, but it is known that the -_temperature_ of the Nitro-Sulphuric Acid at the time of immersing the -Cotton influences the result. The soluble variety is produced by _hot_ -acids; the second, or glutinous, by the same acids employed cold, or -only slightly warm. The best temperature appears to be from 130° to 155° -Fahrenheit; if it rises much beyond that point, the acids act upon and -dissolve the Cotton. - -b. _The physical properties of Collodion affected by the proportions and -purity of the Solvents._--Pyroxyline of the varieties termed B and C -dissolves freely in a mixture of Ether and Alcohol; but the characters -of the resulting solution vary with the relative proportions of the two -solvents. - -When the Ether is in large excess, the film is inclined to be strong and -tough, so that it can often be raised by one corner and lifted completely -off the plate without tearing. It is also very contractile, so that a -portion of the Collodion poured on the hand draws together and puckers the -skin as it dries. If spread upon a glass plate in the usual way, the same -property of contractility causes it to retract and separate from the sides -of the glass. - -These properties, produced by Ether in large proportion, disappear -entirely on the addition of more Alcohol. The transparent layer is now -soft and easily torn, possessing but little coherency. It adheres to the -surface of the glass more firmly, and exhibits no tendency to contract and -separate from the sides. - -From these remarks it will be gathered that an excess of Ether, and a -low temperature in preparing the Pyroxyline, both favour the production -of a contractile Collodion; whilst on the other hand an abundance of -Alcohol, and a hot Nitro-Sulphuric Acid, tend to produce a short and -non-contractile Collodion. - -The physical properties of Collodion are affected by another cause, viz. -by the _strength_ and purity of the solvents, or, in other words, their -freedom from dilution with water. If a few drops of water be purposely -added to a sample of Collodion, the effect is seen to be to precipitate -the Pyroxyline in flakes to the bottom of the bottle. There are many -substances known in chemistry which are soluble in spirituous liquids, but -behave in the same manner as Pyroxyline in this respect. - -The manner in which water gains entrance into the Photographic Collodion -is usually by the employment of Alcohol or Spirit of Wine which has not -been highly rectified. In that case the Collodion is thicker, and flows -less readily than if the Alcohol were stronger. Sometimes the texture of -the film left upon evaporation is injured; it is no longer homogeneous and -transparent, but semi-opaque, reticulated, or honeycombed, and so rotten -that a stream of water projected upon the plate washes it away. - -These effects are to be attributed not to the Alcohol, but to the water -introduced with it; and the remedy will be to procure a stronger spirit, -or, if that cannot be done, to increase the amount of Ether. Collodion -prepared with a large proportion of Ether, and water, but a small quantity -of Alcohol, is often very fluid and structureless at first, adhering to -the glass with some tenacity and having a short texture; but it tends to -become rotten when used to coat many plates successively, the water on -account of its lesser volatility accumulating in injurious quantity in the -last portions. - - -THE COLORATION OF IODIZED COLLODION EXPLAINED. - -Collodion iodized with the Iodides of Potassium, Ammonium, or Zinc, -soon assumes a yellow tint, which in the course of a few days or weeks, -according to the temperature of the atmosphere, deepens to a full brown. -This gradual coloration, due to a development of Iodine, is caused partly -by the Ether and partly by the Pyroxyline. - -Ether may, with proper precautions, be preserved for a long time in a -pure state, but on exposure to the joint action of air and light it -undergoes a slow process of oxidation, attended with formation of Acetic -Acid and a peculiar principle resembling in properties ozone, or Oxygen -in an allotropic and active condition. Iodide of Potassium or Ammonium is -decomposed by Ether in this state. Acetate of the Alkali, and Hydriodic -Acid (HI), being first produced. The ozonized substance then removes -Hydrogen from the latter compound, and liberates Iodine, which dissolves -and tinges the liquid yellow. - -A simple solution of an Alkaline Iodide in Alcohol and Ether does not, -however, become so quickly coloured as Iodized Collodion; and hence it is -evident that the presence of the Pyroxyline produces an effect. It may -be shown that Alkaline Iodides slowly decompose Pyroxyline, and that a -portion of Peroxide of Nitrogen is set free: this body, containing loosely -combined oxygen, tends powerfully to eliminate Iodine, as may be seen by -adding a few drops of the yellow commercial Nitrous acid to a solution of -Iodide of Potassium. - -The _stability_ of the particular Iodide used in Iodizing Collodion, -influences mainly the rate of coloration, though elevation of temperature -and exposure to light are not without effect. Iodide of Ammonium is the -least stable, and Iodide of Cadmium the most so; Iodide of Potassium being -intermediate. Collodion iodized with _pure_ Iodide of Cadmium usually -remains nearly colourless to the last drop, if kept in a cool and dark -place. - -As the presence of free Iodine in Collodion affects its photographic -properties, it may sometimes be necessary to remove it. This is done by -inserting a strip of Silver-foil; which decolorizes the liquid, by forming -Iodide of Silver, soluble in the excess of Alkaline Iodide (p. 42). -Metallic Cadmium, and metallic Zinc, have the same effect. - -When Methylated Spirits are employed in the manufacture of Collodion, -the Iodine first liberated is afterwards either partially or entirely -reabsorbed, the liquid acquiring at the same time an acid reaction to -test-paper. - - -SECTION II. - -_The Chemistry of the Nitrate Bath._ - -The solution of Nitrate of Silver in which the plate coated with iodized -Collodion is dipped, to form the layer of Iodide of Silver, is known -technically as _the Nitrate Bath_. The chemistry of Nitrate of Silver -has been explained at page 13, but there are some points relating to the -properties of its aqueous solution which require a further notice. - -_Solubility of Iodide of Silver in the Nitrate Bath._--Aqueous solution of -Nitrate of Silver may be mentioned in the list of solvents of Iodide of -Silver. The proportion dissolved is in all cases small, but it increases -with the _strength_ of the solution. If no attention were paid to this -point, and the precaution of previously saturating the Nitrate Bath with -Iodide of Silver neglected, the film would be dissolved when left too long -in the liquid. - -This solvent power of Nitrate of Silver on the Iodide is well shown by -taking the excited Collodion plate out of the Bath, and allowing it to dry -spontaneously. The layer of Nitrate on the surface, becoming concentrated -by evaporation, eats away the film, so as to produce a transparent, -spotted appearance. - -In the solution of Iodide of Silver by Nitrate of Silver a _double -salt_ is formed, which corresponds in properties to the double Iodide -of Potassium and Silver in being _decomposed_ by the addition of water. -Consequently, in order to saturate a Bath with Iodide of Silver it is -only necessary to dissolve the total weight of Nitrate of Silver in a -small bulk of water, and to add to it a few grains of an Iodide; perfect -solution takes place, and on subsequent dilution with the full amount of -water, the excess of Iodide of Silver is precipitated in the form of a -milky deposit. - -_Acid condition of Nitrate of Silver._--A solution of _pure Nitrate of -Silver_ is neutral to blue litmus-paper, but that prepared from the -commercial Nitrate has usually an acid reaction; the crystals having been -imperfectly drained from the acid mother-liquor in which they were formed. -Hence, in making a new Bath it is often advisable not only to saturate it -with Iodide of Silver, but to neutralize the free Nitric acid it contains. - -There is also a peculiar condition of Nitrate of Silver crystallized from -a solution of the metal in Nitric Acid, which renders it quite unfit for -photographic purposes (see p. 101). It is thought to depend upon the -presence of an oxide of Nitrogen, possibly of Nitrous Acid, and the remedy -is to dry the crystals very strongly, or, better still, to fuse them at a -moderate heat: mere neutralization with Carbonate of Soda does not suffice. - -In melting Nitrate of Silver great care should be taken not to raise the -heat so high as to decompose the salt, or a basic Nitrite will be formed, -which affects the properties of the solution (p. 13): fused Nitrate of -Silver ought, when cold, to be quite white, and to dissolve perfectly in -water without leaving any residue. The only objection to the employment -of Nitrate of Silver in this form is the facility with which it may be -adulterated with Nitrates of Potash and Soda, the presence of which would -lessen the available strength of the Bath. - -The Nitrate Bath, although perfectly neutral when first prepared, may -become _acid_ by continued use, if Collodion containing much _free -Iodine_ be constantly employed. In that case a portion of Nitric Acid is -liberated, thus:-- - - Nitrate of Silver + Iodine - = Iodide of Silver + Nitric Acid + Oxygen. - -When Collodion is iodized entirely with alkaline Iodides, it liberates -Iodine by keeping; and hence the occasional use of Ammonia may be required -to remove acidity from the Bath. But since the introduction of the Iodide -of Cadmium, which preserves the Collodion nearly or quite colourless, the -necessity for neutralizing Nitric Acid in the Bath has ceased. - -_Alkaline condition of the Bath._--By "alkalinity" of the Bath is meant -a condition in which the blue tint is rapidly restored to reddened -litmus-paper. This indicates that an Oxide of some kind is present in -solution, which, by combining with the acid in the reddened paper, -neutralizes it and removes the red colour. - -If a small portion of caustic Potash or Ammonia be added to a strong -solution of Nitrate of Silver, it produces a brown precipitate, which is -Oxide of Silver. - - Ammonia + Nitrate of Silver - = Oxide of Silver + Nitrate Ammonia. - -The solution however, from which the precipitate has separated, is not -left in a neutral state, but possesses a faint alkaline reaction. Oxide -of Silver and Carbonate of Silver are also _abundantly_ soluble in water -containing Nitrate of Ammonia; which salt is continually accumulating in -the Bath when compounds of Ammonium are used for iodizing. - -An alkaline Bath is perhaps of all conditions the one most fatal to -success in photography. It leads to that universal darkening of the film -on applying the developer to which the name of "fogging" has been given. -Hence care must be used in adding to the Bath substances which tend to -make it alkaline. - -Collodion containing free Ammonia, often sold in the shops, gradually does -so. The use of Potash, Carbonate of Soda, Chalk, or Marble, to remove free -Nitric Acid from the Bath, has the same effect; and hence, when they are -employed, a trace of Acetic acid must afterwards be added. - -The mode of testing a bath for alkalinity is as follows:-- a strip of -porous blue litmus-paper is taken and held to the mouth of a bottle of -glacial Acetic acid until it becomes reddened; it is then placed in the -liquid to be examined and left for ten minutes or a quarter of an hour. If -Oxide of Silver be present in solution, the original blue colour of the -paper will slowly but gradually be restored. - -_Occasional formation of Acetate of Silver in the Nitrate Bath._--In -preparing a new Bath, if the crystals of Nitrate of Silver are acid, it is -usual to add an alkali in small quantity. This removes the Nitric Acid, -but leaves the solution faintly alkaline. Acetic Acid is then dropped in, -which, by combining with the Oxide of Silver, forms Acetate of Silver. - -Acetate of Silver is not formed by the simple addition of Acetic Acid to -the Bath, because its production under such circumstances would imply the -liberation of Nitric Acid; but if an alkali be present to neutralize the -Nitric Acid, then the double decomposition takes place, thus-- - - Acetate of Ammonia + Nitrate of Silver - = Acetate of Silver + Nitrate of Ammonia. - -Acetate of Silver is a white flaky salt, sparingly soluble in water. It -dissolves in the Bath only in small proportion, but yet sufficiently to -affect the Photographic properties of the film (see p. 111 and 117). The -observance of the following simple rules will obviate its production in -injurious quantity:--_First_, when it is required to remove free Nitric -Acid from a bath _not containing Acetic Acid_ a solution of Potash or -Carbonate of Soda may be dropped in _freely_; but the liquid must be -filtered before adding any Acetic Acid, otherwise the brown deposit of -Oxide of Silver will be taken up by the Acetic Acid, and the Bath will -be charged with Acetate of Silver. _Secondly_, in dealing with a Bath -containing both Nitric and Acetic Acids, employ an alkali _much diluted_ -(Liquor Ammoniæ with 10 parts of Water), and add a single drop at a -time, coating and trying a plate between each addition; the Nitric Acid -will neutralize itself before the Acetic, and with care there will be no -formation of Acetate of Silver in quantity. - -_Substances which decompose the Nitrate Bath._--Most of the common -metals, having superior affinity for Oxygen, separate the Silver from a -solution of the Nitrate; hence the Bath must be kept in glass, porcelain, -or gutta-percha, and contact with Iron, Copper, Mercury, etc., must -be avoided, or the liquid will be discoloured, and a black deposit of -metallic Silver precipitated. - -All developing agents, such as Gallic and Pyrogallic Acids, the Protosalts -of Iron, etc., blacken the Nitrate Bath, and render it useless by reducing -metallic Silver. - -Chlorides, Iodides, and Bromides produce a deposit in the Bath; but the -solution, although weakened, may again be used after passing through a -filter. - -Hyposulphites, Cyanides, and all fixing agents decompose Nitrate of Silver. - -Organic matters, generally, reduce Nitrate of Silver, either with or -without the aid of light. Grape Sugar, Albumen, Serum of Milk containing -caseine, etc., blacken the Bath, even in the dark. Alcohol and Ether -act more slowly, and produce no injurious effect unless the liquid is -constantly exposed to light. - -These facts indicate that the Nitrate Bath containing volatile organic -matters must be preserved in a dark place; also that it should be kept -exclusively for sensitizing the Collodion plates, and not used in floating -papers intended for the printing process. - -_Changes in the Nitrate Bath by use._--The solution of Nitrate of Silver -employed in exciting the Collodion film gradually decreases in strength, -but not so quickly as the Bath used in sensitizing papers for printing. If -the amount of Nitrate be allowed to fall as low as twenty grains to the -ounce of water, the decomposition will be imperfect, and the film will be -pale and blue, even with a highly iodized Collodion. - -A gradual accumulation of Ether and Alcohol also takes place in the Bath -after long use, in consequence of which the developing solutions flow -less readily upon the Collodionized plates, and oily stains are apt to be -produced. - -Diminished sensitiveness of the Iodide film is sometimes traced to -impurities in the Bath, when it is very old, and has been much used. These -are probably of an organic nature and may often be partially removed -by agitation with kaolin, or animal charcoal. The latter however is -objectionable, being usually contaminated with _Carbonate of Lime_, which -makes the Bath alkaline; or (in the case of _purified_ animal charcoal) -with traces of Hydrochloric Acid, which liberate Nitric Acid in the Bath. -Even the kaolin may as a preliminary precaution be washed with dilute -Acetic Acid to remove Carbonate of Lime if any should be present. - - -SECTION III. - -_The Conditions which affect the Formation and Development of the Latent -Image in the Collodion process._ - -It will be necessary to preface the observations contained in this Section -by defining two terms which are frequently confounded with each other, but -are in reality of distinct meaning. These terms are "Sensitiveness" and -"Intensity." - -By Sensitiveness is meant a facility of receiving impression from very -feeble rays of light, or of receiving it quickly from brighter rays. - -Intensity, on the other hand, relates to the appearance of the finished -Photograph, independently of the time taken to produce it,--_to the degree -of opacity of the image_, and the extent to which it obstructs transmitted -light. - -It will be seen as we proceed that the conditions necessary to obtain -extreme sensitiveness of the Iodide film are different from, and often -opposed to, those which give the maximum intensity of image. - - -CAUSES WHICH INFLUENCE THE SENSITIVENESS OF IODIDE OF SILVER ON COLLODION. - -Some of the most important are as follows:-- - -a. _The presence of free Nitrate of Silver._--When the sensitive film is -removed from the Nitrate Bath, the Iodide of Silver is left in contact -with excess of Nitrate of Silver. The presence of this compound is not -_essential_ to the action of the light, since, if it be removed by washing -in distilled water, the image may still be impressed. In such a case -however the effect is produced slowly, and a longer exposure in the Camera -is required. - -The sensitiveness of the Iodide film does not increase uniformly with the -amount of the excess of Nitrate of Silver, as measured by the strength of -the Bath. It is found that no advantage in this respect can be gained by -using a proportion of Nitrate of Silver greater than 30 or 35 grains to -the ounce of water, although solutions of three times this strength have -been sometimes employed. - -It has been asserted that a chemically pure Iodide of Silver, which is -unaffected in colour by the direct action of light, is also incapable of -receiving the invisible image in the Camera; and that the sensitiveness of -a washed Collodion film is due to a minute quantity of Nitrate of Silver -still remaining. Iodide of Silver in the state in which it is thrown down -on diluting with water a strong solution of the salt known as the double -Iodide of Potassium and Silver,--and which must, from the mode of its -preparation, be free from Nitrate of Silver,--is quite insensitive; but -this form of Iodide differs from the other in colour, and not only so, but -is likely to contain an excess of Iodide of Potassium. The application -of a solution of Nitrate of Silver to this compound at once renders it -sensitive to light. - -b. _Free acids in the Nitrate Bath._--Strong oxidizing agents, such as -Nitric Acid, greatly diminish the sensibility of the film, and hence the -importance of removing the free acid often met with in commercial samples -of the Nitrate of Silver. The effect of even a single drop of strong -Nitric Acid in an eight-ounce Nitrate Bath will be appreciable; and when -the proportion is increased to one drop per ounce, it will be difficult to -obtain a rapid impression. - -Acetic Acid has far less effect upon the sensitiveness than Nitric -Acid, and being found useful during the development of the image is -commonly employed; but when great rapidity is desired, it should be added -cautiously, and in a proportion very much less than that in the solution -known as the Aceto-Nitrate of Silver, which contains about one drop of the -glacial acid to each grain of Nitrate of Silver. - -c. _Addition of certain organic matters._--It has long been remarked that -the use of bodies like Albumen, Gelatine, Caseine, etc., which combine -with Oxides of Silver, retard the action of light upon Iodide of Silver; -and the recent observations of the Author enable him to confirm this -statement. It is probable that one cause, amongst others, of the great -sensibility of the Collodion film is due to the fact that Pyroxyline is -a substance peculiarly indifferent to the Salts of Silver, exhibiting -no tendency to reduce them to the metallic state; and it is proved by -experiment that the addition of Grape Sugar, or of the resinous body, -Glycyrrhizine, which resembles Albumen in causing a white precipitate in -strong solution of Nitrate of Silver, renders necessary a longer exposure -in the Camera. Alkaline Citrates have a still more marked effect, as also -have Tartrates, Oxalates, etc. - -d. _Impurities in the soluble Iodides._--Commercial Iodide of Potassium -often contains _Iodate_ of Potash, which is found to have a retarding -effect upon the action of light; also Carbonate of Potash, which, in -Collodion, produces Iodoform,[14] and in the paper processes, where -"Aceto-Nitrate" is used for sensitizing, forms Acetate of Silver. Iodoform -has a marked influence in diminishing the sensitiveness of Iodide of -Silver; Acetate of Silver may perhaps increase it a little by securing the -absence of free Nitric Acid (p. 117). Iodide of Potassium prepared by the -process in which Sulphuretted Hydrogen and Alcohol are used, and having -a smell of Garlic, contains probably Xanthate of Potash, and is nearly -useless for Photography. - -[14] See the Vocabulary, Part III., Art. Iodoform. - -Commercial Iodide of Cadmium is a purer salt than the Iodide of -Potassium, and may be advantageously substituted for it; but it possesses -the property of coagulating Albumen, and hence cannot be employed in -conjunction with that substance. - -e. _Presence of free Iodine._--Both in the waxed paper and the Collodion -processes, the solutions often contain a small quantity of free Iodine. -This Iodine, in contact with the Nitrate of Silver of the Bath, produces -a mixed Iodide and _Iodate_ of Silver, and liberates Nitric Acid. It thus -retards the sensitiveness of the film in proportion to the quantity of -Iodine present. Collodion of a full yellow colour is perceptibly less -sensitive than the same rendered colourless; and when enough Iodine has -been liberated to give a red or brown tint, double the original exposure -will probably be required. - -If brown Collodion be much used, the Nitrate Bath may by degrees become -sufficiently contaminated with free Nitric Acid to interfere with the -sensitiveness of the film; but if colourless or lemon-yellow tinted -Collodion be employed, this evil need not be anticipated. - -Certain substances may be added to coloured Collodion which possess the -property of counteracting the retarding influence of the free Iodine, -such, for instance, as the Oils of Cloves, Cinnamon, etc.; they probably -act in virtue of their affinity for Oxygen, by preventing the formation -of Iodate of Silver. In colourless Collodion they produce little or no -effect, neither do they remove the insensitiveness of the film when -dependent upon a too acid condition of the Nitrate Bath. - -f. _Addition of Bromide or Chloride to Collodion._--In the Daguerreotype -a very exalted state of sensibility is obtained by exposing the silvered -plate first to the vapour of Iodine, and afterwards to that of Bromine or -Chlorine; but this rule does not apply to the Collodion process, which -differs essentially in principle. Soluble Bromides added to Collodion -lessen its sensibility to an appreciable extent, as also do Chlorides. -This rule however may perhaps be liable to an exception when artificial -light is used, which contains a greater proportion of the rays of small -refrangibility, known to act more powerfully upon the Bromide than upon -the Iodide of Silver (p. 66). - -g. _Density of the sensitive film._--When the proportion of soluble Iodide -in the iodizing solution is too great, the film is very dense, and the -Iodide of Silver is apt to burst out upon the surface, and fall away in -loose flakes into the Bath. This condition, which is highly unfavourable -to sensitiveness, is very common in Collodion, and constitutes what is -termed "over-iodizing." The Iodide, in fact, is formed in such a case too -much upon the surface, and consequently, when the fixing agent is applied, -the image not being retained by the film, is washed off and lost. - -On the other hand, the sensibility of the film is not lessened by reducing -the amount of Iodide in Collodion to a minimum, if all the solutions are -neutral; but the pale blue films formed by a dilute Collodion, and which -almost rival the Daguerreotype itself in delicacy, are nearly useless in -practice; for if free Iodine or other bodies of a retarding nature are -present in any quantity, either in the Collodion or in the Bath, they -almost destroy the action of a weak light, producing a far more injurious -effect than if the film were more yellow and opaque. - -h. _Impurities in Ether and Alcohol._--Pure Ether should be neutral to -test-paper, but the commercial samples of this article have usually either -an acid or an alkaline reaction. The frequent occurrence of a peculiar -oxidizing principle in Ether has also been pointed out (p. 85). Each of -these three conditions is injurious to sensitiveness; the first and last -by liberating Iodine when alkaline Iodides are used; and the second, -by producing Iodoform under the same circumstances. In this case the -Collodion remains colourless, but gives inferior results. - -The Author has also observed that Ether which has been redistilled from -the residues of Collodion may contain a volatile principle (probably a -compound Ether?) which produces a retarding effect upon the action of -light. - -Commercial Spirit of Wine is not always uniform in composition, as -sufficiently evidenced by the test of smell. It may contain "fusel oil" or -other volatile substances, which become milky on dilution with water, and -are believed to injure the quality of the spirit for Photographic use. - -i. _Relative proportions of Ether and Alcohol in Collodion._--It was -shown at p. 84 that the addition of Alcohol to Collodion lessens the -contractility of the film, and renders it soft and gelatinous. This -condition is favourable to the formation of the invisible image in the -Camera, the play of affinities being promoted by the loose manner in which -the particles of Iodide are held together. It is therefore usual to add to -Collodion as much Alcohol as it will bear without becoming glutinous, or -leaving the glass; the exact quantity required varying with the strength -of the spirit or its freedom from dilution with water. - -k. _Decomposition in the Collodion._--Collodion iodized with the metallic -Iodides generally, excepting the Iodide of Cadmium, becomes brown and -loses sensitiveness in the course of a few days or weeks. If the free -Iodine, the cause of the brown colour, be removed, the greater part, -but not the whole, of the sensitiveness is regained. The experiments of -the Author, and of others, have proved that a solution of Pyroxyline in -contact with an unstable iodide, slowly undergoes decomposition, the -result of which is that Iodine is set free, and an equivalent quantity of -the base remains in union with certain organic elements of the Collodion. - -Decomposition also gradually ensues when iodized Collodion is placed in -contact with reducing agents, such as Proto-iodide of Iron, Gallic Acid, -Grape Sugar, Glycyrrhizine, etc., so that these combinations do not retain -a constant sensibility for any length of time. Even plain Collodion -uniodized cannot be preserved many months without a small but perceptible -amount of change. - -l. _Decomposition in the Nitrate Bath._--A Collodion Nitrate Bath which -has been much used, often gives a less sensitive film than when newly -made. It is known also that many organic substances which reduce Nitrate -of Silver, if added to the Bath, produce a state which is favourable to -sensitiveness whilst the decomposition is taking place, but is eventually -unfavourable; hence the solution will be injured by adding either Gallic -or Pyrogallic Acid, and by organic matters generally if exposed to light. - -_Recapitulation._--The conditions most favourable to extreme sensitiveness -of the Iodide of Silver on Collodion may be condensed as follows:--perfect -neutrality of the solutions employed; a soft, gelatinous state of the -film; absence of Chlorides and other salts which precipitate Nitrate of -Silver; an undecomposed Collodion, containing no organic matter of that -kind which is precipitated by basic Acetate of Lead, and combines with -oxides of Silver. - - -THE CONDITIONS WHICH AFFECT THE DEVELOPMENT OF THE LATENT IMAGE. - -The general theory of the development of a latent image by means of a -reducing agent, having been simply explained in the third Chapter, may -now be more fully examined in its application to the Iodide of Silver on -Collodion. - -a. _The presence of free Nitrate of Silver essential to the -development._--This subject has already been mentioned (p. 36). A -sensitive Collodion plate, carefully washed in distilled water, is still -capable of receiving the radiant impression in the Camera, but it does not -admit of development until it has been redipped in the Bath, or treated -with a reducing agent to which Nitrate of Silver has been added: and if -the proportion of free Nitrate of Silver on a Collodion film be too small, -the image will be feeble or altogether imperfect in parts, with patches of -green or blue, due to deficient reduction. - -b. _Comparative strength of deducing Agents._--No increase of power in a -developer will suffice to bring out a perfect image on an under-exposed -plate, or upon a film containing too little Nitrate of Silver. But there -is considerable difference in the length of time which the various -developers require to act. Gallic Acid is the most feeble, and Pyrogallic -Acid the strongest, producing at least four times more effect than an -equal weight of the crystallized Protosulphate of Iron, and twenty times -more than the Protonitrate of Iron. - -c. _The effect of free Acid upon the development._--Acids tend to retard -the reduction of the image as well as to diminish the sensibility of the -film to light. Nitric Acid especially does so, from its powerful oxidizing -and solvent properties. The effect of Nitric Acid is particularly seen -when the film of Iodide of Silver is very blue and transparent, and the -quantity of Nitrate of Silver retained upon its surface small. Under such -circumstances the proper development of the image may be suspended, and -spangles of metallic Silver separate. This indicates that the quantity of -the acid should be diminished, or the strength of the Nitrate Bath and of -the reducing agent be increased, as a counterpoise to the retarding action -of acid upon the development. - -Acetic Acid also moderates the rapidity of development, but it has not -that tendency altogether to suspend it possessed by Nitric Acid. It is -therefore usefully employed, to enable the operator to cover the plate -evenly with liquid before the development commences, and to preserve the -white parts of the impression from any accidental deposit of metallic -Silver due to irregular action of the reducing agent. - -On comparing the retarding effects of free acid upon the light's action, -and upon the development, we see that the former is the most marked,--that -a small quantity of Nitric Acid produces a more decided influence upon the -impression of the image in the Camera than upon the bringing out of that -image by means of a developer. - -d. _Accelerating effect of certain organic matters._--Organic bodies, -like Albumen, Gelatine, Glycyrrhizine, etc., which combine chemically -with oxides of Silver, and were shown in the last Section to lessen the -sensitiveness of the Iodide film,--facilitate the development of the -image, producing often a dense deposit of a brown or black colour by -transmitted light. - -In the same way, viz. by a retention of organic matter, may partly be -explained the fact, that the image developed by Pyrogallic Acid, although -proved by the application of tests to contain no more than an equal -quantity of Silver, possesses greater opacity by transmitted light, than -that resulting from the use of protosalts of Iron: and in the case of -the Collodion itself the same rule applied--if it be pure, it is liable -to give a less vigorous impression than when by long keeping a partial -decomposition has taken place, and products have been formed which -combine with reduced Oxide of Silver more easily than the unaltered -Pyroxyline. - -e. _Molecular conditions affecting Intensity._--The physical structure -of the Collodion film is thought to exert an influence upon the mode in -which the reduced Silver is thrown down during the development. A short -and almost powdery state, such as Collodion iodized with the alkaline -iodides acquires by keeping, is considered favourable, and a glutinous, -coherent structure unfavourable, to density. This is certainly the case -when the film is allowed to dry before development, as in the process -with desiccated Collodion and, to some extent, in the Oxymel preservative -process. - -The mode of conducting the development also affects the density; a rapid -action tending to produce an image of which the particles are finely -divided and offer a considerable resistance to the passage of light, -whilst a slow and prolonged development often leaves a metallic and almost -crystalline deposit, comparatively translucent and feeble. - -The writer has observed, that with certain samples of Collodion the -image is much enfeebled by keeping the plate for a considerable time,--a -quarter of an hour or longer,--after sensitizing, but before development. -This effect is not the result of the Nitrate of Silver having partially -drained away, since a second dip in the Nitrate Bath immediately before -applying the Pyrogallic Acid, does not remedy it. An alteration of -molecular structure may therefore be the correct explanation, and if so, a -contractile Collodion would suffer more than one possessing less coherency. - -The actinic power of the light at the time of taking the picture, -influences the appearance of the developed image; the most vigorous -impressions being produced by a strong light acting for a short time. On a -dull dark day, or in copying badly lighted interiors, the photograph will -often lack bloom and richness, and be blue and inky by transmitted light. - -f. _Development of images upon Bromide and Chloride of Silver._--Of the -three principal Salts of Silver, the Iodide is the most sensitive to -light, but the Bromide and Chloride, under some conditions, are more -easily developed and give a darker image. In the Collodion process the -difference is principally seen when organic bodies, like Grape Sugar, -Glycyrrhizine, etc., are introduced in order to increase the intensity; a -far more decided effect being produced by adding both Glycyrrhizine and a -portion of Bromide or Chloride, than by using the Glycyrrhizine alone.[15] - -[15] See the Author's Paper on the chemical composition of the -photographic image, in the eighth Chapter. - -g. _The intensity of the image affected by the length of exposure._--This -point has been briefly alluded to in the third Chapter. If the exposure in -the Camera be prolonged beyond the proper time the development takes place -rapidly but without any intensity, the picture being pale and translucent. -The effects produced by over-action of the light are particularly seen -when the Nitrate Bath contains Nitrite of Silver, or Acetate of Silver; -the image being frequently in such a case dark by reflected light, and -red by transmitted light,--more nearly resembling in fact a photographic -print, developed on paper prepared with Chloride of Silver. When Collodion -plates are coated with honey without previously removing the free Nitrate -of Silver, a slow reducing action is set up, which may give rise to the -characteristic appearance above referred to, after development. Other -organic substances, such as biliary matter, etc., will act in the same way. - -h. _Certain conditions of the Bath which affect development._--Attention -may be called to a peculiar state of the Nitrate Bath, in which the -Collodion image developes unusually slowly, and has a dull grey metallic -appearance, with an absence of intensity in the parts most acted on by -the light. This condition, which occurs only when using a newly mixed -solution, is thought by the Author to depend upon the presence of an -Oxide of Nitrogen retained by the Nitrate of Silver. It is removed -partially by neutralizing the Bath with an alkali, more perfectly so by -adding an excess of alkali followed by Acetic Acid; but most completely by -carefully _fusing_ the Nitrate of Silver before dissolving it. - -Commercial Nitrate of Silver has sometimes a fragrant smell, similar to -that produced by pouring strong Nitric Acid upon Alcohol. When such is the -case, it contains organic matter, and produces a Bath which yields red and -misty pictures. - -Nitrate of Silver which has been sufficiently strongly fused to decompose -the Salt, and produce a portion of the basic Nitrite of Silver exhibits -great peculiarity of development, the image coming out instantaneously and -with great force. This condition is exactly the reverse of that produced -by the presence of acids, in which the development is slow and gradual. - -In summing up the different conditions of the Nitrate Bath which affect -the development of the image, as many as _four_ might be mentioned, each -of which gives a more rapid reduction than the one which precedes it. -These are--the acid Nitrate Bath, the neutral Bath, the Bath of strongly -fused Nitrate of Silver, and the Bath containing _Ammoniacal_ Nitrate of -Silver, which is quite unmanageable, and produces an instantaneous and -universal blackening of the film on the application of the developer. - -Greater intensity of image is commonly obtained in a Nitrate Bath which -has been a long time in use, than in a newly mixed solution: this may be -due to minute quantities of organic matter dissolved out of the Collodion -film, which, having an affinity for Oxygen, partially reduce the Nitrate -of Silver; and also to the accumulation of Alcohol and Ether in an old -Bath producing a short and friable structure of the film. - -i. _Effect of Temperature on Development._--Reduction of the oxides of -noble metals proceeds more rapidly in proportion as the temperature -rises. In cold weather it will be found that the development of the image -is slower than usual, and that greater strength of the reducing agent and -more free Nitrate of Silver is required to produce the effect. - -On the other hand, if the heat of the atmosphere be excessive, the -tendency to rapid reduction will be greatly increased, the solutions -decomposing each other almost immediately on mixing. In this case the -remedy will be to use Acetic Acid _freely_ both in the Bath and in the -developer, at the same time lessening the quantity of Pyrogallic Acid, and -omitting the Nitrate of Silver which is sometimes added towards the end of -the development. - -Also in the case of films which are to be kept for a long time in a -sensitive condition by means of honey, etc., the modifying influence of -temperature must be observed, and the quantity of free Nitrate of Silver -left upon the film be reduced to a minimum if the thermometer stands -higher than usual. - - -SECTION IV. - -_On certain Irregularities in the Developing Process._ - -The characteristics of the proper development of a latent image are--that -the action of the reducing agent should cause a blackening of the Iodide -in the parts touched by light, but produce no effect upon those which have -remained in shadow. - -In operating both on Collodion and paper however there is a liability to -failure in this respect; the film beginning, after the application of the -developer, to change in colour to a greater or less extent over the whole -surface. - -There are two main causes which produce this state of things:--the first -being due to an irregularity in the action of the light; the second to a -faulty condition of the chemicals employed. - -If from a defect in the construction of the instrument, or from other -causes which will be pointed out more particularly in the Second Part -of this work, diffused white light gains entrance into the Camera, it -produces indistinctness of the image by affecting the Iodide more or less -universally. - -The luminous image of the Camera not being perfectly pure, mere -_over-exposure_ of the sensitive plate will usually have the same effect. -In such a case, when the developer is poured on, a faint image first -appears, and is followed by a general cloudiness. - -The clearness of the developed Collodion picture is much influenced by the -condition of all the solutions employed, but particularly so by that of -the Nitrate Bath. If this liquid be in the state termed alkaline (p. 88), -it will be impossible to obtain a good picture; and even when neutral, -care and avoidance of all disturbing causes will be required to prevent -a deposition of Silver upon the shadows of the image: especially so when -Nitrite of Silver or Acetate of Silver are present, both of these salts -being more easily reduced than the Nitrate of Silver. - -The use of _Acid_ is the principal resource in obviating cloudiness of the -image. Acids lessen the facility of reduction of the Salts of Silver by -developing agents (p. 98), and hence when they are present the metal is -deposited more slowly, and only on the parts where the action of the light -has so modified the particles of Iodide as to favour the decomposition: -whereas if acids be absent or present in insufficient quantity, the -equilibrium of the mixture of Nitrate of Silver and reducing agent which -constitutes the developer is so unstable, that any rough point or sharp -edge is likely to become a centre from which the chemical action, once -started, radiates to all parts of the plate. - -Various acids have been employed, such as Acetic acid, Citric acid, -Tartaric acid, etc. Nitric acid is the most effectual of all, but is -seldom used, because, although the image can often be developed with great -clearness when the Bath contains a small quantity of Nitric acid, yet -such a condition is not favourable to _intensity_; on the other hand, -films which are prone to irregular reduction, such as those prepared in a -chemically neutral bath or a bath containing Acetate or Nitrite of Silver, -are likely to give the greatest vigour of impression. Hence, when this -quality is desired, the use of Nitric Acid will be adopted cautiously. - -The state of the Collodion must be attended to as well as that of the -bath; it should be either acid or neutral, not alkaline. Colourless -Collodion may be used successfully as a rule, but sometimes a little -free Iodine is advantageously added. Care should be taken in introducing -organic substances, many of which dissolve out into the bath, and spoil it -for giving clear pictures. Glycyrrhizine, however, which is recommended -to produce intensity of Negatives, has no effect of that kind, and may be -employed with safety. - -The condition of the developing agent is a point of importance in -producing clear and distinct pictures. The Acetic acid, which is advised -in the formulæ, cannot be omitted or even lessened in quantity without -danger. This is particularly the case in hot weather or under any other -condition which favours reduction, such as neutrality of the bath, etc.; -at all times, in fact, when the solutions of Pyrogallic acid and Nitrate -of Silver decompose each other with unusual rapidity. - -In addition to the points now mentioned, viz. the state of the Bath, -of the Collodion, and of the developer, the reader should also study -the remarks made in the Third Section of Chapter III. on the effect -of _surface conditions_ in modifying the deposition of vapour and of -metallic Silver: he will then in all probability experience but little -difficulty in dealing with those numerous irregularities in the action -of the developing fluid, which often prove the greatest hindrance to the -successful practice of the Collodion process. - - - - -CHAPTER VII. - -ON POSITIVE AND NEGATIVE COLLODION PHOTOGRAPHS. - - -The terms "Positive" and "Negative" occur so frequently in all works upon -the subject of Photography, that it will be impossible for the student to -make progress without thoroughly understanding their meaning. - -A Positive may be defined to be a Photograph which gives a natural -representation of an object, as it appears to the eye. - -A Negative Photograph, on the other hand, has the lights and shadows -reversed, so that the appearance of the object is changed or negatived. - -In Photographs taken upon _Chloride of Silver_, either in the Camera or by -superposition, the effect must necessarily be Negative; the Chloride being -_darkened by luminous rays_, the lights are represented by shadows. - -The following simple diagrams will make this obvious. - -[Illustration: Fig. 1. Fig. 2. Fig. 3.] - -Fig. 1 is an opaque image drawn upon a transparent ground; fig. 2 -represents the effect produced by placing it in contact with a layer of -sensitive Chloride and exposing to light; and fig. 3 is the result of -copying this negative again on Chloride of Silver. - -Fig. 3 therefore is a Positive copy of Fig. 1, obtained by means of a -Negative. By the first operation the tints are reversed; by the second, -being reversed again, they are made to correspond to the original. The -possession of a Negative therefore enables us to obtain Positive copies of -the object, indefinite in number and all precisely similar in appearance. -This capability of multiplying impressions is of the utmost importance, -and has rendered the production of good Negative Photographs of greater -consequence than any other branch of the Art. - -The same Photograph may often be made to show either as a Positive or -as a Negative. For instance, supposing a piece of silver-leaf to be cut -into the shape of a cross and pasted on a square of glass, the appearance -presented by it would vary under different circumstances. - -[Illustration: Fig. 1. Fig. 2.] - -Fig. 1 represents it placed on a layer of black velvet; fig. 2 as held -up to the light. If we term it Positive in the first case, _i. e._ -by reflected light, then it is Negative in the second, that is, by -transmitted light. The explanation is obvious. - -Therefore to carry our original definition of Positives and Negatives -a little further, we may say, that the former are usually viewed by -reflected, and the latter by transmitted, light. - -All Photographs however cannot be made to represent both Positives and -Negatives. In order to possess this capability, it is necessary that a -part of the image should be transparent, and the other opaque _but with a -bright surface_. These conditions are fulfilled when the Iodide of Silver -upon Collodion is employed, in conjunction with a developing agent. - -Every Collodion picture is to a certain extent both Negative and Positive, -and hence the processes for obtaining both varieties of Photographs are -substantially the same. Although however the general characters of a -Positive and a Negative are similar, there are some points of difference. -A surface which appears perfectly opaque when looked down upon, becomes -somewhat translucent on being held up to the light; hence, to give the -same effect, the deposit of metal in a Negative must be proportionally -thicker than in a positive; otherwise the minor details of the image, will -be invisible, from not obstructing the light sufficiently. - -With these preliminary remarks, we are prepared to investigate more -closely the _rationale_ of the processes for obtaining Collodion Positives -and Negatives. All that refers to paper Positives upon Chloride of Silver -will be treated in a subsequent Chapter. - - -SECTION I. - -_On Collodion Positives._ - -Collodion Positives are sometimes termed _direct_, because obtained by a -single operation. The Chloride of Silver, _acted upon by light alone_, -is not adapted to yield direct Positives, the reduced surface being dark -and incapable of representing the lights of a picture. Hence a developing -agent is necessarily employed, and the Iodide of Silver substituted for -the Chloride, as being a more sensitive preparation. Collodion Positives -are closely allied in their nature to Daguerreotypes. The difference -between the two consists principally in the surface used to sustain the -sensitive layer, and the nature of the substance by which the invisible -image is developed. - -In a Collodion Positive the lights are formed by a bright surface of -reduced Silver, and the shadows by a black background showing through the -transparent portions of the plate. - -Two main points are to be attended to in the production of these -Photographs. - -First, to obtain an image distinct in every part, _but of comparatively -small intensity_.--If the deposit of reduced metal be too thick, the -dark background is not seen to a sufficient extent, and the picture in -consequence is deficient in shadow. - -Secondly, to _whiten_ the surface of the reduced metal as much as -possible, in order to produce a sufficient contrast of light and shade. -Iodide of Silver developed in the usual way presents a dull yellow -appearance which is sombre and unpleasing. - -_The Collodion and Nitrate Bath for Positives._--Good Positives may be -obtained by diluting down a sample of Collodion with Ether and Alcohol -until it gives a pale bluish film in the Bath. The proportion of Iodide -of Silver being in that case small, the action of the high lights is less -violent, and the shadows are allowed more time to impress themselves. The -dilution lessens the amount of Pyroxyline in the Collodion at the same -time with the Iodide, which is an advantage, the slight and transparent -films always giving more sharpness and definition in the picture. - -The employment of a very thin film for Positives is not however always a -successful process. The particles of the Iodide of Silver being closely -in contact with the glass, unusual care is required in cleaning the -plates in order to avoid stains; and the amount of free Nitrate of Silver -retained upon the surface of the film being small, circular patches of -imperfect development are liable to occur, unless the reducing agent be -scattered evenly and perfectly over the surface. Also if free Iodine or -organic substances which have a retarding effect on the action of light -are present to a considerable extent, the Collodion will not work well -with a small proportion of Iodide. The Author found in experimenting on -this subject that with perfectly pure Collodion and a _neutral_ Bath most -vigorous impressions were produced when the density of the film had been -so far reduced by dilution that scarcely anything could be seen upon the -glass; but with Collodion strongly tinted with Iodine, or with a Bath -containing Nitric Acid, it was necessary to stop the dilution at a certain -point or the film became absolutely insensitive to feeble radiations of -light, and the shadows could not be brought out by any amount of exposure. -In this case, by adding more Iodide a better effect was obtained. - -A thicker Collodion may be used for positives if a little free Iodine be -added, for the purpose of diminishing intensity and keeping the shadows -clear during the development. This process is easier to practice than the -last, but does not always give the same perfect definition. - -No organic substance of the class to which Glycyrrhizine and Sugars belong -should be added to Collodion which is to be used for Positives. By so -doing the image would be rendered intense, and the high lights liable to -solarization, _id est_, a dark appearance by reflected light. - -_The Nitrate Bath._--If the materials are pure, the Nitrate Bath may -advantageously be diluted down at the same time with the Collodion, when -Positives are to be taken; but the employment of a very weak Nitrate -Bath (such as one of 20 grains to the ounce), although highly useful -in obviating excess of development, has some disadvantages; it becomes -necessary to exclude free Nitric Acid, and to avoid the employment of a -Collodion too highly tinted with Iodine. On the other hand, with a strong -Nitrate Bath, and a tolerably dense film of Iodide of Silver, a better -result is often secured by the use of Nitric Acid. The sensitiveness of -the plates is impaired, but at the same time the intensity is diminished, -and the picture shows well upon the surface of the glass. - -A new Bath is better for taking Positives than one which has been a long -time in use. The latter often causes _haziness_ and irregular markings on -the film during the action of the developer. This is due partly to the -accumulation of Alcohol and Ether in the Bath, which causes the solution -of Sulphate of Iron to flow in an oily manner; and partly to a reduction -of the Nitrate of Silver by organic matter. - -The presence of _Acetate of Silver_ is objectionable in a Positive Nitrate -Bath as producing solarization and intensity of image; hence those -precautions which obviate its formation must be adopted (p. 89). - -If fused Nitrate of Silver be used for the Positive Nitrate Bath, it is -very important that the fusion should not be carried too far, or the -solution would contain a basic Nitrite of Silver, and yield an intense, -solarized, and misty image. - -_The Developers for Collodion Positives._--Pyrogallic Acid when used with -Acetic Acid, as is usual for negative pictures, produces a surface which -is dull and yellow. This may be obviated by substituting Nitric Acid in -small _quantity_ for the Acetic. The surface produced by Pyrogallic Acid -with Nitric Acid is lustreless, but very white, if the solution be used -of the proper strength. On attempting to increase the amount of Nitric -Acid the deposit becomes metallic, and the half-tones of the picture are -injured; Pyrogallic Acid, although an active developer, does not allow -of the addition of mineral acid to the same extent as the Salts of Iron. -It requires also, when combined with Nitric Acid, a fair proportion of -Nitrate of Silver on the film, or the development will be imperfect in -parts of the plate. - -_Sulphate of Iron._--The Protosalts of Iron were first employed in -Photography by Mr. Hunt. The Sulphate is a most energetic developer, and -often brings out a picture when others would fail. To produce by means of -it a dead white tint with absence of metallic lustre, it may be used in -conjunction with Acetic Acid, and in a somewhat concentrated condition, so -as to develope the picture quickly. - -The addition of _Nitric Acid_ to Sulphate of Iron modifies the -development, making it more slow and gradual, and producing a bright -sparkling surface of reduced Silver. Too much of this acid however must -not be used, or the action will be irregular. The Nitrate Bath also must -be tolerably concentrated, in order to compensate for the retarding effect -of Nitric Acid upon the development. The blue and transparent films of -Iodide of Silver, formed in a very dilute Nitrate Bath, are not well -adapted for Positives to be developed in this way. They are injured by the -acid, and the development of the image becomes imperfect. - -_Protonitrate of Iron._--This salt, first used by Dr. Diamond, is -remarkable as giving a surface of brilliant metallic lustre without any -addition of free acid. Theoretically, it may be considered as closely -corresponding to the Sulphate of Iron with Nitric Acid added. There are -however slight practical differences between them, which are perhaps in -favour of the Protonitrate. - -The reducing powers of _Protoxide_ of Iron appear to be in inverse ratio -to the strength of the acid with which it is associated in its salts; -hence the _Nitrate_ is by far the most feeble developer of the Protosalts -of Iron. - -The rules already given for the use of Sulphate of Iron acidified with -Nitric Acid, apply also to the Nitrate of Iron; the proportion of free -Nitrate of Silver must be large, and the film of Iodide of Silver not too -transparent. - -In developing direct Positives either by Pyrogallic Acid or the Salts of -Iron, the colour of the image will be found liable to some variation; -the character of the light, whether bright or feeble, and the length of -exposure in the Camera, affecting the result. - -_A Process for whitening the Positive Image by means of Bichloride of -Mercury._--In place of brightening the Positive image by modifying the -developer, it was proposed some time since by Mr. Archer to effect the -same object by the use of the salt known as _Corrosive Sublimate_, or -Bichloride of Mercury. - -The image is first developed in the usual way, fixed, and washed. It is -then treated with the solution of Bichloride, the effect of which is to -produce almost immediately an interesting series of changes in colour. The -surface first _darkens considerably_, until it becomes of an ash-grey, -approaching to black; shortly it begins to get lighter, and assumes a -_pure white_ tint, or a white slightly inclining to blue. It is then -seen, on examination, that the whole substance of the deposit is entirely -converted into this white powder. - -The _rationale_ of the reaction of Bichloride of Mercury appears to be, -that the Chlorine of the mercurial salt divides itself between the Mercury -and the Silver, a portion of it passing to the latter metal and converting -it into a Protochloride. The white powder is therefore probably a compound -salt, as is further evidenced by the effects produced on treating it with -various reagents. - - -SECTION II. - -_On Collodion Negatives._ - -As in the case of a direct Positive we require an image which is _feeble_ -though distinct, so, on the other hand, for a negative, it is necessary to -obtain one of considerable intensity. In the Chapter immediately following -the present, it will be shown that in using glass Negatives to produce -Positive copies upon Chloride of Silver paper, a good result cannot -be secured unless the Negative is sufficiently dark to obstruct light -strongly. - -_The Collodion and Nitrate Bath for Negatives._--A Collodion containing -a very small portion of Iodide and yielding a blue transparent film in -the Bath is not well adapted for taking Negatives. Pale opalescent films -often give too little intensity in the high lights, and, unless the -Nitrate Bath be acid, do not admit of being exposed in the Camera for -the proper length of time without cloudiness and indistinctness of image -being produced under the action of the developer. The effect known as -"solarization of negatives," _i. e._ a red and translucent appearance -of the highest lights, is also more liable to occur when operating with -a very pale film. On the other hand, if the layer of Iodide be too -yellow and creamy, the half-tones of the image will often be imperfectly -developed, so that a middle point between these extremes is the best. - -A pure and newly prepared Collodion, although highly sensitive to -light, does not always give, with one application of the developer, a -sufficiently vigorous image to serve as a negative matrix; and this -particularly in the most brightly illuminated parts, such as the sky in a -landscape photograph, or the white borders of an engraving. But on keeping -the Collodion for some weeks or months it becomes yellow, if iodized with -the alkaline iodides, and a decomposition takes place in it, as before -shown (p. 97), which lessens the rapidity of action, but adds to the -intensity of the negative. - -Grape Sugar may be employed for the purpose of giving intensity to newly -mixed Collodion: also Glycyrrhizine, which is a resinous body extracted -from the root of Liquorice; but as both substances have an effect in -lessening the sensitiveness and keeping qualities of the fluid, they -should be used cautiously. In taking portraits in the open air, on bright -days, and with a Bath which has been mixed for a considerable time, it -will rarely be found that the intensity will be deficient; and especially -so if the developer be applied a second time to the film with a few drops -of solution of Nitrate of Silver added. In landscape Photography however, -or in copying engravings, where extreme sensitiveness is not an object, -the Glycyrrhizine may sometimes be added with advantage in order to obtain -perfect opacity of the blacks. - -When the use of this substance is resorted to, the mode of iodizing the -Collodion appears to be of importance, the increase of intensity being -greater with the Iodide of Cadmium than with the Iodides of the Alkalies; -the latter probably exercising a decomposing action. An addition of a -Bromide or a Chloride to the Collodion in small quantity has also a marked -effect in adding to the intensity when Glycyrrhizine is used with alkaline -Iodides (p. 101). - -Substances which produce intensity of the Collodion image have often, -if added in too large quantity, a tendency to lower the half-tone, and -prevent the darker parts of the picture from being sufficiently brought -out. The print from the Negative is then pale and white, or "chalky" as -it is termed, in the high lights. Collodion in this condition is often -preferred by the beginner, from the facility with which the Negatives -are obtained, but it does not give the finest results. An excess of -Glycyrrhizine in Collodion has also the effect of interfering with the -precipitation of the Iodide of Silver, producing a blue and smoky film -which is nearly useless for Negatives. - -A judicious employment of free Iodine in Collodion which has been -previously intensified with Glycyrrhizine, has a remarkable effect in -improving the gradation of tone. The excessive opacity of the high lights -is diminished, and hence the operator is enabled by a longer exposure of -the sensitive plate to bring out the shadows and minor details of the -image with great distinctness. Collodion prepared in this manner is too -slow to be used for portraits, excepting in a strong light, but often -gives an image with great roundness and stereoscopic effect. - -The Iodine and the liquorice sugar employed conjointly, tend also to -preserve the clearness of the plates under the influence of the developer, -and to give sharpness to the lines and dots of engravings, etc., which, -with a new and sensitive Collodion, are often imperfectly rendered. These -advantages will be appreciated by the operator who has failed from -working with a too feeble Collodion; but it must be borne in mind, that -all substances acting as intensifiers have a bad effect when the state of -the film is not such as to call for their employment. - -The Proto-iodide of Iron has been recommended as an addition to Negative -Collodion. In the Nitrate Bath it forms, in addition to Iodide of Silver, -Protonitrate of Iron, an unstable substance and a developer. The use of -Iodide of Iron gives great sensibility, but it is difficult to preserve -it pure and unchanged. It also decomposes the Collodion in the course of -a few hours, becoming itself peroxidized, and producing an insensitive -condition of film. In addition to this, the negatives taken by the aid of -Iodide of Iron are commonly of an inferior kind, the reduction being too -marked in the high lights; so that its employment is of doubtful utility. - -_The Nitrate Bath._--This should be prepared from Nitrate of Silver which -has been melted at a moderate heat (see pp. 13 and 101). If this point be -neglected, the best Collodion will sometimes fail in producing an intense -negative. - -Acetic Acid must be added in minute quantity, to preserve the solution -from a too ready reduction by the Alcohol and Ether of the Collodion. -Also, unless the Nitrate of Silver be quite pure and free from organic -matter (p. 104), clear pictures will not be obtained without the use of -Acid. - -Acetate of Silver has often been advised as an addition to the Negative -Nitrate Bath. It is produced by dropping into the solution an alkali, such -as Ammonia, followed by Acetic Acid in excess. The Negatives are rendered -blacker and more vigorous by this proceeding, but especially so when the -Bath is contaminated with Nitric Acid; which neutralizes itself at the -expense of the Acetate of Silver, thus:-- - - Acetate of Silver + Nitric Acid - = Nitrate of Silver + Acetic Acid. - -As a rule, it will be better to avoid adding Acetate of Silver to the -Bath, since with, pure melted Nitrate of Silver no Nitric Acid can be -present, and perfect intensity is easily obtained. When the Bath is -saturated with Acetate of Silver, it is in a more reducible state, and -hence unless the glass plates are very perfectly cleaned, black lines -and markings, the results of irregular action, will be produced on the -application of the developer to the film (p. 104). Solarization, or -reddening by over-exposure, is also promoted by the presence of Acetate of -Silver. - -_Developing solutions for Negatives._--The Protosalts of Iron are not -usually employed in developing Negative impressions. They are liable -to yield a violet-coloured image, which cannot easily be rendered more -intense by continuing the action. - -Gallic Acid is too feeble for developing Collodion pictures. Pyrogallic -Acid is much superior, and may be used of any strength, according to -the effect desired. When the light is bad, the temperature low, and -the Negative developes slowly and appears blue and inky by transmitted -light, the proportion of the reducing agent should be increased. But with -an intense Collodion, on a clear summer's day, the finest gradation is -obtained with a weak solution, which does not begin to act until the plate -has been evenly covered. A strong developer might in such, a case produce -too much opacity in the highest lights, and would probably occasion stains -of irregular reduction. - -_Modes of strengthening a finished impression which is too feeble to be -used as a Negative._--The ordinary plan of pushing the development cannot -be applied with advantage after the picture has been washed and dried. In -that case, if it is found to be too feeble to print well, its intensity -may be increased by one of the following methods.-- - -It must be premised however, that the same degree of excellence is not to -be expected in a Negative Photograph which has been improperly developed -in the first instance and more especially if the exposure to light was -too short. Any "instantaneous Positive" may be rendered sufficiently -intense for a Negative, but in that case the shadows are almost invariably -imperfect. - -1. _Treatment of the image with Sulphuretted Hydrogen or Hydrosulphate of -Ammonia._--The object is to convert the metallic Silver into _Sulphuret -of Silver_, and if this could be done it would be of service. The mere -application of an Alkaline Sulphuret has however but little effect upon -the image, excepting to darken its surface and destroy the Positive -appearance by reflected light; the structure of the metallic deposit being -too dense to admit of the Sulphur reaching its interior. - -Professor Donny ('Photographic Journal,' vol. i.) proposes to obviate this -by first converting the image into the white Salt of Mercury and Silver by -the application of Bichloride of Mercury, and afterwards treating it with -solution of Sulphuretted Hydrogen or Hydrosulphate of Ammonia. Negatives -produced in this way are of a brown-yellow colour by transmitted light, -and opaque to chemical rays to an extent which would not, _à priori_, have -been anticipated. - -2. _MM. Barreswil and Davanne's process._--The image is converted into -Iodide of Silver by treating it with a saturated solution of Iodine in -water. It is then washed--to remove the excess of Iodine,--exposed to -the light, and a portion of the ordinary developing solution, mixed with -Nitrate of Silver, poured over it. The changes which ensue are precisely -the same as those already described; the whole object of the process being -to bring the metallic surface back again into the condition of Iodide of -Silver modified by light, that the developing action may be commenced -afresh, and more Silver deposited from the Nitrate in the usual way. - -3. _The process with Bichloride of Mercury and Ammonia._--The image is -first converted into the usual white double Salt of Mercury and Silver -by the application of a solution of the Corrosive Sublimate. It is then -treated with Ammonia, the effect of which is to _blacken_ it intensely. -Probably the alkali acts by converting Chloride of Mercury into the black -Oxide of Mercury. In place of Ammonia, a dilute solution of Hyposulphite -of Soda or Cyanide of Potassium may be used, with very similar results. - - - - -CHAPTER VIII. - -ON THE THEORY OF POSITIVE PRINTING. - - -The subject of Collodion Negatives having been explained in the previous -Chapter, we proceed to show how they may be made to yield an indefinite -number of copies with the lights and shadows correct as in nature. - -Such copies are termed "Positives," or sometimes "Positive prints," to -distinguish them from direct Positives upon Collodion. - -There are two distinct modes of obtaining photographic prints;--first by -development, or, as it is termed, _by the Negative process_, in which -a layer of Iodide or Chloride of Silver is employed, and the invisible -image developed by Gallic Acid; and second, by the direct action of light -upon a surface of Chloride of Silver, no developer being used. These -processes, involving chemical changes of great delicacy, require a careful -explanation. - -The action of light upon Chloride of Silver was described in Chapter II. -It was shown that a gradual process of darkening took place, the compound -being reduced to the condition of a coloured _subsalt_; also, that the -rapidity and perfection of the change were increased by the presence of -excess of Nitrate of Silver, and of organic matters, such as Gelatine, -Albumen, etc. - -We have now to suppose that a sensitive paper has been prepared in this -way, and that a Negative having been laid in contact with it, the -combination has been exposed to the agency of light for a sufficient -length of time. Upon removing the glass, a Positive representation of the -object will be found below, of great beauty and detail. Now if this image -were in its nature fixed and permanent, or if there were means of making -it so, without injury to the tint, the production of Paper Positives would -certainly be a simple department of the Photographic Art; for it will be -found that with almost any Negative, and with sensitive paper however -prepared, the picture will look tolerably well on its first removal from -the printing-frame. Immersion in the bath of Hyposulphite of Soda however, -which is essentially necessary in order to fix the picture, produces -an unfavourable effect upon the tint; decomposing the violet-coloured -Subchloride of Silver, and leaving behind a red substance which appears -to be united to the fibre of the paper, and, when tested, reacts in the -manner of a Suboxide of Silver. - -Other chemical operations are therefore required to remove the -objectionable red colour of the print, and hence the consideration of the -subject is naturally divided into two parts; first, the means by which -the paper is rendered sensitive, and the image impressed upon it;--and -secondly, the subsequent fixing and _toning_, as it may be termed, of the -proof. - -The present Chapter will also include, in two additional Sections, a -condensed account of the most important facts relating to the properties -and the mode of preservation of photographic prints. - - -SECTION I. - -_The Preparation of the Sensitive Paper._ - -In this Section the general theory of the preparation of Positive paper, -in so far as it affects the tone and intensity of the print, will be -described; the reader being referred to the second division of the Work -for the formulæ required. - -_The Preparation of the Sensitive Paper._--The conditions which are -required for producing a sharp and well defined print are--that an even -layer of Chloride of Silver should exist upon the very surface of the -paper, and that the particles of this Chloride should be in contact with -a sufficient excess of Nitrate of Silver. These points have been already -referred to at an early part of the Work (p. 19). - -The material used for _sizing_ the paper is of importance. English papers -are usually sized with Gelatine, which is a photographic agent, and acts -chemically in forming the image. Foreign papers on the other hand being -sized with starch only, require an addition of Gelatine, Caseine, or -Albumen, to retain the Salt at the surface of the paper, and to assist -in producing the picture: if otherwise, the print will be flat and -"mealy," as it is termed. Albumen especially produces a beautifully smooth -surface, and is advantageously employed in printing small portraits and -stereoscopic subjects. - -The uniform surface distribution of the Chloride of Silver is sometimes -interfered with by a faulty structure of the paper, causing it to absorb -liquids unevenly, and in consequence the pictures, when removed from the -printing frame, appear _spotted_. Another cause producing the same effect, -is the employment of too weak a solution of Nitrate of Silver, or the -removal of the sheet from the Nitrate bath before the Chloride of Ammonium -has been perfectly decomposed; it is thus rendered unequally sensitive at -different portions of the surface, and the prints have the characteristic -marbled appearance above referred to. - -A sufficient excess of Nitrate of Silver being essential, it is important -to bear in mind, that the quantity of this salt eventually remaining in -the paper, is much influenced by the manner in which the solution is -applied. If it be laid on by _floating_, then the proportion of Nitrate to -that of Chloride of Sodium should be about as 3 to 1 (the atomic weights -are nearly as 5 to 2); but if the plan of brushing or spreading with a -glass rod be adopted, 7 to 1 or 8 to 1 will not be too much. - -_The Darkening of the Sensitive Paper by Light._--The operator should be -familiar with the changes of colour which indicate the progress of the -reduction of the sensitive layer. Much in this respect depends upon the -kind of organic matter used, but there is always a regular sequence of -tints; in the case of a paper prepared simply with Chloride of Ammonium -and Nitrate of Silver, it is as follows: pale violet, violet-blue, -slate-blue, _bronze_ or copper-colour. When the _bronzed_ stage is -reached, there is no further change. On immersion in the fixing bath of -Hyposulphite, the violet tones due to Subchloride of Silver are destroyed, -and the print assumes a red or brown colour, which is deepest and most -intense in the parts where the light has acted longest. - -Hence we see, that, to produce a good print, it is essential that the -Negative should possess considerable intensity in the dark parts. Pale -and feeble Negatives yield proofs which are wanting in vigour, and have -a flat and indistinct appearance. The combination cannot be exposed to -light for a sufficient length of time to bring about the requisite degree -of reduction of the Chloride of Silver; and hence the deepest shadows of -the resulting Positive are not sufficiently dark, and there is _a want of -contrast_ which is fatal to the effect. - -A good Negative should be so opaque as to preserve the lights of the -printed image beneath clear, _until the darkest shades are about to pass -into the bronze or coppery condition_. If the amount of intensity be less -than this, the finest effect cannot be obtained. - - -CONDITIONS AFFECTING THE SENSITIVENESS OF THE PAPER AND THE INTENSITY OF -THE IMAGE. - -Some of the principal of these are as follows:-- - -a. _The Strength of the Salting Bath._--The sensibility of the paper is -regulated up to a certain point by the amount of salt[16] used in the -preparation. The quantity of alkaline Chloride determines the amount of -Chloride of Silver; and with a proper excess of Nitrate of Silver, papers -are to a certain point more sensitive in proportion as they contain more -of the Chloride. - -[16] The difference in the atomic weights of the various soluble Chlorides -used in salting must be borne in mind. Ten grains of Chloride of Ammonium -contain as much Chlorine as eleven of Chloride of Sodium, or twenty-two -grains of Chloride of Barium. (See the Vocabulary, Part III.) - -Highly sensitized papers darken rapidly, and pass very completely into the -bronze stage. Those containing less Chloride darken more slowly, and do -not become bronzed with the same intensity of light. A Photographic print, -formed upon paper highly salted and sensitized, is usually vigorous, with -great contrast of light and shade; particularly so when the printing is -conducted in a strong light. Hence it will be an advantage, with a feeble -Negative, and in dull weather, to _double_ the ordinary quantity of Salt, -whereas in the case of an intense Negative, and with direct sunlight, the -deep shadows will be too much bronzed unless the quantity of Chloride and -Nitrate of Silver in the paper be kept low. - -In proportion as Photographic papers are highly salted and sensitized, -they become more prone to change colour spontaneously in the dark. - -b. _Proportion of Nitrate of Silver._--The compound on which a positive -print is formed is a Chloride, or an organic Salt of Silver, _with -an excess of Nitrate of Silver_. Nothing is gained by increasing the -proportion of Chloride of Sodium, unless at the same time an addition be -made to the quantity of free Nitrate in the sensitizing Bath. - -A surface of Chloride of Silver with a bare excess of Nitrate, darkens on -exposure, but it does not reach the bronzed stage; the action appearing to -stop at a certain point. On placing the print in Hyposulphite of Soda, it -becomes very red and pale, and when tinted, looks cold and slaty, without -depth or intensity. - -c. _The sensitiveness and intensity affected by substituting the Oxide -of Silver for the Nitrate._--Many operators employ a solution of Oxide -of Silver in Ammonia[17] or Nitrate of Ammonia, in preparing Chloride of -Silver paper. By doing so, a great increase of sensitiveness, and also of -intensity of image, is obtained. This will be understood if we remember -that the action of light in producing the print is of a reducing nature. -Hence the substitution of Oxide for Nitrate of Silver facilitates the -decomposition; just as _Ammonio-Nitrate_ of Silver is more readily reduced -by Gallic or Pyrogallic Acid than the simple Nitrate (see p. 31). - -[17] The chemistry of Ammonio-Nitrate of Silver is explained in the -Vocabulary, Part III. - -Ammonio-Nitrate paper has the disadvantage of soon _discolouring_ when -kept; but it is very serviceable in printing during the winter months. -The proportion of Chloride in the salting Bath may, if desired, be -considerably reduced; the intensity of action being greatly exalted by the -use of the Oxide of Silver. - -d. _Employment of organic matters._--Those recommended in this work -are--Albumen, Gelatine, and Iceland Moss. Albumen adds much to the -sensibility of the paper, and gives very fine surface definition. A less -amount of Chloride is required than in the case of plain paper simply -salted, the glutinous character of Albuminous liquids causing more of -the fluid to be retained upon the surface of the paper, and the animal -matter assisting the reduction. By varying the proportion of salt, both -feeble and intense Negatives may be printed successfully upon albuminized -paper. No process gives better results, either as regards sensitiveness, -or in faithfully rendering all the finer details of the Negative, than the -process with Albumen. - -Iceland Moss, when boiled in water, yields a mucilaginous liquid which is -conveniently employed as a vehicle for Chloride of Silver; it increases -the sensitiveness of the paper and gives additional power of bronzing, -by assisting to reduce the free Nitrate of Silver. Many other organic -matters, tending to absorb oxygen, would act in the same way. - -_Gelatine_ is used in positive printing; it is analogous to Albumen in -composition, and, like it, forms a red compound with Suboxide of Silver. -It is serviceable in keeping the print at the surface of the paper, but -does not alter the sensibility or the general appearance of the finished -picture so greatly as Albumen. - -e. _Impurities in Nitrate of Silver._--Nitrate of Silver used for -Photographic printing should be free from even a trace of Protonitrate of -Mercury, since it is known that the precipitation of Chloride of Mercury -prevents the darkening of Chloride of Silver by light. - -The peculiar condition of Nitrate of Silver spoken of at page 101, in -which it is thought to contain Oxides of Nitrogen, is likely to interfere -with Photographic printing. This is probably the explanation of a -faulty state of the Nitrate solution, in which it yields red and feeble -positives, and does not darken in colour in exciting albuminized paper. -The remedy will be, to fuse the Nitrate of Silver at a moderate heat -before dissolving it. - - -THE COLOUR OF THE IMAGE INFLUENCED BY THE PREPARATION OF THE SENSITIVE -PAPER. - -This subject should be studied by those who desire to print with taste. -By introducing a few simple modifications into the mode of preparing the -sensitive paper, almost any variety of tint may be obtained. - -The tendency of the "toning" process, to which the print is afterwards to -be submitted, is to darken the colour, and, if gold be used, to give a -shade of _blue_. Hence, if the Positive be printed of a red tone, it will -change in the gold Bath to a purple; whereas if left, after exposure to -light and fixing, of a dark brown or sepia tint, it passes by toning into -a pure black. - -The Positive should look warm and bright on its removal from the printing -frame; but the tint which remains after immersion in Hyposulphite of Soda -is the proper colour of the simply fixed print. - -The following points may be mentioned as affecting the colour and general -appearance of the picture. - -a. _The proportions of Salt and Nitrate of Silver._--Highly salted and -sensitized papers give a _darker_ image than those which, containing a -small proportion of Chloride of Silver, are less sensitive to light. -Hence in printing upon paper weakly sensitized, in order to bring out the -finer details of a highly intense negative, we find the image unusually -red after fixing, and of a brown or mulberry colour when toned. The above -remarks apply also in some degree to the strength of the Nitrate Bath, and -especially so when no organic matter excepting Gelatine is employed,--in -such a case the image will be _darker_ after fixing, if the proportion of -free Nitrate of Silver be large. - -b. _Effect of Oxide of Silver on the colour._--Prints formed upon -Ammonio-Nitrate papers highly salted are of a sepia colour after fixing, -and usually of a pure black or a purple-black when toned. With the -increased facility of reduction by light afforded by use of _Oxide_ of -Silver, there is also less redness in the print. But if the quantity of -salt used in preparing the paper be reduced to a minimum (one grain to -the ounce or less), for the sake of economy or to improve the half-tone, -then the usual red colour returns, and the Positive is brown or purple -after toning, in place of black. Thus by employing a solution of Oxide of -Silver, the operator is enabled, without the addition of organic matter, -to print Positives of a pleasing variety of tint, combined with a peculiar -softness and delicacy, which cannot easily be obtained with the simple -Nitrate of Silver. - -c. _The colour affected by organic matter._--Albumen is coagulated by -Nitrate of Silver, and forms a permanent gloss upon the paper. The -sensitive albuminized paper darkens in the sun to a chocolate-brown -colour, which becomes very red on immersion in the Hyposulphite. The -finished prints are clear and transparent; usually of a brown tone, or -with a shade of purple when the gold Bath is newly made and active; pure -blacks are not easily obtained. - -Iceland Moss affects the colour of the proof to a certain extent, but less -than Albumen; the finished prints are nearly black if the paper is highly -salted. - -The Gelatinous sizing used for the English papers, and obtained by boiling -hides in water, and hardening the product by an admixture of Alum, has -a _reddening_ influence upon reduced Silver salts, analogous to that of -Albumen, or of Caseine, the characteristic animal principle of milk. -Positives printed upon English paper, commonly assume some shade of brown -more or less removed from black; the darker tones being more readily -obtained upon the foreign papers. - -Citrates and Tartrates have a marked effect upon the colour of prints. -Paper prepared with Citrate, in addition to Chloride of Silver, darkens to -a fine purple colour which changes to brick-red in the fixing Bath. The -Positives, when toned, are usually of a violet-purple or of a bistre tint, -with a general aspect of warmth and transparency. - - -SECTION II. - -_The Processes for Fixing and Toning the Proof._ - -This part of the operation is one to which great attention should be -paid, in order to secure bright and lasting colours: it involves more of -delicate chemical change than perhaps any other department of the Art. - -The first point requiring explanation is the process of fixing; to which -(p. 41) brief reference has already been made. The methods adopted to -improve the tint of the finished picture will then be described. - - -CONDITIONS OF A PROPER FIXING OF THE PROOF. - -This subject is not always understood by operators, and consequently they -have no certain guide as to how long the prints should remain in the -fixing Bath. - -The time occupied in fixing will of course vary with the strength of -the solution employed; but there are simple rules which may be usefully -followed. In the act of dissolving the unaltered Chloride of Silver -in the proof, the fixing solution of Hyposulphite of Soda converts it -into Hyposulphite of Silver (p. 43), which is soluble in an _excess_ of -Hyposulphite of Soda. But if there be an insufficient excess,--that is, if -the Bath be too weak, or the print removed from it too speedily,--then the -Hyposulphite of Silver is not perfectly dissolved, and begins by degrees -to _decompose_, producing a brown deposit in the tissue of the paper. -This deposit, which has the appearance of yellow spots and patches, is -not usually seen upon the surface of the print, but becomes very evident -when it is held up to the light, or if it be split in half, which can be -readily done by gluing it between two flat surfaces of deal, and then -forcing them asunder. - -_The reaction of Hyposulphite of Soda with Nitrate of Silver._--In order -to understand more fully how _decomposition_ of Hyposulphite of Silver may -affect the process of fixing, the peculiar properties of this salt should -be studied. With this view Nitrate of Silver and Hyposulphite of Soda may -be mixed in equivalent proportions, viz. about twenty-one grains of the -former salt to sixteen grains of the latter, first dissolving each in -separate vessels in half an ounce of distilled water. These solutions are -to be added to each other and well agitated; immediately a dense deposit -forms, which is Hyposulphite of Silver. - -At this point a curious series of changes commences. The precipitate, at -first white and curdy, soon alters in colour: it becomes canary-yellow, -then of a rich orange-yellow, afterwards liver-colour, and finally black. -The _rationale_ of these changes is explained to a certain extent by -studying the composition of the Hyposulphite of Silver. The formula for -this substance is as follows:-- - - AgO S{2}O{2}. - -But AgO S{2}O{2} plainly equals AgS, or Sulphuret of Silver, and SO{3}, or -Sulphuric Acid. The acid reaction assumed by the supernatant liquid is due -therefore to Sulphuric Acid, and the black substance formed is Sulphuret -of Silver. The yellow and orange-yellow compounds are earlier stages of -the decomposition, but their exact nature is uncertain. - -The instability of Hyposulphite of Silver is principally seen when it is -in an isolated state: the presence of an excess of Hyposulphite of Soda -renders it more permanent, by forming a double salt, as already described. - -In fixing Photographic prints, this brown deposit of Sulphuret of Silver -is very liable to form in the Bath and upon the picture; particularly -so when the _temperature_ is high. To obviate it, observe the following -directions:--It is especially in the reaction between _Nitrate of Silver_ -and Hyposulphite of Soda that the blackening is seen; the Chloride and -other _insoluble_ Salts of Silver being dissolved, even to saturation, -without any decomposition of the Hyposulphite formed. Hence if the -print be washed in water to remove the soluble Nitrate, a very much -weaker fixing Bath than usual may be employed. But if the proofs are -taken at once from the printing frame and immersed in a dilute Bath of -Hyposulphite (one part of the salt to six or eight of water), _a shade of -brown_ may often be observed to pass over the surface of the print, and -a large deposit of Sulphuret of Silver soon forms as the result of the -decomposition. On the other hand, with a strong Hyposulphite Bath there is -little or no discoloration, and the black deposit is absent. - -The print must also be left for a sufficient time in the fixing bath, -or some appearance of brown patches,[18] visible by transmitted light, -may occur. Each atom of Nitrate of Silver requires _three_ atoms of -Hyposulphite of Soda to form the _sweet and soluble double salt_, and -hence, if the action be not continued sufficiently long, another compound -will be formed almost tasteless and insoluble (p. 44). Even immersion in -a new Bath of Hyposulphite of Soda does not fix the print when once the -yellow stage of decomposition has been established. This yellow salt is -insoluble in Hyposulphite of Soda, and consequently remains in the paper. - -[18] The writer has noticed that when sensitive paper is _kept for some -time_ before being used for printing, these yellow patches of imperfect -fixation are very liable to occur. The Nitrate of Silver appears gradually -to enter into combination with the organic matter of the size of the -paper, and cannot then be so easily extracted by the fixing bath. - -In fixing prints by Ammonia the Author has found that the same rule may be -applied as in the case of Hyposulphite of Soda, viz. that if the process -be not properly performed, the white parts of the print will appear -_spotted_ when held up to the light, from a portion of insoluble Silver -Salt remaining in the paper. Prints imperfectly fixed by Ammonia are also -usually brown and discoloured upon the surface of the paper. - -More exact directions as to the strength of the fixing bath and the time -occupied in the process, will be given in the Second Part of the Work; -at present it may be noticed only that _Albuminized_ paper, from the -horny nature of its surface-coating, requires a longer treatment with the -Hyposulphite than the plain paper. - - -THE SALTS OF GOLD AS TONING AGENTS FOR PHOTOGRAPHIC PRINTS. - -The Salts of Gold have been successfully applied to the improvement of the -tones obtained by simply fixing the proof in Hyposulphite of Soda. The -following are the principal modes followed:-- - -_M. Le Grey's Process._--The print, having been exposed to light until -it becomes very much darker than it is intended to remain, is washed in -water to remove the excess of Nitrate of Silver. It is then immersed in a -dilute solution of Chloride of Gold, acidified by Hydrochloric Acid. The -effect is to reduce the intensity considerably, and at the same time to -change the dark shades to a violet or bluish tint. After a second washing -with water, the proof is placed in plain Hyposulphite of Soda, which fixes -it and alters the tone to a pure black or a blue-black, according to the -manner of preparing the paper and the time of exposure to light. - -The _rationale_ of the process appears to be as follows:-- the Chlorine, -previously combined with Gold, passes to the reduced Silver Salt; it -bleaches the lightest shades, by converting them again into white -Protochloride of Silver, and gives to the others a violet tint more or -less intense according to the reduction. At the same time metallic Gold -is deposited, the effect of which is not visible at this stage, since the -same violet tint is perceived when a solution of _Chlorine_ is substituted -for Chloride of Gold. - -The Hyposulphite of Soda subsequently employed, decomposes the violet -Subchloride of Silver, and leaves the surface of a black tint, due to the -Gold and the reduced Silver Salt. - -M. Le Grey's process is objectionable on account of the excessive -over-printing required. This however is to a great extent obviated by a -modification of the process in which an _alkaline_ instead of an acid -solution of the Chloride is employed; one grain of Chloride of Gold is -dissolved in about six ounces of water, to which are added twenty to -thirty grains of the common Carbonate of Soda. The alkali moderates the -violence of the action, so that the print washed with water and immersed -in the Gold Bath, is less reduced in intensity, and does not acquire the -same _inky_ blueness. On subsequent fixing in the Hyposulphite, the tint -changes from violet to a dark chocolate-brown, which is permanent. - -_The Tetrathionate and Hyposulphite of Gold employed in toning._--After -the discovery of Le Grey's mode, it was proposed, as an improvement, -to add Chloride of Gold to the fixing solution, so as to obviate the -necessity of using two Baths. The print, in that case, although darkened -considerably, is less reduced in intensity, and the same amount of -over-printing is not required. The chemical changes which ensue are -different from before: they may be described as follows:-- - -Chloride of Gold, added to Hyposulphite of Soda, is converted into -Hyposulphite of Gold, Tetrathionate of Gold, and (if the Chloride of Gold -be free from excess of acid) a red compound, containing more of the metal -than, either of the others, but the exact nature of which is uncertain. -Each of these three Gold Salts possesses the property of darkening the -print, but not to the same extent. The activity is less as the stability -of the salt is greater, and hence the red compound, which is so highly -unstable that it cannot be preserved many hours without decomposing and -precipitating metallic Gold, is far more active than the Hyposulphite of -Gold, which, when associated with an excess of Hyposulphite of Soda, is -comparatively permanent. - -When rapidity of colouring is an object it will therefore be advisable to -add Chloride of Gold to the fixing Bath of Hyposulphite rather than an -equivalent quantity of Sel d'or; and by dropping a little Ammonia into the -Chloride of Gold so as to precipitate "fulminating gold"[19] (a compound -which dissolves in Hyposulphite of Soda with considerable formation of the -unstable red salt), the activity of the Bath will be promoted. - -[19] Read the observations on the Explosive Properties of Fulminating Gold -in the Vocabulary, Part III. - -The Author explains the action of these Salts of Gold upon the Positive -print as follows:--they are unstable, and contain an excess of Sulphur -loosely combined; hence, when placed in contact with the image, which -has an affinity for Sulphur, the existing compound is broken up, and -Sulphuret of Silver, Sulphuric Acid, and metallic Gold are the results. -That a minute proportion of Sulphuret of Silver is formed seems certain; -but the change must be superficial, as the stability of the print is very -little lessened when the process is properly performed. - -_Sel Or employed as a toning agent._--This process, which was communicated -to the 'Photographic Journal' by Mr. Sutton of Jersey, has been found -serviceable. - -The prints are first washed in water, to which is added a little Chloride -of Sodium, to decompose the free Nitrate of Silver. They are then immersed -in a dilute solution of "Sel d'or," or double Hyposulphite of Gold and -Soda, which quickly changes the tint from red to purple without destroying -any of the details or lighter shades. Lastly, the Hyposulphite of Soda is -employed to fix the print in the usual way. - -This process differs theoretically from the last in some important -particulars. The toning solution is applied to the print _before fixing_, -which experience proves to have an important influence upon the result, -it having been found that when the print is previously acted upon by -Hyposulphite of Soda, the rapidity of deposition of the Gold is interfered -with;--thus, a dilute solution of Sel d'or colours a print rapidly, but if -to this same liquid a few crystals of Hyposulphite of Soda be added, the -picture becomes red and may be kept in the Bath for comparatively a long -time without acquiring the purple tones. - -As Hyposulphite of Soda in excess lessens the action of the Sel d'or, so -on the other hand the addition of an acid increases it. The acid does -not precipitate _Sulphur_, as might be expected from a knowledge of the -reaction of Hyposulphite with acid bodies (p. 137), but it favours the -reduction of metallic Gold. Hence it is usual to add a little Hydrochloric -Acid to the toning solution of Sel d'or, to increase the rapidity and -perfection of the colouring process. - - -THE CONDITIONS WHICH AFFECT THE ACTION OF THE FIXING AND TONING BATH OF -GOLD AND HYPOSULPHITE OF SODA. - -Although the process of toning Positives by Sel d'or is very certain in -its results and gives good tints, yet, as involving a somewhat greater -expenditure of time and trouble, it is not at present universally adopted. -The ordinary plan of fixing and toning in one bath has been proved to -yield permanent prints if the proper precautions are observed, but it is -quite necessary, in order to ensure success, that the conditions by which -its action is modified should be understood. The more important of these -are as follows:-- - -a. _The_ AGE _of the Bath._--When Chloride of Gold is added to -Hyposulphite of Soda, several unstable salts are produced, which decompose -by keeping. Hence the solution is very active during the first few days -after mixing; but at the expiration of some weeks or months, if not used, -it becomes almost inert, a reddish deposit of Gold first forming, and -eventually a mixture of black Sulphuret of Silver and Sulphur, the former -of which often adheres to the sides of the bottle in dense shining laminæ. - -When the Bath is constantly kept in use there is a loss of Gold, which, -although it is less perceived than it otherwise would be, from the fact -that sulphuretting principles are formed (see next page) capable of -replacing the Gold as toning agents--yet makes the Bath work more slowly, -and hence over-printing is required. - -b. _Presence of free Nitrate of Silver upon the surface of the -proof._--This produces an accelerating effect, as may be shown by soaking -the print in salt and water, to convert the Nitrate into Chloride of -Silver; the action then takes place more slowly. - -The free Nitrate of Silver increases the instability of the Gold salts; -but if present in too great an excess, it is apt to cause a decomposition -of Hyposulphite of Silver, and consequent yellowness in the white parts -of the proof. It is therefore particularly recommended to wash the print -in water before immersing it in the fixing and toning Bath. - -c. _Temperature of the solution._--In cold weather, the thermometer -standing at 32° to 40°, the Bath works more slowly than usual; whereas -in the height of summer, and especially in hot climates, it occasionally -becomes quite unmanageable. The best temperature for operating -successfully appears to be about 60° to 65° Fahrenheit; if higher than -this the solutions must be employed more dilute. - -d. _Addition of Iodide of Silver._--Some operators associate Iodide with -Chloride in the preparation of sensitive paper for printing. Another -source of the same salts is the admixture of a portion of the fixing Bath -used for Negatives with the Positive toning solution. The presence of -Iodides in the fixing and toning Bath is injurious: when in large excess, -they dissolve the image, or produce yellow patches of Iodide of Silver on -the lights; in smaller quantity, the deposition of the Gold is hindered, -and the action proceeds more slowly. Bromides and Chlorides have not the -same effect. - -e. _Mode of preparing the paper._--The rapidity of toning varies with -causes independent of the Bath: thus, plain paper prints are toned more -quickly than prints upon albuminized paper, and the use of English paper -sized with Gelatine retards the action. Foreign papers rendered sensitive -with Ammonio-Nitrate tone the most quickly. - -_On certain states of the fixing and toning Bath which are injurious to -the proofs._--The object of using the Hyposulphite Bath is to fix the -proof and to tone it by means of Gold. But it is a fact familiar to the -photographic chemist, that Positives can also be toned by a sulphuretting -action, and that the colours so obtained are not very different from -those which follow the employment of Gold.[20] Now the Hyposulphite of -Soda is a substance which can be very readily made to yield up Sulphur -to any bodies which possess an affinity for that element, and as the -reduced Silver compound in the print has such an affinity, there is -always a tendency to absorption of Sulphur when the proofs are immersed -in the Bath. Consequently in many cases a sulphur toning-process is -set up, and as the picture is improved by it in appearance, losing its -brick-red colour and assuming a purple shade, it was at first adopted -by Photographers. Experience however has shown that colours brightened -in this way are less permanent than others, and are liable to fade -unless kept perfectly dry. Hence the process will be discarded by all -careful operators, and the object will be to avoid sulphuration as far -as possible. This can be done to a great extent, and, when the Bath is -properly managed, the prints will be toned almost entirely by Gold, and -will, with care, be permanent. - -[20] For a more detailed account of the toning process by Sulphur, see the -Third Section of this Chapter, page 145. The instability of sulphuretted -prints is shown in the fourth Section. - -Some of the conditions which facilitate a sulphuretting action upon the -proof are as follows:-- - -a. _The addition of an Acid to the Bath._--It was at one time common to -add a few drops of Acetic Acid to the fixing Bath of Hyposulphite of -Soda, immediately before immersing the proofs. The Bath then assumes an -opalescent appearance in the course of a few minutes, and, when this -milkiness is perceptible, the print begins to _tone_ rapidly and becomes -nearly black. - -The chemical changes produced in a Hyposulphite Bath by addition of acid, -may be explained thus:--The acid first displaces the feeble Hyposulphurous -acid from its combination with Soda. - - Acetic Acid + Hyposulphite Soda. - = Acetate Soda + Hyposulphurous Acid. - -Then the Hyposulphurous Acid, _not being a stable substance when -isolated_, begins spontaneously to decompose, and splits up into -Sulphurous Acid--which remains dissolved in the liquid, communicating the -characteristic odour of burning Sulphur--and _Sulphur_, which separates in -a finely divided state and forms a milky deposit.[21] - -[21] From the Vocabulary, Part III., it will be seen that commercial -Chloride of Gold usually contains _free Hydrochloric Acid_; hence -a considerable deposit of Sulphur takes place on adding it to the -Hyposulphite solution, and the liquid must not be used immediately. - -Observe therefore that free acids of all kinds must be excluded from the -fixing Bath, or, if inadvertently added, the liquid must be set aside for -some hours until the Hyposulphurous Acid has decomposed, and, the Sulphur -having settled to the bottom, the Bath has regained its original neutral -condition.[22] - -[22] The chemical reader will understand the decomposition of free -Hyposulphurous Acid by the following equation:--S{2}O{2} = SO{2} and S. - -b. _Decomposition of the Bath by constant use._--It has long been known -that a solution of Hyposulphite of Soda undergoes a peculiar change in -properties when much used in fixing. When first prepared it leaves the -image of a red tone, the characteristic colour of the reduced Silver -Salt, but soon acquires the property of darkening this red colour by a -subsequent communication of Sulphur. Hence a simple fixing Bath becomes at -last an active toning bath, without any addition of Gold. - -This change of properties will be found more fully explained in the -abstract of the Author's researches given in the next Section (p. 156). At -present we remark only that it is due principally to a reaction between -Nitrate of Silver and Hyposulphite of Soda, attended with decomposition -of Hyposulphite of Silver (p. 130); and hence, if the prints are washed -in water before immersion in the Bath, the solution will be less quickly -liable to change. - -Many operators state that the toning Bath having at first been prepared -with Chloride of Gold, no further addition of this substance will be -required. This no doubt is correct, but in such case the proofs will at -last be toned by Sulphur more than by Gold, and will not possess the -same stability; the Bath will also, after long use, be found to acquire -a distinct _acid_ reaction to test-paper, the acidity being due to a -peculiar principle generated by decomposing Hyposulphite of Silver, and -which is shown to have an injurious action upon the print (p. 158). To -avoid this the solution should be kept _neutral to test-paper_ by means of -a drop of Ammonia, if required; and when it begins to be exhausted, and -does not tone (quickly) a print from which the free Nitrate of Silver has -been removed by washing, a fresh quantity of Chloride of Gold should be -added. - -c. _Tetrathionate in the Hyposulphite Bath._--The Author has shown that -the Tetrathionates, which are analogous to the Hyposulphites, have an -active sulphuretting action upon Positive prints (see the papers in the -next Section). Very fine colours can be obtained in this way; but toning -by Sulphur having been proved to be wrong in principle, the formulæ given -in the first two editions of this Work have been omitted.[23] - -[23] The preparation of a toning bath by Tetrathionate, without Gold, is -described in the next Section, but it is not recommended for practical use. - -The bodies which produce Tetrathionate when added to a solution of -Hyposulphite of Soda, and hence are inadmissible in the toning process, -are as follows:--Free Iodine, Perchloride of Iron, Chloride of Copper, -Acids of all kinds (in the latter case the acid first produces Sulphurous -Acid, and the Sulphurous Acid, if present in any quantity, by reacting -upon Hyposulphite of Soda, forms Tetrathionate and Trithionate of Soda). - -Chloride of Gold also produces a mixed Tetrathionate of Gold and Soda when -added to the fixing Bath (p. 133); but as the quantity of Chloride used -is small, the prints are far less sulphuretted than in the case of toning -Baths prepared by Tetrathionate without Gold. - - -SECTION III. - -_The Author's Researches in Photographic Printing._ - -Having been long engaged in conducting experiments upon the composition -and properties of the reduced material forming the Photographic image, and -especially with a view of determining the exact conditions under which the -picture may be considered permanent, the Author has thought it advisable -to give the results of these researches in the form of an abstract of the -original papers read at the meetings of the Photographic Society. - -A previous perusal of these papers will put the reader in possession of -the principal facts upon which are founded the precautions advised in the -next Section for the preservation of Photographic prints. In order to keep -the Work as nearly as possible within its original limits, and also for -the purpose of distinguishing the present Section from the others, as one -referring principally to scientific details, the type has been reduced to -the size of that used in the Appendix. - - -ON THE CHEMICAL COMPOSITION OF THE PHOTOGRAPHIC IMAGE. - -The determination of the chemical nature of the Photographic image in -its various forms is a point of much importance, both as indicating the -conditions required for the preservation of works of art of that class, -and also as a guide to the experimenter in selecting bodies likely to have -an effect as chemical agents in Photography. - -It has been stated by some who have given attention to the subject, -that the image is formed in all cases of pure metallic Silver, and that -any observable variations in its colour and properties, are due to a -difference in the molecular arrangement of the particles. But this -hypothesis, although involving much that is correct, yet does not contain -the whole truth, for it is evident that the chemical properties of the -Photographic image often bear no resemblance to those of a metal. One -Photograph may also differ essentially from another, so that we are led -to infer the existence of two varieties, the first of which is less of a -metallic nature than the second. - -In investigating the subject, the principal point appeared to be to -examine the action of light upon Chloride of Silver, and afterwards -to associate the Chloride with organic matter in order to imitate the -conditions under which Photographs are obtained. - -The following is an epitome of the conclusions arrived at:-- - -_Action of Light upon Chloride of Silver._--The process is accompanied -by a separation of Chlorine, but its product is not a mere mixture of -Chloride of Silver and Metallic Silver; if it were so, we cannot suppose -that the darkening would take place beneath the surface of Nitric Acid, -which it is found to do. A definite Subchloride of Silver seems to be -formed, the most important property of which is its decomposition by -fixing agents, such as Ammonia, and Hyposulphite of Soda, both of which -destroy the violet colour, dissolving out Protochloride of Silver, and -leaving a small quantity of a grey residue of metallic Silver. - -Inasmuch therefore as all Photographic pictures require fixing, we may -conclude that if they could be produced upon pure and isolated Chloride -of Silver (which however is not the case), they would consist solely of -metallic Silver. - -_Decomposition of organic Salts of Silver by Light._--Compounds of Oxide -of Silver with organic bodies, are as a rule darkened by exposure to -light, but the process does not always consist in a simple reduction to -the metallic state. This assertion is proved by the employment of the -following tests. - -a. _Mercury._--Little or no amalgamation takes place on triturating the -darkened salt with this metal. - -b. _Ammonia and fixing agents._--These usually produce only a limited -amount of action. Thus, the Albuminate of Protoxide of Silver is perfectly -soluble in Ammonia; but after having been reddened by exposure to light, -it is little or not at all affected. - -c. _Potash._--Animal matters coagulated by Nitrate of Silver, and reduced -by the sun's rays, are dissolved by boiling Potash, the solution being -clear and of a blood-red colour. Metallic Silver, it is presumed, if -present, would remain insoluble. - -d. _Boiling Water._--Gelatine treated with Nitrate of Silver and exposed -to light, loses its characteristic property of dissolving in hot water. -This experiment is conclusive. - -The above facts justify us in supposing the existence of combinations of -organic matter with a low Oxide of Silver; and analysis indicates further -that the relative proportion of each constituent in these compounds may -vary. For instance, when Citrate of Silver is reduced by light, and acted -on with Ammonia, a black powder remains, which was found to contain as -much as 95 per cent, real Silver; but Albuminate of Silver treated in the -same way yields on analysis less of metallic Silver, and more volatile and -carbonaceous matter. - -The use of _Ammonio_-Nitrate of Silver in preparing the salt tends also -to increase the relative quantity of metal left in the compound after -reduction and fixing. The length of time during which the light has acted, -has also a modifying effect of the same kind,--the product of reduction by -a powerful light being more nearly in the state of metal, and containing -less both of Oxygen and organic matter. - -_Action of Light upon Chloride of Silver associated with organic -matter._--Photographs formed on Chloride of Silver alone, would, after -fixing, consist of metallic Silver, but such a process could not be -carried out in practice. The addition of organic matter is absolutely -necessary in order to increase the sensitiveness, and to prevent the -image from being dissolved in the Bath of Hyposulphite of Soda. The blue -Subchloride of Silver is decomposed by fixing, a very scanty proportion of -grey metallic Silver remaining insoluble; but the red compound of Suboxide -of Silver with organic matter is almost unaffected by Hyposulphite of -Soda, or Ammonia. - -The increase of sensitiveness and intensity produced by the use of organic -matter is accompanied also by a change in the composition of the picture; -the image losing the metallic character which it possesses when formed on -pure Chloride of Silver, and resembling in every respect the product of -the action of light upon organic Salts of Silver. - -There are certain characteristic tests which may usefully be employed in -distinguishing the metallic image from what may be termed the organic or -non-metallic image. One of these tests is Cyanide of Potassium. An image -formed upon pure Chloride of Silver, although pale and feeble, may, after -fixing, be immersed ill dilute solution of Cyanide of Potassium without -injury. But a photograph on Chloride of Silver supported by an organic -basis, is much acted upon by Cyanide of Potassium, quickly losing its -finer details. - -A second test is the Hydrosulphate of Ammonia. If no organic matter be -employed, the image becomes darker and more intense by treatment with a -soluble Sulphuret; whilst the non-metallic image, formed on an organic -surface, is quickly bleached and faded. The action of Sulphur upon the -image is indeed a mode of determining the real quantity of Silver present. -When existing in a very finely divided layer, Sulphuret of Silver often -appears yellow; but in a thicker layer it is black. Hence the colour -of the Photograph, after treatment with Sulphuretted Hydrogen, is an -indication of the proportion of metal present, and the reason of the -organic image becoming so perfectly faded is because it contains a minimum -of Silver in relation to the intensity. We see, therefore, that the -addition of organic matter to Chloride of Silver does not so much increase -the actual quantity of Silver reduced by light, as it adds to its opacity -by associating other elements with the Silver, and altogether modifying -the composition of the image. - -The employment of _oxidizing agents_ shows also that in an ordinary -Photographic process by the direct action of light, other elements besides -Silver assist in forming the image: the pictures being found to be easily -susceptible of oxidation, whereas the metallic image formed on pure -Chloride of Silver resists oxidation. - -_Composition of_ developed _images._--By exposing sensitive -layers of the Iodide, the Bromide, and the Chloride of Silver to the light -for a short time only, and subsequently developing with Gallic Acid, -Pyrogallic Acid, and the protosalts of Iron, a variety of images may be -obtained, which differ from each other materially in every important -particular, and a comparison of which assists the determination of the -disputed point. - -The appearance and properties of the developed Photograph are found to -vary with the existence of the following conditions. - -1st. _The surface used to sustain the sensitive layer._--There is a -peculiarity in the image formed on _Collodion_. Collodion contains -Pyroxyline, a substance which behaves towards the salts of Silver in a -manner different from that of most organic bodies, exhibiting no tendency -to assist their reduction by light. Hence Chloride of Silver on Collodion -darkens far more slowly than the same salt upon Albumen, and the image, -after fixing, is feeble and metallic. Iodide of Silver on Collodion, -exposed and developed, gives usually a more metallic image, with less -intensity, than Iodide of Silver upon Albumen, or on paper sized with -Gelatine. By adding to the Collodion a body which has an affinity for low -oxides of Silver, such for instance as Glycyrrhizine, the opacity of the -developed image is increased. - -2nd. _The nature of the sensitive salt._--When Iodide of Silver is used -to receive the latent impression, the image after development, although -lacking intensity of colour by reflected light, is more nearly in the -condition of metallic Silver than if Bromide or Chloride of Silver -be substituted; and of the three salts, the Chloride gives the most -intensity, with the least quantity of metallic Silver. This rule applies -especially when organic matters, Gelatine, Glycyrrhizine, etc., are -present. - -3rd. _The developing agent employed._--An organic developing agent like -Pyrogallic Acid may be expected to produce a Collodion image more intense, -but less metallic, than an inorganic developer, such as the Protosulphate -of Iron. - -4th. _The length of time during which the light has acted._--Over-action -of the light favours the production of an image which is dark by -reflection and brown or red by transmission, corresponding in these -particulars to what may be termed the non-metallic image containing an -oxide of Silver. - -5th. _The stage of the development._--The red image first formed on the -application of the developer to a gelatinized or albuminized surface of -Iodide of Silver is less metallic, and more easily injured by destructive -tests, than the black image, which is the result of prolonging the action. -Developed photographs which are of a bright red colour after fixing, -correspond in properties to images obtained by the direct action of light -on paper prepared with Chloride of Silver, more nearly than to Collodion, -or even to fully developed Talbotype Negatives. - -To conclude the Paper, the following may be offered in the way of -recapitulation:--An image consisting of metallic silver, as a rule, -reflects white light, and shows as a positive when laid on black velvet; -but a non-metallic organic image is dark, and represents the shadows of a -picture. Collodion positives developed with protosalts of Iron are nearly -or quite metallic. Photographs on Albumen or Gelatine less so than those -on Collodion. Developed Photographs contain more Silver than others, if -the development has been prolonged. The half shadows of the image in a -Positive Print are especially liable to suffer under injurious conditions, -since they contain the Silver in a less perfect state of reduction.[24] - -[24] The Author omits, in this place, all mention of molecular conditions -affecting intensity, inasmuch as at the present time nothing positive -has been determined with regard to them. It is however known that in the -use of the protosalts of Iron as developing agents, the appearance of -the image is much influenced by the rapidity with which the reduction is -effected--the particles of Silver being larger and more metallic when the -development is conducted slowly. The process of electro-plating and other -chemical operations of a similar kind prove that the physical properties -of metals precipitated from solutions of their salts, vary greatly with -the degree of fineness and arrangement of their particles. - - -ON THE VARIOUS AGENCIES DESTRUCTIVE TO PHOTOGRAPHIC PRINTS. - -_Action of Sulphuretting Compounds upon Positive Prints._--It was first -noticed by Mr. T. A. Malone, that the most intense Photograph might be -destroyed by acting upon it with solution of Sulphuretted Hydrogen or a -soluble Sulphuret, for a sufficient length of time. - -The changes produced by a sulphuretting compound acting upon the red image -of a simply fixed print are these:--the colour is first darkened, and a -degree of brilliancy imparted to it; this is the effect termed "toning." -Then the warm tint by degrees alters to a colder shade, the _intensity_ of -the whole image is lessened, and the half-tones turn yellow. Lastly, the -full shadows pass also from black to yellow, and the print fades. - -Now in this peculiar reaction we notice the following points of interest. -If at that particular stage at which the print has reached its maximum of -blackness, it be raised partially out of the liquid and allowed to project -into the air, the part so treated becomes yellow before that which remains -immersed. Again, if a print toned by Sulphur be placed in a pan of water -to wash, after the lapse of several hours it is apt to assume a faded -appearance in the half-tones. The full shadows, in which the reduced -Silver salt is thicker and more abundant, retain their black colour for -a longer time, but if the action of the sulphuretting Bath be continued, -every portion of the print becomes yellow. - -These facts prove that _Oxygen_ has an influence in accelerating the -destructive action of the Sulphur compounds upon Positive prints; and this -idea is borne out by the results of further experiments, for it is found -that moist Sulphuretted Hydrogen has little or no effect in darkening the -colour when every trace of air is excluded. When prints are washed in -water they are exposed to the influence of the dissolved air which water -always contains, and hence the change from black to yellow is produced.[25] - -[25] Further remarks upon the action of damp air upon Positives toned by -Sulphur are given at p. 153. - -There are some substances which facilitate the yellow degeneration of -Positives toned by Sulphur, a knowledge of which will be useful: they -are--1st, powerful oxidizers, such as Chlorine, Permanganate of Potash, -and Chromic Acid; these, even when highly diluted, act with great -rapidity: 2nd, bodies which dissolve Oxide of Silver, as soluble Cyanides, -Hyposulphites, Ammonia; also _acids_ of various kinds, and hence the -frequency of yellow finger impressions upon old sulphuretted prints, which -are probably caused by a trace of organic (Lactic?) Acid left by contact -with the warm hand. - -It was at one time supposed that the Photograph in the stage at which it -appears _blackened_ by Sulphur, consisted of Sulphuret of Silver, and that -this black Sulphuret became yellow by absorption of Oxygen and conversion -into Sulphate. MM. Davanne and Girard, who examined the subject, thought -that there might be two isomeric forms of Sulphuret of Silver, a black -and a yellow form; the former of which passing gradually into the latter -produced the fading of the impression. But neither of these views are -correct; for it is proved by careful experiment, that the Sulphuret of -Silver is a highly stable compound, not prone to oxidize, and, further, -that the change of colour from black to yellow has no reference to a -modification of this salt. The truth appears to be that the image whilst -in the black stage contains other elements besides Sulphur and Silver, but -when it has become yellow by the continued action of the sulphuretting -compound, it is then a true Sulphuret. - -_Comparative permanence of Photographs under the action of -Sulphur._--_Developed_ Positives, as a rule, stand better than those -printed by direct exposure to light; but much depends upon the nature of -the negative process followed; and hence no general statement can be made -which will not be liable to many exceptions. The mode of conducting the -development must not be overlooked. The prints, which become very red in -the Hyposulphite fixing Bath from the action of the developer having been -stopped at too early a period, are often sulphuretted and destroyed even -more readily than a vigorous sun-print obtained by direct exposure to -light. - -A point of even greater importance is _the nature of the sensitive -surface_ which receives the latent image. It is the print _developed upon -Iodide of Silver_ which especially resists sulphuration. In that case, -not only is the preliminary toning effect of the Sulphur more slow than -usual, but the impression cannot be made to fade by any continuance of the -action. It loses much of its brilliancy, and is reduced in intensity, but -it is not so completely destroyed as to be useless. The reason of this, as -shown in the last paper, depends upon the fact that the Talbotype proofs -contain the largest amount of Silver in the image. - -The employment of Gold in toning does not render an ordinary sun-print -as permanent as a Positive developed upon Iodide of Silver. The deep -shadows of the picture are protected by the Gold, but the lighter shades -not so perfectly. Hence after the Sulphur has acted, in place of the -universal yellow and faded aspect presented by the simple untoned print, -the Positive fully toned by Gold has black shadows with yellow half-tones. -Therefore, whilst recommending the use of Gold as a toning agent, it does -not seem advisable to lay too much stress upon it as a preservative from -the destructive action of Sulphur. - -_Exposure of Positive Prints to a Sulphuretting Atmosphere._--In testing -the action of a solution of Sulphuretted Hydrogen upon paper Positives, -it did not appear that the conditions under which the prints were placed -bore a sufficiently close resemblance to the case of Positives exposed -to an atmosphere contaminated with _minute traces_ of the gas; and this -more particularly because it is known that _dry_ Sulphuretted Hydrogen has -comparatively little effect upon Photographic Prints. - -The experiments were therefore repeated in a somewhat different form. -A number of Positives (about three dozen) printed in various ways, -were suspended in a glass case, measuring 2-1/2 feet by 21 inches, -and containing 7-1/2 cubic feet of air; into which was introduced, -occasionally, a few bubbles of Sulphuretted Hydrogen, just sufficient to -keep the air of the chamber smelling perceptibly of the gas. A polished -Daguerreotype plate was hung up in the centre, to serve as a guide to the -progress of the sulphuretting action. - -By the second day the metal plate had acquired a faint yellow hue, -not easily seen except in certain positions; but the Positives were -unaffected. At the expiration of three days the majority of the pictures -exhibited no signs of change, but a few untoned prints of a pale red -colour, some of which had been printed by development, and others by -direct exposure to light, had perceptibly darkened. - -After the eighth day, the action, appearing to progress more slowly -than at first, was stopped, and the prints removed. The general results -obtained were as follows:-- - -The Daguerreotype plate had become strongly tarnished with a film of -Sulphuret of Silver, which appeared yellowish-brown in some parts and -steel-blue in others. The Positives were, as a rule, toned to a slightly -colder shade, but many of them had scarcely changed. - -No obvious difference was observed between prints developed on paper -prepared with Chloride of Silver, and others printed by direct exposure to -light; but in all cases the prints obtained by those methods which give a -very red image after fixing, were the first to show the change of colour -due to sulphuration, the proofs submitted to the test having all been -previously toned with Gold. - -Effect of Oxidizing Agents upon Positive Prints.--It appeared of -importance to ascertain to what extent Photographic Prints are susceptible -of oxidation; on account of the atmospheric influences to which they are -necessarily exposed. In experimenting upon this subject the following -results have been obtained. - -Powerful oxidizers destroy Positive Prints rapidly; the action usually -commencing at the corners and edges of the paper, or at any isolated -point, such as a metallic speck or particle of extraneous matter, which -can serve as a centre of chemical action. This same fact is often noticed -in the fading of Positives by long keeping, and therefore since other -destructive actions (with the exception of that of Chlorine) do not appear -to follow the same rule, it is an argument in addition to others which can -be adduced, that Photographic Prints are frequently destroyed by oxidation. - -Air which has been _Ozonized_ by Phosphorus, and in which blue -litmus-paper becomes reddened, quickly bleaches the Positive image. Oxygen -gas, obtained by voltaic decomposition of acidified water and which should -contain Ozone, did not appear to have an equal amount of effect, the -action being comparatively slight, or altogether wanting. - -_Peroxide of Hydrogen_ obtained in solution, and in conjunction with -Acetate of Baryta, by adding Peroxide of Barium to dilute Acetic Acid,[26] -bleaches darkened Positive paper; but the effect is slow, and does not -take place to a very perceptible extent if the liquid be kept alkaline to -test-paper. - -[26] Hydrochloric Acid, which is usually recommended in place of Acetic -Acid, cannot be employed in this experiment; it seems to cause a -liberation of free Chlorine, which bleaches the print instantly. - -Nitric Acid applied in a concentrated form acts immediately upon the -darkened surface, bleaching every part of the print with the exception -of the bronzed shadows, which usually retain a slight residual colour. A -solution of Chromic Acid is still more active. This liquid may usefully be -applied to distinguish prints toned by Sulphur from others toned by Gold; -the presence of metallic Gold protecting the shadows of the picture in -some measure from the action of the acid. The solution should be prepared -as follows:-- - - Bichromate of Potash 6 grains. - Strong Sulphuric Acid 4 minims. - Water 12 ounces. - -A solution of Permanganate of Potash is an energetic destroyer of paper -positives; and, as it is a neutral substance, may conveniently be employed -in testing the relative capability of withstanding oxidation possessed -by different Photographic Prints. The solution should be dilute, of a -pale pink hue, and the Positives must be moved occasionally, as the first -effect is to decolorize a great portion of the liquid, the Permanganate -oxidizing the size and organic tissue of the paper. After an immersion -of twenty minutes to half an hour, varying with the degree of dilution, -the half-tones of the picture begin to die out, and the full shadows -become darker in colour; the bronzed portions of the print withstand the -action longer, but at length the whole is changed to a yellow image much -resembling in appearance the Photograph faded by Sulphur. - -_Comparative permanence of Photographs treated with Permanganate of -Potash._--Developed prints prepared by a Negative process withstand the -action better than others. But to this rule there are exceptions; much -depending upon the time of exposure to light, and the extent to which -the development is carried. Those prints which, being exposed for a -short time, and afterwards strongly developed, become dark in colour and -vigorous in outline, are more permanent than others which having been -over-exposed and under-developed, lose their dark colour and become red -and comparatively faint in the Hyposulphite fixing Bath. - -Positives developed upon a surface of _Chloride_ of Silver on plain paper -do not resist the oxidizing action so perfectly as those on Iodide of -Silver. Prints developed upon paper prepared with Serum of Milk containing -Caseine stand better than those on plain paper. - -Of prints obtained by the ordinary process of direct exposure to light, -those on plain paper are the first to fade, the oxidizing action being -most seen upon the _half-tones_. The use of _Albumen_ gives a great -advantage. Developed prints on Albumen stand far better than the same -upon plain paper; and even the Albuminized sun prints are less injured by -the Permanganate than the best of the Negative prints prepared without -Albumen. Caseine has the same effect, but to a less extent; and as Serum -of Milk almost invariably contains uncoagulated Caseine, its efficacy is -thus explained. - -The manner of toning the print is a point of importance; previous -sulphuration in an old Hyposulphite Bath always facilitating the oxidizing -action. - -_Action of Chlorine upon Positive Prints._--Aqueous solution of Chlorine -destroys the Photographic image, changing it first to a violet tint -(probably Subchloride), and subsequently obliterating it by conversion -into white Chloride of Silver. The impression, although invisible, -remains in the paper, and may be developed in the form of yellow or brown -Sulphuret of Silver by the action of Sulphuretted Hydrogen. It also -becomes visible on exposure to light, and assumes considerable intensity -if the paper be previously brushed with free Nitrate of Silver. Sulphate -of Iron produces no effect upon the invisible image of Chloride of Silver; -but Gallic or Pyrogallic Acid, rendered alkaline by Potash, converts it -into a black deposit. - -The Action of Chlorine water usually commences at the edges and corners of -the print, in the same manner as that of oxidizing agents. The proofs upon -Albumen are the least readily injured, and next, those developed on Iodide -of Silver. - -_Hydrochloric Acid._--The liquid acid of sp. gr. ·116, even when free from -Chlorine, acts immediately upon the half-tones of a positive print, and -destroys the full shadows in the course of a few hours; a slight residual -colour however usually remains in the darkest parts. The prints developed -on Iodide of Silver are the most permanent. - -_Sulphuric, Acetic Acids, etc._--Acids of all kinds appear to exert an -injurious influence upon Positive prints, and especially so upon the -half-tones of the image, the effect varying with the strength of the acid -and the degree of dilution with water. Even a vegetable acid like Acetic -gradually darkens the colour and destroys partially or entirely the faint -outlines of the picture. - -_Bichloride of Mercury._--The most important particulars relating to -the action of this test upon Photographs are well known. The image is -ultimately converted into a white powder, and hence, in the case of a -Positive print, it becomes invisible; immersion in Ammonia or Hyposulphite -of Soda however restores it in a form often resembling in tint the -original impression. A point worthy of note is the protective effect of a -deposit of Gold, which is very marked, the proof, after toning, resisting -the action of the Bichloride for comparatively a long time. - -_Ammonia._--The effect of Ammonia upon a print is rather to _redden_ the -image than to destroy it; the half-tones become pale and faint, but they -do not disappear. Toning with Gold enables the proof to resist the action -of the strongest solution of Ammonia, and hence Ammonia may safely be -employed as a fixing agent after the use of the Sel d'or Bath. - -_Hyposulphite of Soda._--A concentrated solution of Hyposulphite of Soda -exercises a gradual solvent action upon the image of Photographic Prints, -at the same time tending to communicate Sulphur and to darken the colour -of the impression. A faint yellow outline of Sulphuret of Silver usually -remains after the solution of the image is completed. - -Developed prints of all kinds, but in particular the Talbotype proofs -upon Iodide of Silver, are less readily dissolved by Hyposulphite of Soda -than those obtained by the direct action of light. There is also a slight -difference between plain and Albuminized prints, which is in favour of the -former, the albuminized paper always losing somewhat more by immersion in -the Hyposulphite Bath than plain Chloride paper sensitized by Nitrate of -Silver. - -_Cyanide of Potassium._--The solvent action of Cyanide of Potassium is -most energetic upon Photographs formed on paper. These images, whether -developed or not, do not withstand the test so well as the impressions on -Collodion. Albuminized proofs are also somewhat more easily affected than -prints on simple Chloride paper sensitized with Nitrate or Ammonio-Nitrate -of Silver. - -_Heat, moist and dry._--Long-continued boiling in distilled water has -a reddening action upon Positive Prints. The image becomes at length -pale and faint, resembling a print treated with Ammonia before toning. -A deposit of Gold upon the image lessens, but does not altogether -neutralize, the effect of the hot water. If the boiling be long continued, -the violet-purple tone often imparted by the Gold invariably gives place -to a chocolate-brown, which appears to be the most permanent colour. -Prints _developed_ by Gallic Acid upon paper prepared with Serum of Milk -or with a Citrate, suffer as much as others obtained by direct action of -light. Ammonio-Nitrate prints on highly salted paper, which become nearly -black when toned with Gold, retain their original appearance the most -perfectly; a slight diminution of brightness being the only observable -difference after long boiling in water. Albumen proofs, and prints on -English papers, or foreign papers prepared with Serum of Milk, Citrates, -Tartrates, or any of those bodies which _redden_ the reduced Salt, are, -as a rule, rendered lighter in colour, and pass from purple to brown when -boiled in water. - -Dry heat has an opposite effect to that of hot water, usually _darkening_ -the colour of the image. On exposing a plain paper print simply fixed, and -thoroughly freed from Hyposulphite of Soda by washing, to a current of -heated air, it changes gradually from red to dark brown, in which state -it continues until the temperature rises to the point at which the paper -begins to char, when it resumes its original red tone, becoming at the -same time faint and indistinct. - -_The Products of Combustion of Coal-gas a cause of Fading._--Coal-gas -contains Sulphur compounds, which in combustion are oxidized into -Sulphurous and Sulphuric Acids; other substances of a deleterious nature -may also be present. A plate of polished silver suspended in a glass tube, -through which was directed the current of heated air rising from a small -gas jet, became tarnished with a white film in the course of twenty-four -hours. Positive prints exposed to the same, absorbed moisture and faded; -the action resembling that of oxidation, in being preceded by a general -darkening in colour. Of four prints exposed, an Iodide-developed print was -the least injured, and next, a print upon Albuminized paper. - - -ON THE ACTION OF DAMP AIR UPON POSITIVE PRINTS. - -In order to ascertain this point, more than six dozen Positives, printed -on every variety of paper, were mounted in new and perfectly clean -stoppered glass bottles, at the bottom of each of which was placed a -little distilled water, to keep the contained air always moist. They were -removed at the expiration of three months, having been kept during that -time, some in the dark, and others exposed to the light. As the prints -were prepared by various methods, toned in different ways, and mounted -with or without substances likely to exercise a deleterious action, this -series of experiments will possess considerable value in determining some -of the intrinsic causes of fading of Positives.[27] - -[27] For a more detailed account of the experiments, see the original -paper in the 'Photographic Journal,' vol. iii. - -The general results obtained were as follows:--Positives which had been -_simply fixed_ in Hyposulphite of Soda remained quite uninjured. Whether -developed by Gallic Acid on either of the three Salts of Silver usually -employed, or printed by direct action of light, the result was the same. -Hence we may infer that the darkened material which forms the image of -Photographic Prints does not readily oxidize in a damp atmosphere. - -_Toned_ Positives were found in many cases to be less permanent than -Positives simply fixed. This was especially the case when the toning had -been effected by _Sulphur_; all the sulphuretted prints, fixed in solution -of Hyposulphite which had been long used, became yellow in the half-tones -when exposed to moisture. Positives fixed and toned in Hyposulphite -containing Gold were variously affected; some prepared when the solution -was in an active state being unchanged, others losing a little half-tone, -and others, again, fading badly. These latter were prepared in a Bath -which had lost Gold and acquired sulphuretting properties; and it was -noticed that they were more injured by the action of boiling water than -those Positives which proved to be permanent under the influence of the -moisture. - -Toning by means of Chloride of Gold appeared to be highly satisfactory, -but the number of prints operated upon was small. The Sel d'or process -also did not injure the integrity of the image, no commencing yellowness -or bleaching of half-tones being visible after exposure to the moist air. - -This series of experiments confirmed the statement made in a former paper, -that some tints obtained in Positive printing are more permanent than -others. Violet tones produced by Sulphur invariably passed into a dull -brown by the action of the moist air; and even when Gold was employed -in toning, these same purple colours were usually _reddened_. This was -especially the case when English papers were used, or foreign papers -re-sized with Serum of Milk containing Caseine. The chocolate-brown tints -which best stand the action of boiling water, and in particular those upon -Ammonio-Nitrate paper, were least affected by the damp air; and indeed it -was evident that the two agents, viz. moist air and hot water, acted alike -in tending to _redden_ the print, although the latter did so in the most -marked manner. - -It seemed also, from the results of these experiments, to be a point of -great importance that the size should be removed from the print in order -to render it indestructible by damp air. This was evidently seen in two -cases where Positives, toned in an old Hyposulphite and Gold Bath, were -divided into halves, one of which was treated with a strong solution of -Ammonia. The result was that the halves in which the size was allowed -to remain, faded, whilst the others were comparatively uninjured. The -Albumen proofs especially suffered when the size was left in the paper, a -destructive mouldiness forming, and fading the picture. The use of boiling -water obviated this, and the prints so treated remained clean and bright. -A partial decomposition of Albumen however occurred in some cases even -when hot water was used, the gloss disappearing from the paper in isolated -patches. With _Caseine_ substituted for Albumen there was also a loss of -half-tone; thus seeming to indicate that both these animal principles, -although stable under ordinary conditions, will, even when coagulated by -Nitrate of Silver, decompose if kept long in a moist state. - -The use of improper substances for mounting proved to be another -determining cause of fading by oxidation. Those bodies which combine -with Oxide of Silver, are likely upon theoretical grounds to destroy the -half-tones of the image; and it was found, that if the picture were left -in contact with Alum, Acetic Acid, etc., or with the substances which -generate an acid by fermentation, such as paste or starch, it invariably -faded. - -The supposed accelerating influence of _Light_ upon the fading of -Positives was not confirmed by these experiments, as far as they extended. -Many of the bottles containing the Photographs were placed outside the -window of a house with a southern aspect during the whole of the three -months with the exception of two or three weeks, but no difference -whatever could be detected between Positives so treated and others kept -in total darkness. It will be proper however that this part of the -investigation should be repeated, allowing a longer time. - -An examination of the various modes employed for coating Positives, in -order to exclude the atmosphere, showed that many of them were not fitted -to fulfil the purpose intended. Waxed prints faded quite as much when -exposed to moisture as others not waxed. White wax is a substance often -adulterated, and Oil of Turpentine has been shown to contain a body -resembling Ozone in properties, and possessing the power of bleaching a -dilute solution of Sulphate of Indigo. Spirit varnish applied to the -surface of the picture after re-sizing with Gelatine was plainly superior -to white wax, but nevertheless it did not obviate the fading effect of the -moisture upon an unstable Positive which had been toned by sulphuration. -Its protective influence is therefore limited. - - -ON THE CHANGE IN COMPOSITION WHICH HYPOSULPHITE OF SODA EXPERIENCES BY USE -IN FIXING PAPER PROOFS.[28] - -[28] These observations are condensed and re-arranged from the papers -published by the Author in the 'Photographic Journal' for September and -October, 1854. - -It was remarked by Photographers at an early period that the properties of -the Fixing Bath of Hyposulphite of Soda became altered by constant use; -that it gradually acquired the power of _darkening_ the colour of the -Positive image. This change was at first referred to the accumulation of -_Salts of Silver_ in the Bath, and hence directions were given to dissolve -a portion of blackened Chloride of Silver in the Hyposulphite in preparing -a new solution. - -Careful experiments performed by the Author convinced him that an error -had been entertained; since it was found that the simple solution of -Chloride of Silver in Hyposulphite of Soda had no power of yielding -the black tones. But it afterwards appeared that if the fixing Bath, -containing dissolved Silver Salts, were set aside for a few weeks, a -_decomposition_ occurred in it, evidenced by the formation of a black -deposit of Sulphuret of Silver; and _then_ it became active in toning the -proofs. - -The presence of this deposit of Sulphuret of Silver indicated that a -portion of Hyposulphite of Silver had spontaneously decomposed, and, -knowing the products which are generated by the spontaneous decomposition -of this salt, a clue to the difficulty was afforded. One atom of -Hyposulphite of Silver includes the elements of one of Sulphuret of Silver -and one of Sulphuric Acid. Sulphuric Acid in contact with Hyposulphite of -Soda produces _Sulphurous Acid_ by a process of displacement; and Plessy -has shown that Sulphurous Acid reacts upon an excess of Hyposulphite -of Soda, forming two of that interesting series of Sulphur compounds -designated by Berzelius the "Polythionic Acids." - -It appeared therefore probable, upon theoretical grounds, that the Penta-, -Tetra-, and Trithionates might produce some effect in the Hyposulphite -fixing Bath. Upon making the trial these expectations were verified; and -it was found that Tetrathionate of Soda added to Hyposulphite of Soda -yielded a fixing and toning Bath quite equal in activity to that produced -by means of Chloride of Gold. - -It may be useful to review for an instant the composition of the -Polythionic series of acids; it is thus represented:-- - - Sulphur. Oxygen. Formulæ. - Dithionic or Hyposulphuric Acid 2 atoms 5 atoms S{2}O{5} - Trithionic Acid 3 " 5 " S{3}O{5} - Tetrathionic Acid 4 " 5 " S{4}O{5} - Pentathionic Acid 5 " 5 " S{5}O{5} - -The amount of _Oxygen_ in all is the same, that of the other element -increases progressively; hence it is at once evident that the highest -member of the series might _by 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 exception of the -Hyposulphuric. The Alkaline Salts of these acids are more unstable than -the acids themselves; a solution of Tetrathionate of Soda becomes milky in -the course of a few days from deposition of Sulphur, and, if tested, is -then found to contain _Tri_thionate and eventually _Di_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 imparting 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 displacing _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 solution 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 continues to do so for several -days. At the same time the liquid acquires an acid reaction to test-paper, -and produces effervescence 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 Photographers, 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 -effect of the acid fixing Bath upon the Positive proofs. Tetrathionate of -Soda added to solution of Hyposulphite 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 becoming 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 effect 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 reaction with -Hyposulphite of Soda likewise elicited the important fact that _alkalies_ -decompose the unstable sulphuretted principle. If the Bath be treated -with Potash or Carbonate of Soda, an alkaline _Sulphuret_ appears to be -gradually formed, which precipitates Sulphuret of Silver, and in the -course of a few days the liquid returns 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 process of spontaneous change going forward, which issues in -a deposition of Sulphur and Sulphuret of Silver, and a partial loss of -sulphuretting 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 Hyposulphite, 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 brown_, 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 result, but -it cannot be used for "Ammonio-Nitrate" prints; the addition of an alkali -precipitating Sulphuret of Iron. - - Strong solution of Perchloride of Iron 6 fluid drachms. - Hyposulphite of Soda 4 ounces. - Water 8 ounces. - Nitrate of Silver 30 grains. - -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, which it does -in a few minutes, add the Nitrate of Silver, stirring briskly. Perfect -solution 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 Perchloride of Iron should be prepared by boiling Peroxide 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 produce a -portion of Hyposulphite of Silver in the bath; the presence of a Silver -Salt having been found to modify the tint of the Positives, and to prevent -their quickly turning yellow. - - -SECTION IV. - -_On the Fading of Photographic Prints._ - -For many years subsequent to the discovery of the process 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. - -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 subject. 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 Photographic Prints, and -the precautions which should be taken to ensure their permanency. The -chemistry of the subject having been fully explained in the last Section, -it will suffice to refer the reader to its pages for more detailed -information. - -_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 Positives printed more than ten years ago, which have not perceptibly -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 frequently 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 -Photographers. - -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 change, cannot but lead to the inference that the 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 important 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 Sulphur, and destroys the print in the same way and quite -as effectually 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, _begins to get darker 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 extend 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 running 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, _four or five 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 Paper._--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 indistinct. This kind of fading often commences at the corners 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 susceptible 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 -Resin, 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 carbonate, without injury to the tint, has -the additional advantage of carrying out the last traces of Hyposulphite -of Soda, and also the germs of _fungi_, which if allowed to remain 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[29] which would tend in time to produce a yellow mark. - -[29] The writer has 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! - -c. _Moisture as a cause of fading._--Although. Photographs properly -printed are not readily injured by damp air (p. 153), yet as there are -_impurities_ of various 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! No pictures will long survive such treatment, and Photographs, like -engravings and water-colour paintings, require common care to be exercised -in their preservation. - -d. _The modes of Mounting the Proof._--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. Flour paste is especially injurious, and many cases -of fading have been traced to this cause. The addition of Bichloride of -Mercury, which is often made to prevent 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 Photographs, 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 Photographers 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. 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 with 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 Photograph carefully prepared may yet suffer injury from deleterious -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 exposure 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 precautionary measure, that Photographic Pictures be -protected 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 sparkling 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 of itself -will be sufficient, but in other cases it may perhaps be useful to apply a -layer of spirit or gutta-percha varnish. The use of wax, resin, and such -bodies is likely, by introducing impurities, to act injuriously rather -than otherwise. - -g. _Decomposition of Pyroxyline a source of Injury to Collodion -Photographs._--Collodion Positives and Negatives 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 commences at -rough edges and isolated points, leaving the centre, as a rule, the last -affected. On examination, numerous 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 compounds 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 coating of varnish. - -_Comparative Permanence 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 subsequent -development with Gallic Acid, may be expected to be more permanent than -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 precipitating 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 -otherwise 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 Collodion, 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 _sulphuration_, 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 theoretical grounds, toning by an alkaline -solution of Chloride of Gold (p. 132), and fixing by Ammonia, is the best -process; but the employment of Sel d'or, which gives a more agreeable -colour and has not been found practically to injure 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 -maintaining the 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 -Hyposulphite 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 Sulphuretted 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 -immersed whilst the liquid is in a milky state from precipitation 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 improvement 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 decomposition 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 obtained by -sensitizing plain paper, free from animal matters, 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 permanence of Positives._--The tests for Hyposulphite -of Soda are not sufficiently delicate to indicate with certainty when the -process of washing 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 permanence 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 following 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. - -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 commencing yellowness, -it is almost certain that the Positive will not last for any considerable -length of time. - - - - -CHAPTER IX. - -ON THE THEORY OF THE DAGUERREOTYPE AND TALBOTYPE PROCESSES, ETC. - - -SECTION I. - -_The Daguerreotype._ - -It was not the original intention of the Author to include a description -of the Daguerreotype Process within the limits of the present Work. The -Daguerreotype is a branch of the Photographic Art so distinct from the -others, that, in manipulatory details, it bears very little analogy to -them; a slight sketch of the theory of the process may not however 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 strengthening 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 -possessed by the others, viz. the presence of soluble Nitrate of Silver in -contact with the particles of Iodide of Silver. - -It is important to remember that the 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 Daguerreotype 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 -excess 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 brilliant -metallic lustre. This preliminary operation of polishing is one of great -practical importance, and the troublesome 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 cardboard or a thin sheet of wood, previously -soaked in solution 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 prismatic -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 -repeated; the steel-grey disappears, and the yellow and rose-colours -recur. The deposit of Iodide of Silver gradually 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 produced by the light is concerned. An inspection -of a sensitive 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 -Goddard that the sensibility 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 Bromo-Iodide of Silver, so called, is uncertain, -and has not been proved to bear any analogy to that of the mixed salt -obtained by decomposing a solution of Iodide and Bromide of Potassium with -Nitrate 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 circumstances retards the -effect. - -_The Development and Properties of the Image._--The latent 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 Quicksilver, is a metallic fluid which boils at 662° Fahrenheit. We -are not however to suppose that the iodized plate is subjected to the -vapour of Mercury at a temperature at all approaching 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 Daguerreotype image. -Unfortunately, the quantity of material 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 metallic 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 -superimposed. - -It is a remarkable fact that an image more or less resembling that -developed by Mercury can be obtained 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 crystals when highly -magnified. Its composition is uncertain. - -For all practical purposes the production of the Daguerreotype 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 vapour be omitted. - -_M. Ed. Becquerel'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 surface be first exposed to -white light for a sufficient time to impress a latent image, and then -_afterwards_ to the yellow 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 _Bromo_-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 by 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 Hyposulphite of Gold, containing about one part of the salt -dissolved 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 Collodion image--is _darkened_ by Hyposulphite of -Gold. - -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 II. - -_Theory of the Talbotype and Albumen Processes._ - -_The Talbotype or Calotype._--This process, as practiced by many at the -present time, is almost identical with that originally described by Mr. -Fox 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 perfectly 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 -solutions 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 Potassium 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 an -instant upon this liquid and dried; they are then removed 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 primrose colour, and is perfectly insensitive to light. Even exposure -to the sun's rays produces no change, thus indicating that an excess of -Nitrate of Silver is essential to the visible darkening of Iodide of -Silver by 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 apparently -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 exposure of five to eight minutes in the Camera is -the average time with an ordinary view lens. - -The picture is developed with a saturated solution of 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 unnecessarily strong, the invisible image being more easily -developed 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 possesses 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, without involving 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 simplicity 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 directions 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 (see 'Photographic Journal,' vol. ii. p. 231). - -The sheets of paper, having been properly waxed, are soaked for _two -hours_ in a solution containing Iodide and 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 immersion -is required. The iodizing formulæ of the French Photographers have been -encumbered by the addition of a variety of substances which appear to -introduce complications 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 was 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 quantity 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 reduce 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 development 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 always 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 -of time during which Gallic Acid and Nitrate of Silver may remain mixed -without decomposing, is much lessened by using vessels which are dirty -from having been before employed for a similar purpose. The black deposit -of Silver exercises a _catalytic_ (καταλυσις, decomposition by contact) -action upon the freshly-mixed portion, and hastens its discoloration. - -The Waxed Paper process is exceedingly simple and inexpensive,--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 frequently taking an exposure of 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 weather, it will in all probability continue to be -extensively followed. The prints which have been sent to the Exhibition -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 -Photography with distinctness. - -_The Albumen process upon Glass._--The process with Albumen originated in -a desire to obtain a more even surface 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 convenient 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 off for use, in the same manner as in the preparation 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 to blister and separate from the 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 washing, and the Albumen exercising a direct -retarding influence 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 distinctness in the -shadows and minor details, and are admirably 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 circumstances. - -_M. Taupenot's Collodio-Albumen process._--This is a recent 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 Albumen upon a plate _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 receives the image, is -promoted by its resting upon a substratum of Iodide rather than upon the -inert surface of the glass. In this view, if the theory be correct, the -lower particle of Iodide of Silver promotes the molecular disturbance of -the upper, itself remaining unchanged. - -Other experimenters, pursuing the subject further, have asserted that -a successful result may be obtained by coating 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. - - - - -PRACTICAL DETAILS OF THE COLLODION PROCESS. - -[Illustration] - - - - -CHAPTER I. - -PREPARATION OF COLLODION. - - -This includes--the soluble Paper;--the Alcohol and Ether;--and the -iodizing compounds. - -The formulæ for Negative and Positive Collodion, and for the Nitrate Bath -and developing fluids, are given in the second Chapter. - -THE SOLUBLE PAPER. - -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.[30] 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. - -[30] 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. Munktell." - -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 -best in cases where large quantities of the material are operated on, but -the amateur is recommended to begin by trying the Nitre 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 succession. - -a. _Directions for mixing according to fixed rule._--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 following 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 Peroxide 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 evaporation. -Sulphate of Lead and Bisulphate of Potash 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 making the soluble Pyroxyline, may be stated thus:-- - - HO NO{5}, 2 (HO SO{3}) + 3-1/2 HO - -or - - Atoms. Atomic weight. - Nitric Acid 1 | 54 - Sulphuric Acid 2 | 80 - Water 6-1/2 | 58 - | --- - | 192 - -Having found the percentage of real acid which is present,[31] the -following calculation will give the relative weights of the ingredients -required to produce the formula:-- - - Let { _a_ = percentage of real Nitric Acid, - { _b_ = " " Sulphuric Acid, - - then 5400/_a_ = quantity of Nitric Acid, - - 8000/_b_ = " Sulphuric Acid, - - 192 - 5400/_a_ - 8000/_b_ = " Water. - -[31] Tables are given in the Appendix for calculation by specific gravity; -but direct analysis of the acids is the most certain. - -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. - -The weight 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 scale-pan, 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 8000/76·65 = 104·3 grains of Oil of Vitriol. - 5400/65·4 = 82·5 " Nitric Acid - 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 } 1920 grains. - Nitro-Sulphuric Acid } - -Having prepared the acid mixture of a definite strength by the above -formula, the paper must be immersed according to directions given at page -191. - -b. _Process for mixing Nitro-Sulphuric Acid, the strength of the two acids -not having been previously determined._--Take a strong sample of Nitric -Acid (the yellow Nitrous acid, so called, succeeds well), and mix it with -Oil of Vitriol as follows:-- - - Sulphuric Acid 10 fluid drachms, - Nitric Acid 10 " - -Now immerse a thermometer and note the temperature;[32] 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. - -[32] In the preparation of soluble cotton, and indeed in all 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. - -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. -Remove 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-Sulphuric 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,[33] 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. - -[33] Observe that the Ether be pure; if it contains too much water and -Alcohol, it will not dissolve the Pyroxyline, or will yield an opalescent -solution. - -There is a third condition of Pyroxyline, different from either of the -above, which may be puzzling:--the fibres of the Cotton mat together very -slightly or not at all on immersion, and the washing proceeds tolerably -well; the compound formed is scarcely explosive, and dissolves imperfectly -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 convenient 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 obtained. Commercial -Nitre often contains a large quantity of _Chloride of Potassium_, 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 destroy 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 crystallization; still, in many cases, -a little water is retained 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 Nitre, powdered and dried, 600 grains. - -This quantity is equivalent to 1-1/4 ounce Troy or Apothecaries' -weight;--and to 1-1/4 ounce Avoirdupois weight _plus_ 54 grains. Place -this in a teacup or any other convenient vessel, and pour upon it. - - Water 1-1/2 fluid drachms - mixed with Oil of Vitriol 12 " - -Stir well with a glass rod for two or three minutes, until all -effervescence 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 the 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. - - -GENERAL DIRECTIONS FOR IMMERSING, WASHING, AND DRYING THE PYROXYLINE. - -The mixture of Sulphuric Acid and Nitre requires to be used immediately -after its preparation, as it solidifies into a stiff mass on cooling; but -the 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 Sulphuric Acid is used, as in the case of a weak -sample of Nitric Acid, the Cotton should be removed at the expiration of -six or seven minutes, as there is a tendency to partial solution of the -Pyroxyline in the acid mixture under those 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, solution of the Cotton may take place if -the thermometer indicates 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 -quantity of water and allowed to remain, the rise in temperature and -weakening of the acid mixture might do mischief. - -The washing should be continued for at least a quarter 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, resulting -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 thoroughly washed. It is however a safe plan to -place the Pyroxyline in running water and allow it to remain for several -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. - - -RECAPITULATION OF THE GENERAL CHARACTERS OF PYROXYLINE 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 proper strength._--No agglutination 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 weak._--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 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 solvents contain too much water. - -In using Swedish Paper in place of Cotton, the Pyroxyline formed in too -weak a Nitro-Sulphuric Acid is usually insoluble in Ether and Alcohol, and -burns slowly like unchanged paper. - -By studying these characters, and at the same time bearing in mind that _a -drachm and a half of water_ in the quantities of acid given in the formula -(p. 188) will suffice to cause the difference, the operator will overcome -all difficulties. - - -PURIFICATION OF THE SOLVENTS REQUIRED FOR 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, containing a certain percentage of -Alcohol and of water; specific 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 neither Alcohol nor water; in this case -the specific gravity should not be higher than ·720. Fourth, "Methylated" -Ether, manufactured at a lower price than the others. - -Rectified Ether of 750° is not to be depended on, inasmuch as the -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 Photographic purposes, -are as follows:--a peculiar and disagreeable 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 Alcohol 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 dealing 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 severe -injuries have been occasioned from this cause. - -_Purification of Ether by redistillation from a caustic or carbonated -alkali._--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 washed Ether; shake the bottle again many times, -in order that the water--a small portion of which is still present 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 will probably be observed that the liquid has become -yellow, and that a flocculent deposit has formed in small quantity. -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 otherwise, 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 temperature. - -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 liberation 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 Collodion -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; consequently when Spirits of Wine are -agitated with Carbonate 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 shaken frequently, the action is complete, and the -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 containing about -90 per cent, of absolute Alcohol, the remaining 10 per cent, being water. -The specific gravity at 60° Fahrenheit should be from ·815 to ·825; -commercial Spirit of Wine being ·836 to ·840. - - -PREPARATION OF THE IODIZING COMPOUNDS IN A STATE OF PURITY. - -These are the Iodides of Potassium, Ammonium, and Cadmium. The properties -of each are more fully described in Part III. - -a. _The Iodide of Potassium._--Iodide of Potassium, as sold in the -shops, is often contaminated with various impurities. 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 crystals, -which are strongly alkaline to test-paper, and become 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 yellow 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 employed is not pure, but contains -traces of free Chlorine, the Iodide of Silver must be well washed 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. - -_Iodate of Potash_ is a fourth impurity often found in Iodide of -Potassium: to detect it, add a drop of dilute Sulphuric Acid, or a crystal -of Citric Acid, to the solution of the Iodide; when, if much Iodate be -present, the liquid will become yellow from liberation of free Iodine. The -rationale 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 (IO{5}) be also present, it decomposes the -Hydriodic Acid first formed, oxidizing the Hydrogen into Water (HO), and -setting 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 Iodate. As Iodate 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 Iodate 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, Iodate, -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 prepared by adding -Carbonate of Ammonia to Iodide of Iron, but more easily by the following -process:--A strong solution of Hydrosulphate of Ammonia is first made, by -passing Sulphuretted Hydrogen gas into Liquor Ammoniæ. 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 decomposition of the Hydrosulphate of -Ammonia is complete, 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 effervescence. The -effervescence being over, the liquid is still acid to test-paper, from -excess of Hydriodic Acid; it is to be cautiously neutralized with Ammonia, -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 -with Iodide of Zinc; the crystals being imperfectly formed and slowly -liberating Iodine when 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. - - - - -CHAPTER II. - -FORMULÆ FOR SOLUTIONS REQUIRED IN THE COLLODION PROCESS. - - Section I.--Solutions for direct Positives. - Section II.--Solutions for Negative Photographs. - - -SECTION I. - -_Formulæ for Solutions for direct Positives._ - -The solutions are taken in the following order;--The Collodion.--The -Nitrate Bath.--Developing fluids.--Fixing 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 Ammonium 4 grains. - -_Formula No. 2._ - - Rectified 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 " - -If the operator wishes to prepare a stock of the plain Collodion, and to -iodize as required, the last formula will stand thus:-- - - Rectified Ether, ·750 3 fluid ounces. - Alcohol of ·836 2 fluid drachms. - Pyroxyline 8 to 14 grains. - -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 formulæ 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 Ammonium 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 mixture 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 Cadmium 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 opalescent and not too -yellow and creamy. Pale-blue films yield very good Positives, but with -more liability to failure than thicker films (p. 109). - -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 methylated), 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 evaporation of the solvents -being retarded, and the film rendered less liable to become dry before -development. _Anhydrous_ 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 ordinarily obtainable, the Collodion -will often become somewhat 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 insensitive. - - -THE NITRATE BATH. - - Nitrate of Silver 30 grains. - Nitric Acid 1/20 minim, or Acetic - Acid (glacial) 1/6 minim. - Alcohol 15 minims. - Distilled water 1 fluid ounce. - -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 substitutes when it cannot be obtained. - -The Bath must be saturated with Iodide of Silver, and Nitric Acid -neutralized if it be present. Nitrate of Silver however which has -undergone fusion is free from Nitric Acid. - -Weigh out the total quantity of crystals of Nitrate required for the Bath, -and dissolve in about two parts of water. Then take a quarter of a grain -of Iodide of Potassium to each 100 grains of Nitrate, 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 re-dissolved. -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_, Nitric Acid is present, to neutralize which, add -solution of Potash or Carbonate of Soda (not Ammonia) until a distinct -turbidity, remaining after agitation, 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 impurities, 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 Nitrate to the ounce, which will be found -sufficient to maintain the strength nearly at the original point. - -The common practice of occasionally dropping Ammonia or Potash into the -solution, to remove Nitric Acid liberated by free Iodine in the Collodion, -is not recommended (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. - - -THE DEVELOPING FLUIDS. - -Either of the three following formulæ may be used, according 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. - Nitric Acid 1 drop. - Water 1 fluid ounce. - -FORMULA No. 3. - - Solution of Protonitrate of Iron 1 fluid ounce. - Alcohol 20 minims. - -In all these formulæ, if distilled water is not at hand, read the -directions in the Vocabulary, Part III., Article "Water," for the best -substitute. - -_Remarks upon these Formulæ._--_Formula No. 1_ is the most simple, since -the solution can be 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 combine 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 _per_salt. When it is too weak, add more of the -Protosulphate. The muddy deposit which settles to the bottom of the Bath -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 Protonitrate 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, immediately before use, will augment the -energy of development when blue and green spots occur. - -_Formula No. 3_, or Protonitrate of Iron, does not require 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 preparing 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, _powdered_, 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 -different. The atomic weights of Nitrate of Baryta and Nitrate of Lead are -as 131 to 166; consequently 300 grains of the former are equivalent to 380 -grains of the latter. - - -THE FIXING SOLUTION. - - Cyanide of Potassium 2 to 12 grains. - Common Water 1 fluid ounce. - -Cyanide of Potassium is usually preferred to Hyposulphite 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 vertical 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 saturated 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. - - -SECTION II. - -_Formulæ, etc., for Negative Solutions._[34] - -[34] 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. - -THE COLLODION. - -FORMULA No. 1. - - Purified Ether, sp. gr. ·720 5 fluid drachms. - Purified Alcohol, sp. gr. ·825 3 fluid drachms. - Soluble Pyroxyline 4 to 8 grains. - Pure Iodide of Cadmium or Ammonium 4 to 5 grains. - -FORMULA No. 2. - - Rectified Ether, sp. gr. ·750 6 fluid drachms. - Alcohol, sp. gr. ·836 2 fluid drachms. - Soluble Pyroxyline 4 to 8 grains. - Iodide of Potassium or Ammonium 4 grains. - -When the Collodion and Iodizing mixture are kept separate, 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 Nitro-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 -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 surface, 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 obtain 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 volatility 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 allowed to subside, the clear -portion being decanted for use. The Iodide of Potassium is to be finely -powdered, and digested 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 working with an old, -coloured Collodion, finding that it gives more intensity. It has been -shown at page 97 that a peculiar 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 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 Methylated) in the proportion of 5 grains to the ounce: -this solution 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 Collodion 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 sensitiveness 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 Nitro-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 condition of the Ether and the heat of the weather. - -Plain Collodion may retain its properties unimpaired for five or six -months, sometimes much 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, observing 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 Rain-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 atmosphere. 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 Collodion prepared from Spirits nearly -anhydrous, and iodized with Iodide of Cadmium, the full quantity of Acetic -Acid 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 preparation, 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 -diluted down when required for use. - -The following is a good formula:-- - - Pyrogallic acid 12 grains. - Beaufoy's Acetic acid 1 fluid ounce. - -To one drachm add seven drachms of water. - - -THE FIXING LIQUID. - - Cyanide of Potassium 2 to 12 to 20 grains. - Water 1 fluid ounce. - - or, Hyposulphite of Soda 1/2 ounce. - Water 1 fluid ounce. - -For remarks on the Cyanide of Potassium Fixing Bath, see the last Section, -page 207. - - - - -CHAPTER III. - -MANIPULATIONS OF THE COLLODION PROCESS. - - -These may be classed under five heads:--Cleaning the Plates.--Coating with -Iodide of Silver.--Exposure in the Camera.--Developing the image.--Fixing -the image.--In addition to this, the present Chapter will include in -separate 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 Photography. 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 compression -during the printing process. - -The patent plate answers better than any other description of glass; but -if it cannot be procured, the "flatted crown glass" may be substituted. - -Before washing the glasses, each square should be roughened 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. - -In cleaning glasses, it is not sufficient, as a rule, to wash them simply -with water; other liquids are required to remove _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 expressly for that -purpose; they are best made of a material 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 containing a -little Carbonate of Soda. - -After wiping the glass carefully, complete the process by polishing with -an old silk handkerchief, avoiding contact with the skin of the hand. -Some object to _silk_, 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 angular position -and _breathe_ upon it. The importance of attending to this simple rule -will be at once seen by referring to the remarks made at page 39. In the -Honey preservative 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 blister during the development -and washings. Caustic Potash, sold by the druggists under the name of -"Liquor Potassæ," is very efficacious, or in place of it, a warm solution -of "washing Soda" (Carbonate of Soda). Liquor Potassæ, 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 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 allowed 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 OF 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 window, 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 fogging. - -It is often convenient to illuminate by means of a candle screened -by yellow glass. A dark orange yellow, approaching 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 particles 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 striæ upon the -film. In taking small portraits and stereoscopic subjects, 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. - -[Illustration] - -The Collodion, having been iodized some hours previously, 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 remove any particles of dust which may have -subsequently collected. If it be a plate of moderate size, it may be -held by the corners in a horizontal position, between the forefinger and -thumb of the left hand. The Collodion is to be poured on steadily until a -circular pool is formed, extending nearly to the edges of the glass. - -[Illustration] - -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. 4. 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 -surface. The operation of coating a plate with Collodion 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 -adhering to it. Photographers term this _setting_, and when it takes -place it is a sign that the time has come for submitting it to the action -of the Bath. - -If the film be lowered into the Nitrate before it has set, the effect is -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 appearance, and is paler in some parts than -in others. - -No rule can be given as to the exact time which ought to elapse: it varies -with the temperature of the atmosphere, and with the proportions of Ether -and of Pyroxyline; 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 horizontal line corresponding to the surface -of the liquid will be formed. Then place the cover upon the vertical -trough[35] 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. - -[35] 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. - -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 -containing but little Alcohol. Continue to move the plate until the liquid -flows off in a uniform sheet, when the decomposition 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 hand -for a few seconds upon blotting paper, to drain off as much as possible of -the solution of Nitrate of Silver.[36] It is then wiped on the back with -filtering-paper, placed in a clean and dry slide, and is ready for the -Camera. - -[36] This blotting-paper must be frequently changed, or stains will be -produced at the lower edge of the plate during the development. - -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 appearance:--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 Collodion was poured off. This part -remains wet longer than the other, and always suffers the most; horizontal -cracks or marks resembling vegetation are seen, each of which would cause -an irregular action of the developing fluid. On the other hand, the upper -part of the plate must be examined for the pale colour characteristic of -a film which had become too dry before immersion, since the Collodion is -thinner at that point than at any other. - - -EXPOSURE OF THE PLATE IN THE CAMERA. - -After the plate has been rendered sensitive, it should be exposed and -developed with all convenient despatch; the intensity of the Negatives -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.[37] - -[37] See the Second Section of this Chapter. - -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 horizontally. 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 -picture 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 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 finding 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 nature 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 -aperture and short focus, perhaps three seconds, and six seconds, or even -less, may be sufficient. - -In 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 appearance -in three or four seconds, and the film is then immediately washed with -clear water. Whilst in the Bath, 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 concealed 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 cementing 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 developer 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 plate has been developed, it is -washed, 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 unnaturally black -and gloomy appearance, the dark portions of the drapery, etc., being -invisible, the picture has been _under-exposed_. - -On the other hand, in an over-exposed plate, the face is usually pale and -white, and the drapery misty and indistinct. 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 indistinct. - -_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 Negative 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 present -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 the Bath being old and containing Ether, Alcohol must be added to -the solution of Pyrogallic Acid. - -With ordinary Negative Collodion, an addition of Nitrate 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 developing 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 black_, 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 well as a Positive, but by transmitted light -all the minor details 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 Silver, without any appearance (or, at most, an -indistinct one) of an image. By transmitted light the plate may appear 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 converse 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 -Positive, but the features of the sitter are scarcely to be discerned. -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 Collodion 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. - - -FIXING AND VARNISHING 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 Hyposulphite 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; 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 varnish, 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 formula;--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 Varnish 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 off 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. Suggett's patent jet is sometimes -employed, but it has a disagreeable 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 Caoutchouc 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 injured by varnishing; -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 necessarily reversed. - - -SECTION II. - -_Directions for the use of Photographic Lenses._ - -Those who 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 copying living objects, etc. - -A convenient-sized Camera for small portraits is "the half-plate" with a -lens of about 2-1/4 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 portrait 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 6-1/2 by 4-3/4 -inches, and has a diameter of about 3-1/4 inches. It will take larger -pictures than the last, but not necessarily in a shorter time; since, -although the aperture for admitting the light is larger, the focal length -is proportionately greater and the light less condensed. - -The "quarter-plate" portrait lens of 1-1/4 inch diameter is useful for -stereoscopic subjects and small portraits; which are usually more sharply -defined when taken with a small lens. - -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 produced by lenses, -observe particularly, that with the portrait combination of full aperture, -and especially when the powers of the glass are rather strained by its -being advanced too near to the sitter,--all objects near to the lens will -be _magnified_, and those more removed will appear diminished; hence, as -the position of the sitter is never quite vertical, the Camera must be -inclined a little _downwards_, 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 becomes 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 practicable. 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 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 aperture 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, -projecting 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 application -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 be -obtained._--Non-Achromatic Lenses are understood 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. Suspend 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.-- - -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 will be proper to take many Photographs at minute -variations 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 versâ_ when the nearest ones are most -sharply defined. - -_The Single Achromatic Lens._--A useful lens for landscape 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 Portrait Lens, viz. simply to cut off the outside portion of -the glass.[38] - -[38] See this subject explained in 'Photographic Journal,' vol. ii. p. 133. - -In taking Photographs of architectural and other subjects 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 convenient to -rule the ground focussing glass with a number of parallel lines in both -directions, which enables the operator at once to see that the position of -the instrument is correct. - - -SECTION III. - -_Mode of copying 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 vibrate 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 essential. 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 -Cadmium, if found wanting in intensity, may be at once rendered fit for -use in copying engravings by adding Glycyrrhizine (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. - - Iodide of Potassium 4 grains. - Bromide of Potassium 1 grain. - -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 copied 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 Gallic Acid. -This yields a Negative which 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 exposing to the light until a copy is obtained. All the details are -faithfully rendered in this way, but it is sometimes difficult to obtain a -Negative sufficiently black to yield a _vigorous_ print. - - -SECTION IV. - -_Rules for taking Stereoscopic Photographs._ - -Binocular pictures of a large size, for the reflecting Stereoscope, 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 at page 231, the 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 second 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 picture 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 4-1/2 inches focus. For portraits, a Camera may advantageously -be fitted with two double-combination lenses, of 1-3/4 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 when the -distance is increased to eight feet. - -Pictures taken with a binocular Camera of this kind, require 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 versâ_. - -Mr. Latimer Clark has devised an arrangement for taking 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. - -[Illustration] - -"A strongly-framed Camera-stand carries a flat table, about 20 inches wide -by 16, furnished with the usual adjustments. 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 opposite ends also carry similar pins, but -these are directed upwards, and fit into two corresponding holes in the -tail-board of the Camera. - -"Now when the Camera is placed upon these pins, and moved to and fro -laterally, the whole system exactly resembles the common parallel ruler. -The two bars form the guides, and the Camera, although capable of free -lateral 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 -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 powdered 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 6-3/4 inches by 3-1/4; and when -coated with Iodide of Silver, the two images are impressed side by side, -the plate being shifted laterally about 2-1/2 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.[39] - -[39] See 'Photographic Journal,' vol. i. page 59. - -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 V. - -_On the Photographic delineation of Microscopic objects._ - -Many specimens of Micro-photography which have been exhibited are -exceedingly elaborate and beautiful; and their production is not -difficult to one thoroughly acquainted 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 properly arranged. - -The object-glass of the ordinary compound Microscope is the only part -actually required in Photography, but it is useful to retain the _body_ -for the sake of the adjustments, and the mirrors used in the illumination. -The _eye-piece_ however, which simply magnifies the image formed by the -object-glass, is not necessary, since the same effect of enlargement 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 apparatus 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 large a surface, and a faint and unsatisfactory -picture is the result. The object should be illuminated by sunlight if -it can be obtained, but a bright diffused daylight will succeed with -low-power glasses, and especially when Positives 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 intensity 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 (see page 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-corrected" for colour, in order to compensate for a little -chromatic aberration in the eye-piece. The violet rays, in consequence -of the over-correction, are projected _beyond_ 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 1/50th of an -inch, or two turns of _their_ fine adjustment; a 2/3rds of an inch, -1/200th of an inch, or half a turn; and a 4/10ths of an inch, 1/1000th -of an inch, or about two divisions of the adjustment. With the 1/4th -and higher powers, the difference between the foci was so small as to be -practically unimportant." - -There is also reason to think that the _kind 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 _Camphine_, 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 2-1/2 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 1-1/2-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 exposure 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 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 process by which very -beautiful Microscopic Photographs have been obtained. He illuminates (in -the absence of sunlight) with the brilliant 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 objectives, and this -independent of the angular aperture of the glass. - - - - -CHAPTER IV. - -THE PRACTICAL DETAILS OF PHOTOGRAPHIC PRINTING. - - -This Chapter is divided as follows:-- - - Section I.--The ordinary direct process of positive printing. - Section II.--Positive printing by development. - Section III.--The mode of toning Positives by Sel d'or. - Section IV.--On printing enlarged or reduced Positives, - transparencies, etc. - - -SECTION I. - -_Positive Printing by 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 uniform in texture. Many samples of even the finest paper are however -defective, and hence each sheet should be examined separately by holding -it against the light, and if spots or irregularities of texture are seen, -it should be rejected. These spots usually consist of small particles of -brass or iron, which, when the paper is rendered sensitive, decompose the -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 -experienced 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 Ammonio-Nitrate paper. - -Formula I. _Preparation of Albuminized Paper._--This includes the salting -and albuminizing, and the sensitizing with Nitrate of Silver. - -_The Salting and Albuminizing._--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[40] will answer the purpose. To obtain the Albumen, use new-laid -eggs, and be careful that in opening the shell the yolk is not broken; -each egg will yield about one fluid ounce of Albumen. - -[40] If the water contained much Sulphate of Lime, it is likely that the -sensitiveness of the paper would be impaired (?). - -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 -process 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 narrow 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. Albuminous 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 contain 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 however will depend upon the -kind of paper which is employed, certain varieties taking more gloss than -others; Papier Rive, 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 influence of the light. To avoid them, use the eggs -quite fresh, and lower the paper on to the liquid by one steady movement; -if a pause be made, a line will probably be formed. Some papers are not -readily wetted by 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' colour-men, will be found a useful adjunct. Care must be -taken however not to add an excess, or the Albumen 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 off, and pin -it up by two corners. If any circular spots, free from Albumen, are seen, -caused by bubbles of air, replace the sheet for the same length of time as -at first. - -The paper must not be allowed to remain upon the salting 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 with a heated iron, in order to coagulate the -layer of Albumen upon the surface; but this precaution is unnecessary, -since the coagulation 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 application of a heated iron. - -_To render the paper sensitive._--This operation must be conducted by the -light of a candle, or by yellow light. Take of - - Fused Nitrate of Silver 60 grains. - Glacial Acetic Acid 1/3 minim. - Distilled Water 1 ounce. - -Prepare a sufficient quantity of this solution, and lay the sheet upon it -in the same manner as before. Three minutes' contact will be sufficient -with the thin Negative paper, but if the Canson Positive paper be used, -four or five minutes 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 produced -upon the paper, from decomposition of the Nitrate of Silver. When the -sheet is hung up, a small strip of 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 begin 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 preference 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 sensitizing until it is nearly black. The -colour can be removed by Animal Charcoal,[41] 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 Alcohol -to the Nitrate Bath prevents it discolouring with Albumen. - -[41] Common Animal Charcoal contains Carbonate and Phosphate of Lime, the -former of which renders the Nitrate of Silver alkaline; purified Animal -Charcoal is usually acid from Hydrochloric Acid. - -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 Moss[42] 60 grains. - Water 20 ounces. - -[42] Iceland Moss is recommended because the writer finds that Positives -so printed stand the action of destructive tests better than prints on -plain paper, and equal to prints upon Ammonio-Nitrate paper. - -Pour boiling water upon the 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,[43] floated upon the salting Bath in -the same manner as directed for Albumen at p. 243. - -[43] The writer does not recommend the Positive paper of De Canson, -having noticed that prints upon that paper do not withstand the action of -sulphuretting agents so well as others (?). - -Render 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 paper._--Take of - - Chloride of Ammonium 200 grains. - Citrate of Soda[44] 200 " - Gelatine 20 " - Water 20 fluid ounces. - -[44] This salt may be obtained at the operative chemists; or it may be -prepared extemporaneously by neutralizing 112 grains of pure Citric Acid, -free from Tartaric Acid, with 133 grains of the dried Bicarbonate or -"Sesquicarbonate" of Soda, used for effervescing draughts. - -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. - -Render sensitive by floating for three minutes upon a Nitrate Bath of -sixty grains to the ounce of water. - -Formula III. _Ammonio-Nitrate Paper._--This is always prepared without -Albumen, which is dissolved by Ammonio-Nitrate of Silver. Take of - - Chloride of Ammonium 100 grains. - Citrate of Soda 200 " - Gelatine 20 " - Water 20 fluid ounces. - -Dissolve the Gelatine 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 -employed; the Gelatine and Citrate being retained or omitted, 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 Ammonia 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.[45] 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 liquid becomes -perfectly clear, a drop or two of solution of Nitrate 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 -contain 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 Ammonia_ with the alkali, 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 precipitation would -be considerable. From the same reason, 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. - -[45] If the excess of Ammonia does not readily dissolve it, probably the -Nitrate of Silver is impure. - -Ammonio-Nitrate of Silver should be kept 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 Vocabulary, 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 economical mode of -applying Silver solutions. Procure a flat piece of board somewhat smaller -than the sheet to be operated 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 formed; then carry the rod -directly across the sheet, when an even wave of fluid will be spread over -the surface. 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 discoloured. - -_Use of a solution of Oxide of Silver in Nitrate of Ammonia._--The great -objection to the use of Ammonio-Nitrate 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 -precipitated 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 Formulæ._--Albuminized paper is the -most simple and generally useful; it is well fitted for small portraits -and stereoscopic Photographs. The Ammonio-Nitrate Process requires more -experience, but gives excellent results when black tones are required: it -may be used for larger portraits, engravings, etc. - -Plain paper rendered sensitive by floating upon a Bath of Nitrate of -Silver is easier of manipulation than the Ammonio-Nitrate, and will be -found to be better adapted for toning by the Sel d'or Bath (p. 267) than -the Albuminized Paper. - - -PREPARATION OF THE FIXING AND TONING BATH. - -Take of - - Chloride of Gold 4 grains. - Nitrate of Silver 16 grains. - Hyposulphite of Soda[46] 4 ounces. - Water 8 fluid ounces. - -[46] The common kind of Hyposulphite of Soda occurring in yellow and -discoloured masses, is too impure for use in Photography, and requires -re-crystallization. - -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 Gold. - -In place of Nitrate of Silver, recommended in the formula, Chloride of -Silver may be used, but not Iodide of Silver, as the formation of Iodide -of Sodium would be objectionable (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. - -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 Tetrathionate of Soda reacting upon the -Hyposulphite) has subsided. It will be very active at the expiration of a -few 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 economical, 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 PHOTOGRAPHIC PRINTING. - -These include--the exposure to light, or printing properly 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, when a little -experience has been acquired. They may be held together by the wooden -clips sold at the American warehouses at one shilling 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 then 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 Negative and the power of the -actinic rays, as influenced by the season of the year and other obvious -considerations. As a general rule, the best Negatives 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 powerful, 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 proceeded 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 darker than it is intended to remain. The toning Bath dissolves -away the lighter shades, and reduces 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 the toning Bath; -and albuminized paper will require to be printed somewhat more deeply than -plain paper. - -If, on removal from the printing-frame, a peculiar _spotted_ 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 purple with paper prepared with -Chloride and Citrate of Silver, probably the subsequent 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 sufficiently printed, the -Negative is in fault. Ammonio-Nitrate paper highly salted is particularly -liable to this fault of excess of reduction, and especially so if the -light be powerful; hence it is best, in the summer months, not to print -by the direct rays of the Sun. This 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 -exception 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, produce spots. But the Author, for reasons given in the first -part of the Work (pp. 129 and 165), recommends that the print should -first be washed in common water until the soluble Nitrate of Silver has -been removed.[47] This is known to be the case 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. - -[47] This water must be free from Hyposulphite of Soda, or the print will -become discoloured. - -Immediately on coming in contact with the Hyposulphite 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 _pale_ 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 coloration than the -black tones, and therefore the latter require more time. It must be borne -in mind however that prolonged immersion in the Bath is favourable to -sulphuration and yellowness; tending also to render the image unstable -and liable to fade in the half-tones. 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 subject; copies of line engravings, -having but little half-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 minutes, 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 Hyposulphite of Soda to keep -up the strength of the Bath, is useful, 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 water when it can be obtained, and choose a large -shallow vessel exposing a considerable 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 movement, and then pour off the water quite -dry. This point is important, viz. to drain off the last portion of liquid -completely before adding fresh water. Repeat the process of changing at -least five or six times, or more, according to the bulk of water, number -of Prints, and degree of attention paid to them. - -Lastly, proceed to remove the size from the Print by immersion in boiling -water.[48] This process will give some idea of the permanency of the -tints, since, if they become dull and red, _and do not darken on drying_, -the Print is probably toned without Gold. Ammonio-Nitrate and plain paper -Prints prepared on foreign papers by the modes described 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 -objectionable degree. - -[48] The Print must be well washed in cold water, to remove the -Hyposulphite, before using the hot water; or the half-tones will be liable -to be darkened, or changed to incipient yellowness, by sulphuration. This -point is important as regards the permanency. - -The size may also be effectually removed from the Print by the common -Carbonate of Soda used in washing, although the former process is -recommended as the most secure. Dissolve about a handful of the Soda in a -pint of water, and when the milky deposit, if any occurs, has subsided, -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 properly performed, ink will _run_ in attempting to write -upon the back of the finished picture. After removal from the Soda Bath -a second washing will 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 which 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 River water supplied to many parts of London, being -comparatively soft, answers perfectly, and produces no white deposit, if -the proofs are moved about occasionally. - -When the Prints have been thoroughly washed, blot them off between sheets -of porous paper and hang up to 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 Gelatine in hot water, freshly made; -the best Scotch glue answers well. Gum 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 contracting as it dries. - -Caoutchouc dissolved in mineral Naphtha to the consistence of thick glue -or gold-beaters' size, is employed by many for mounting Photographic -Prints; it may be obtained 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 picture; 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 expansion -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 OF CORRESPONDENCE BETWEEN THE FORMULA OF DIFFERENT -OPERATORS. - -The formulæ for Positive printing given in the works on practical -Photography exhibit great variety; and it has been proposed to attempt to -reduce them to more uniform proportions. This cannot however easily be -done, both on account of the difference in the structure and preparation -of the various Photographic papers, and also because 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 _immerse_ 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 quantity 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. Immersion 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 sensitiveness, and yielding a pale red image without proper depth of -shadow. - -But independent of these differences, the chemical nature of the _size_ -employed also influences the toning of the 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 blotting 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 Photographic papers must -therefore be borne in mind, and allowance 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 formula -should alone be used. - - -SECTION II. - -_Positive Printing by Development._ - -Negative 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 -obtain 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 unfavourable conditions. - - -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 associating -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.[49] 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. - -[49] The formula at p. 246 may be modified with advantage: use double the -quantity of Gelatine, and half the amount of Citrate and Chloride. - -The Bath of Aceto-Nitrate is prepared as follows:-- - - Nitrate of Silver 30 grains. - Glacial Acetic Acid 30 minims. - Water 1 fluid ounce. - -Float the papers (Papier Saxe or Papier Rive) 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 ordinary 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 appearance of the -image when it has received a sufficient amount of exposure:--the whole of -the picture should be seen, 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:-- - - Gallic Acid 2 grains. - Water 1 fluid ounce. - -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 immerse 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 dark 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 over-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 by -removing the 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 appearance 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 obtained by the direct -action of light upon the same sensitive 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 _colour_ 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 Milk._--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 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 -besides 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.[50] - -[50] See the Vocabulary, Part III,, Art. "Milk," for further particulars. - -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_ Positives 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. - -Take of - - Iodide of Potassium 160 grains. - Water 20 fluid ounces. - -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. - -Render 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 Negative in a -pressure frame, and expose _to a feeble light_. About 30 seconds will be -an average time upon a dull winter's day, on which it would be impossible -to print at all in the ordinary way. On removing the Negative nothing -whatever is seen upon the paper, the image being strictly invisible in -this process unless the exposure has been carried 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 correctly 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_-exposed proofs develope slowly but show no half-tones, and that -the _over_-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 Hyposulphite Bath. Then fix the Print in a solution -of Hyposulphite 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 effect; and therefore a considerable change of colour in the -Hyposulphite 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. - -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 -sulphuretting 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 PRINTING PROCESS UPON BROMIDE OF SILVER. - -By substituting the Bromide for the Iodide of Silver in the above process, -the proportions and details of manipulation 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 proportion 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 surface picture. The proportion of Bromide may -be five grains to the ounce of Serum. - -These proofs, even when simply fixed in plain Hyposulphite of Soda, are -superior in colour to the Positives printed by the last formula upon -Iodide of Silver; and the permanency is very great if the development be -sufficiently pushed. The use of the Serum of Milk gives an advantage in -resisting the oxidizing influences to which Positives are liable to be -exposed (p. 150). - - -GENERAL REMARKS ON NEGATIVE PRINTING. - -Printing by development should not be attempted until the manipulation of -the ordinary process by direct exposure 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 Iodide 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 Gallo-Nitrate -of Silver will be rapidly discoloured (read the remarks at page 179). - -Stereoscopic Negatives and small portraits are not successfully printed -by development; since it is difficult to obtain the most elaborate -definition, and there is a slight tendency to yellowness in the white -parts. Positives may be developed upon Albumen paper, but the Gallic 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 opposite -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 the paper, the effect of which is to -remove by degrees the acidity of the Nitrate Bath, and to produce -over-development 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. - - -SECTION 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 advantages which will presently be -enumerated. The description 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.) - -In place of making an extemporaneous Hyposulphite 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 solutions 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 Hyposulphite of Soda may -by long keeping become decomposed, with precipitation of Sulphur. The -effect 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 formulæ 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 milky; that 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 removed, 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[51] 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; otherwise -the dark tones would nearly disappear in the fixing Bath, the Hyposulphite -carrying away the Gold with this superficial layer of silver salt. - -[51] The amateur is recommended not to use Albuminized paper in this -process until he has become accustomed to the manipulations; the plain -paper prints being toned with more ease and certainty. - -To prepare the Ammonia Bath, take of - - Liquor Ammoniæ 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 Ammonia -Bath be strong, the proof becomes unusually _pale 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 necessary to darken the room; but -a _bright light_ proceeding from an open door or window should be avoided. - -After using the salt or the Ammonia, 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 Gold 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 -without 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 solution. 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 removed 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. - -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 Ammoniæ, 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, appear 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 by 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 always used soon after mixing, a uniform -and permanent 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 Negative 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 becoming 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 occupied -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. - -A good arrangement for this 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 in 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 object; 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 2-1/2 or 3-1/4 inches diameter -is the best form to use, and the actinic and luminous 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 -_between_ the lenses obviates to some extent the loss of sharp outline -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 reflector. 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 2-1/4 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 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 intensity 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 -Ammonia, 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 following 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 received upon iodized -paper, and developed by Gallic Acid, in the mode described in the second -Section of this Chapter (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 which 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 be _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 wood, on which the -Negative and sensitive plate are retained, separated only by the thickness -of a sheet of paper; and the use of this will be better than holding the -combination 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 -illuminated 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 Sublimate, and again -blackening by dilute Ammonia, as an improvement 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 outline 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 footnote at page 302). - - - - -CHAPTER V. - -CLASSIFICATION OF CAUSES OF FAILURE IN THE COLLODION PROCESS. - - - Section I.--Imperfections in Collodion Photographs. - Section II.--Imperfections in Paper Positives. - - -SECTION I. - -_Imperfections in Negative and Positive Collodion Photographs._ - -The following may be mentioned:--fogging--spots-- markings, etc. - - -CAUSES OF FOGGING OF COLLODION PLATES. - -1. _Over-exposure of the Plate._--This is likely to happen when using the -full aperture of a double combination lens for distant objects brightly -illuminated, the Collodion 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 -the 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 fogging. 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 sitter, 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).--_b._ Constant employment of a -Collodion containing 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 -Ammonia; 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 described 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).--_b._ 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 contaminated 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). - -5. _Faults of the developing solution._--a. Brown and decomposed 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 having 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._ Vapour of Ammonia or -Hydrosulphate of Ammonia, or the products of the combustion of coal-gas, -escaping into the developing room.--_b._ Development of the image by -immersion in solution 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 portion twice.--_c._ Redipping the plate in the Bath -before development: 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 working order. - -Next prepare a sensitive plate, and after draining it for two or three -minutes in a dark place, pour on the developer: wash, fix, and bring out -to the light; if any mistiness is perceptible, either the developing room -is not sufficiently 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 it in the Camera, and proceed -exactly as if taking a picture, 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 development at the point where -the spot is seen; they may be caused by-- - -1. _The use of Collodion holding small particles in suspension._--Each -particle becomes a centre of chemical action, 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. - -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 light, and produces a spot, known by its being always in the -same part of the plate; occasionally the door works too tightly 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 exposure 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. _Insoluble particles in the Pyrogallic Acid._--The solution of -Pyrogallic Acid will not usually require filtering, but if specks of -Metagallic Acid are present, the developer should be passed through -blotting paper before use. - -Spots of Transparency may generally be traced to some cause -_which renders the Iodide of Silver insensible to light at particular -points_, 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 transparent 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 full of holes. - -4. _Use of glasses improperly cleaned._--This cause is perhaps 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 VARIOUS KINDS ON COLLODION PLATES. - -1. _A reticulated appearance on the film after developing._--When this is -universal, it often depends upon the employment of Collodion containing -water. Or, if not due to this cause, the plate may have been immersed too -quickly in the Bath, and the soluble Pyroxyline partially precipitated. - -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.--_b._ Removal of the plate from the Bath before the -Ether upon the surface has been washed away.--_c._ Redipping the plate in -the Nitrate Bath after exposure to light, and pouring on the developer -_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 blotting-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 sufficiently quickly to cover the -surface before the action begins; 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 has 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 development, the action does not always proceed with -regularity. - -6. _Irregular striæ._--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 inferior sample of Pyroxyline made from too hot acids, and are -most seen when using an old Bath. - -[Illustration] - -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 constant use, dilute it with a -little Ether containing an eighth part of Alcohol.--_b._ 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 Pyrogallic Acid.--_c._ Impurities on the woodwork of -the frame ascending the film by capillary attraction. This is a frequent -source of stains. - -10. _Marks from the developer not running up to the edge of the film_ (p. -212). Remedy 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).--_b._ From the Collodion film being too -blue and transparent for Negatives.--_c._ The Collodion newly made from -pure materials (p. 114).--_d._ The plate kept too long between exciting -and development (p. 100).--_e._ The Bath newly prepared from commercial -crystallized Nitrate of Silver (p. 101).--_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 indurated layer of syrup not properly removed before applying the -developer. - -6. _The developer does not run up to the edge of the film._--This is -likely to occur when using Collodion nearly anhydrous; and particularly -so with a new Bath not containing much 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 effectual 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 different. The beauty of these -pictures depends entirely upon their being clean and brilliant, without -fogging, specks, or imperfections 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, Nitric 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 preference to Pyrogallic -Acid. - -2. _The shadows good, but the lights overdone._--The developing fluid may -have been kept on too long; or the object is not properly illuminated (p. -220); or the Collodion is not adapted for Positives. - -3. _The high lights pale and flat, 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 probably 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 developer at one spot, so as to wash away -the Nitrate of Silver; 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 unevenly at different points; -or the amount of Silver in the 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, but spotted when 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 Chloride 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 present 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 following causes -are likely to produce yellowness:--acidity of the fixing and toning Bath -(p. 139),--its action continued 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 frequently 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. - -7. _Markings of a yellow tint in the dark 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 Negative, 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 Nitrate 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 sensitizing._--The -Nitrate Bath is probably alkaline (see page 89). - - - - -CHAPTER VI. - -LANDSCAPE PHOTOGRAPHY ON PRESERVED 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 process 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 intervening stratum of liquid. -The difficulty however of separating the glasses again without tearing the -film, is considerable. - -In the process of Messrs. Spiller and Crookes, the property 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 application of a considerable heat. - -More recently, Honey has been employed by Mr. Shadbolt.[52] This -substance can scarcely be termed deliquescent, 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, superior -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 capable of supplying the deficient -element, both being nearly or quite indifferent to the Salts of Silver. - -[52] A claim has lately been advanced by Mr. Maxwell Lyte to be considered -as the 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. - - -THE HONEY AND OXYMEL KEEPING PROCESSES. - -When the weather is cool, Collodion plates may be preserved 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 dripping it immediately -from the comb. This point is of importance, since if the sample of honey -be of inferior quality, or adulterated, the process may not succeed. The -quantity of water to be added will vary with the consistence 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 Nitrate 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 Honey 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 considered -certain, since the Honey exercises a slow reducing action upon the Nitrate -of Silver. - -The solution of Pyrogallic Acid may be used of the ordinary 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. - -Fix with Hyposulphite of Soda, and wash in the usual way. - -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 _pure_ -Honey, free from mouldiness and fermentation, will, _in cool weather_, -almost certainly ensure success. - -_A modification of the process when the plates are to be kept over four -hours._--In this case the whole, or the greater part of the Nitrate of -Silver must be removed before applying 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 Photography, 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 development -of the image. The preparation of Oxymel is described 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 Collodion, 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 extent a dry -Collodion process. The observations above referred 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 containing common water and the -other diluted and filtered Oxymel. 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 repeated _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 vertically 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 sensitiveness will be considerably less than that of fresh -Collodion: from two to five minutes may be allowed with a Stereoscopic -view lens having a quarter-inch diaphragm. - -Before developing, the film should be gently washed for a few seconds with -common water. Solution of Pyrogallic 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 ordinary 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. - - -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 affected 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 River and of the River -Thames, with which many parts of London 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 produce a stain when 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, such 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. - - -THE COLLODIO-ALBUMEN PROCESS. - -This process, the theory of which has been briefly explained 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 effectual mode of obviating which will be to clean the -glass so that the film adheres with unusual tenacity. - -The Liquor Potassæ 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 Tripoli-water and Nitric Acid may however, if -desired, be substituted:-- - - Tripoli 1 drachm. - Nitric Acid 30 minims. - Water 1 ounce. - -Lay the glass flat on a cloth, and rub the surface carefully with a tuft -of cotton-wool dipped in the Tripoli; then, before 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. - -_Coating 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-contractile, 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 dipping in the Bath, in order to -favour its adherence to the glass. With Collodion prepared from anhydrous -spirits, about half a minute may be given in cool weather. - -_The Nitrate Bath._--Take of - - Fused Nitrate 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 -blotting-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; continue to use it until it -is of a dark sherry-colour, and then treat it with "Kaolin," in the manner -and with the precautions advised at pages 91 and 245. - -_The Iodized Albumen._--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 9 fluid ounces. - Water 3 fluid ounces. - Liquor Ammoniæ 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 quantity 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 minute 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 referred (p. 241). - -The ammoniacal solution of Albumen may be kept for some time in a -stoppered bottle without much decomposition. 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. Return 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 Nitrate of Silver has been properly washed away from the -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 number 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 process nearly or -quite insensitive to light, and may be preserved unchanged for many weeks. - -_Sensitizing the Albumen film._--When the plate has become 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 clouding -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 Stereoscopic single lens, allow about three minutes in the -winter, or one minute and a half in the summer. - -_Development 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 Nitrate of Silver made with forty grains of Nitrate to the ounce of -water must be previously added to each fluid drachm of the Pyrogallic. -The development, in the case of a landscape view taken with sunlight, -commences almost immediately, and may be completed in about ten minutes, -but the time occupied in developing 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 developer should discolour -before the proper intensity has been obtained, pour it off and mix a fresh -quantity. - -_Fixing 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 fixing agent must penetrate the Albumen, to reach the Collodion -beneath. - -Careful washing in water for five or ten minutes removes the excess of -Hyposulphite, and the plate may then be varnished in the usual way. - - -THE DRY COLLODION PROCESS. - -The earlier attempts to employ sensitive Collodion plates in a desiccated -condition were unsuccessful. The film of Pyroxyline shrinks on drying, and -becomes almost impervious to moisture: hence, the developing solution not -penetrating properly, density cannot easily be obtained. We are indebted -to Dr. Hill Norris, of Birmingham, for establishing 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,[53] -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 between 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 permeable by the developer, it is -recommended to coat it whilst moist with a solution of Gelatine. - -[53] See these states of the film more fully described at page 83. - -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 applicable, since it depends entirely for success upon -the peculiar state of the Collodion, resembling in this respect the Oxymel -process already described. - -_Mode of preparing 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 2-1/2 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 point. The Alcohol must now be added, to -facilitate the 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 _paper_; but as it tends to -gelatinize on cooling, the ordinary mode of filtration commonly fails. -The quantity 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 necessary to stand the -vessel in warm water, in order to prevent gelatinization. - -The Collodion plate, thoroughly washed, is to be immersed 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 settling -unequally upon the plate. Those who possess an apparatus 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 bottom 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 inwards; in a quarter of an hour, -or from that to twenty minutes, 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 sensibility remains good for many days, -possibly for weeks or months in cold weather. - -_Exposure in the Camera._--Allow from four to eight times 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 diameter. -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 -flat 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 minims in hot weather. It -is important that no discoloration should occur on mixing these liquids -together, to obviate 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 will 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 before 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 Hyposulphite of Soda, or dilute -solution of Cyanide of Potassium. - -_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 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. - -Blisters, unless of large size, may often be disregarded, as they -disappear on drying. General cloudiness may depend upon the film having -been imperfectly washed. Irregular reduction at certain parts may be due -to the Gelatine setting before the plate has become dry, or to stains -produced by the finger applied to the upper edge of the plate.[54] - -[54] Since the above was written, Mr. Maxwell Lyte has communicated to the -'Photographic Journal' (vol. iii.) a dry process in which a _modified_ -Gelatine 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 -necessity of employing artificial heat in drying the plates is avoided. - - - - -PART III. - -OUTLINES or GENERAL CHEMISTRY. - - - - -OUTLINES or GENERAL CHEMISTRY. - -[Illustration] - - - - -CHAPTER I. - -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 -understanding 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 Nomenclature; 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 substances which cannot, -in the present state of our knowledge, be resolved into simpler forms of -matter. - -The chemical elements are divided into "metallic" and "non-metallic," -according to the possession of certain general characters. - -The following are some of the principal non-metallic elements, with the -symbols employed to designate them, and their atomic weights:[55]-- - - Symbol. Atomic Wt. - Gases. { Oxygen O 8 - { Hydrogen H 1 - { Nitrogen N 14 - { Chlorine Cl 36 - - Solids. { Iodine I 126 - { Carbon C 6 - { Sulphur S 16 - { Phosphorus P 32 - - Liquid. Bromine Br 78 - Unknown. Fluorine F 19 - -The metallic elements are more numerous. The following list includes only -those which are commonly known:-- - - Symbol. Atomic Wt. - - Metals of the { Potassium K 40 - Alkalies. { Sodium Na 24 - - Metals of the { Barium Ba 69 - Alkaline { Calcium Ca 20 - Earths { Magnesium Mg 12 - - Metals { Iron Fe 28 - Proper. { Zinc Zn 32 - { Cadmium Cd 56 - { Copper Cu 32 - { Lead Pb 104 - { Tin Sn 59 - { Arsenic As 75 - { Antimony Sb 129 - - Nobel { Mercury Hg 202 - Metals. { Silver Ag 108 - { Gold Au 197 - { Platinum Pt 99 - -[55] The atomic weights, with the exception of that of Gold, are taken -from the last edition of Brande's 'Manual of Chemistry.' - - -ON THE BINARY COMPOUNDS OF THE ELEMENTS. - -Many of the elementary bodies exhibit a strong tendency 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 indifferent to each other. - -Oxygen is by far the most important in the list of chemical 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 -oxidized; 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 element, 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 chemical energy, the former belonging to a superior class of bases, -viz. the alkaline. - -By studying the properties of an alkali (such as Potash 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 -solution 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 cabbage 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 manner 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 different 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 property of the class. - -_Chemical composition of Acid and Basic Oxides contrasted._--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 compound of Sulphur and -Oxygen; Nitric Acid, of Nitrogen and Oxygen. But the alkali, Potash, 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 { Oil of Vitriol, Sulphur 1 atom, Oxygen 3 atoms. - { Aqua-fortis, Nitrogen 1 " Oxygen 5 " - Bases { Oxide of Silver, Silver 1 atom. Oxygen 1 atom. - { Oxide of Iron, Iron 1 " Oxygen 1 " - -The class of Hydrogen Acids.--Oxygen is so essentially the element which -forms the acidifying principle of acids, that its very name is derived -from that fact (οξυς, acid, and γενναω, 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 by -Hydrogen uniting with elements like Chlorine, Bromine, Iodine, Fluorine, -etc. Thus, Muriatic or Hydrochloric Acid contains Chlorine and Hydrogen; -Hydriodic Acid contains Iodine and Hydrogen. - -Observe, however, that the position held by the Hydrogen in these -compounds, is different from that of the Oxygen 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 constituent. - - -THE TERNARY COMPOUNDS OF THE ELEMENTS. - -As the various elementary substances unite with each other to form Binary -Compounds, so these binary compounds again unite and form _Ternary_ -Compounds. - -Compound bodies however do not, as a rule, unite with simple elements. In -illustration, take the action of Nitric Acid upon Silver, described at -page 12. No effect is produced upon the metal until _Oxygen_ is imparted; -then the Oxide of Silver so formed dissolves in the Nitric 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 compound 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 neutralizing each other and forming "a -salt." - -_Salts_ therefore are ternary compounds produced by the union of acids and -bases; common Salt, formed by neutralizing 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 energetic 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 neutral salt is formed when a strong acid unites with an -energetic 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 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 dissolve 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 being -correct, and that, in the language of strict definition, any substance -is a salt which is produced by the union of an acid with an alkali, -independent 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 between Oxyacids -and Hydracids has already been pointed out (p. 309), the latter having -been shown to consist of Hydrogen 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 following example:-- - - Hydrochloric Acid + Soda - = Chloride of Sodium + Water; - -or, stated more at length,-- - - (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 -elementary 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 _two_ elements; but that in the -Oxyacid Salts, such as Sulphates, Nitrates, Acetates, _three_ are present. -Thus, Nitrate of Silver consists of Nitrogen, Oxygen, and Silver, but -Chloride 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 Sulphuric Acid added, this acid, being powerful in its -chemical affinities, tends to appropriate to itself the alkali; but it -does not remove _Potassium_ 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) _plus_ (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 of Chlorine -gas be passed into a solution containing the same salt as before -mentioned, viz. the Iodide of Potassium, 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 chemical -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 by the Metals._--A strip of Iron dipped in -solution of Nitrate of Silver becomes immediately 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 solution. - - Iron + Nitrate of Silver - = Silver + Nitrate of Iron. - -_Illustrations amongst Binary Compounds._--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 affinities; those of Potassium, -Sodium, Barium, etc. being the most marked. - -As the alkalies displace the weaker bases from their combination with -acids, so the 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 Hydrocyanic -Acid. - -_The order of decompositions affected by the insolubility or the -volatility of the products which may be 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 versâ_. There are -many causes however which interfere to prevent this; one of which is -_volatility_.--- - -The violent effervescence 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 influence 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 solution 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 of Lead, the result is uncertain, because there -are no elements present which can, by interchanging, form an insoluble -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 important element of the two; as, the -Ox_ide_ of Hydrogen, or Water; the Chlor_ide_ of Silver; the Sulph_ide_ -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 _proto_ is applied to that containing -the least Oxygen; _sesqui_ to that with once and a half as much as the -_proto_; _bi_ 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 Sesquioxide of Iron: the Protochloride of Mercury; -the Bichloride of Mercury. In these examples the Sesquioxide of Iron -is also a _Per_oxide, because no higher simple oxide is known, and the -Bichloride of Mercury is a _Per_chloride 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 of Oxygen and Chlorine respectively, and are hence -entitled to the prefix _proto_; but being of minor importance, they are -excepted from the general rule. - -The combinations of metallic elements with each other are termed "alloys;" -or if containing Mercury, "amalgams." - -_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_ic_ Acid. So Nitric Acid is an Oxide -of Nitrogen; Carbonic Acid is an Oxide of Carbon, etc. When there are two -oxides of the same element, both possessing acid properties, the most -important has the termination _ic_, and the other _ous_; as Sulphuric -Acid, Sulphur_ous_ Acid; Nitric Acid, Nitr_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, _Hydro_chloric Acid, or the Chloride of -Hydrogen; _Hydr_iodic Acid, or the Iodide of Hydrogen. - -_Further illustrations of the nomenclature of Binary Compounds._--The -Oxides of Nitrogen, and also of Sulphur, 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 properties, 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. - -The binary compounds of Sulphur with Oxygen all possess 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 confusion which would result from changing -the nomenclature, the new bodies are termed _Hypo_sulphuric and -_Hypo_sulphurous (from ὑπο, _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 Sulph_ates_; Nitric Acid, Nitr_ates_; -but Sulphur_ous_ Acid forms Sulph_ites_, and Nitrous Acid, Nitr_ites_. - -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 _Proto_sulphate of Iron, or Sulphate of the Protoxide; the -_Per_sulphate of Iron, or Sulphate of the Peroxide. - -Many salts contain more than one atom of acid to each atom of base. In -that case, the usual prefixes expressive of quantity are adopted: thus, -the _Bi_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 deposits from solution of Sulphate of Iron, is a -_basic_ Persulphate 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 neutralizing Hydrochloric Acid with Soda, the product -formed is not known as Hydrochlorate of Soda, but as _Chloride of Sodium_; -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, Cl for Chlorine, Cd for Cadmium, -and Cu for Copper. - -The chemical symbol however does not simply represent 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 speaking of the Laws of Combination. - -_Formulæ of Compounds._--In the _nomenclature_ of compounds it is usual -to place the Oxygen or analogous element _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{5}, not NO{5}AgO. - -The juxtaposition of symbols expresses combination; thus, FeO is a -compound of one proportion of Iron with one of Oxygen, or the "Protoxide -of Iron," If more than one equivalent be present, small figures are -placed below the symbols: thus, Fe{2}O{3} represents two equivalents of -Iron united with three of Oxygen, or the "Peroxide of Iron;" SO{3}, one -equivalent of Sulphur with three of Oxygen, 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 SO{3} means two -equivalents of Sulphuric Acid; 3 NO{5}, three equivalents of Nitric Acid. -The interposition of a comma prevents the influence of the large figure -from extending further. Thus, the double Hyposulphite of Soda and Silver -is represented as follows:-- - - 2 NaO S{2}O{2}, AgO S{2}O{2}, - -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 Hyposulphite -of Gold and Soda, or "Sel d'or," may be written thus;-- - - 3 (NaO S{2}O{2}) AuO S{2}O{2} + 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 representing salts which -contain water of crystallization. Thus, the formula for the crystallized -Protosulphate of Iron is written as follows:-- - - FeO SO{3} + 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 SO{3}. - -The _plus_ sign however is often employed in token of simple _addition_, -no combination of any kind being intended. Thus the decomposition which -follows on mixing Chloride of Sodium with Nitrate of Silver may be -written as follows:-- - - NaCl + AgO NO{5} = AgCl + NaO NO{5}; - -that is,-- - - Chloride of Sodium _added to_ Nitrate of Silver. - = Chloride of Silver _and_ Nitrate of Soda. - - -ON EQUIVALENT PROPORTIONS. - -When elementary or compound bodies enter into chemical union with each -other, they do not combine in indefinite 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 _equivalent_ to 36 grains -of Chlorine and to 126 grains of Iodine, seeing that these quantities all -play the same part in combining; and so it is with regard to the other -elements,--to every one of them a figure can be assigned which represents -the number of parts by weight in which that element unites with others. -These figures are the "equivalents" 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 representative of _one -equivalent_ of an element. Thus "O" indicates 8 parts by weight of Oxygen; -"Cl" one equivalent, or 36 parts by weight, of Chlorine; and so with the -rest. - -Observe however that these figures, termed "equivalents," 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, Hydrogen, as being the -lowest of all, is taken as unity, and the others are related to it. - -_Equivalents of Compounds._--The law of equivalent proportions 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 SO{3}, equals -40. The equivalent of Nitrogen is 14, that of Nitric Acid, or NO{5}, 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 equivalent } 170 - of the Nitrate of Silver } - -_Practical application of the Laws of Combination ._--The utility of being -acquainted with the law of combining proportions is obvious when their -nature is understood. As bodies both unite with and replace each other -in equivalents, 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 Nitrate of Silver into _Chloride_ -of Silver, the weight of Chloride of Sodium which will be necessary is -deduced thus:--one equivalent, or 170 parts, of Nitrate of Silver, is -decomposed 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 proportions 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 required 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 dissolved in water, -and an equivalent quantity, viz. 14·5 grains, of the Nitrate of Silver -added, the yellow precipitate, 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. - -It is supposed that all matter is made up of an infinite number of minute -atoms, which are elementary, and do not admit of further division. Each -of these atoms possesses an actual weight, although inappreciable by our -present methods of investigation. Simple atoms, by uniting with each -other, form _compound atoms_; and when these compounds are broken up, the -elementary constituent 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. - -[Illustration: Fig. 1. Fig. 2. Fig. 3.] - -Fig. 1 is a compound atom of Sulphuric Acid, consisting of an atom of -Sulphur united intimately with three of Oxygen; fig. 2 is an atom of -Peroxide of Nitrogen, NO{4}; and fig. 3, an atom of Nitric Acid, composed -of Nitrogen 1 atom. Oxygen 5 atoms, or in symbols NO{5}. - -_The term "atomic weight" substituted for equivalent proportion._--If -we suppose that the simple atoms of different kinds of matter _differ -in weight_, and that this difference is expressed by their equivalent -numbers, the whole laws of combination follow by the simplest reasoning. -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, 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 "combining 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 Sulphuric Acid, or SO{3}, is -necessarily equal to the combined weights of the four simple atoms; _id -est_, 16 + 24 = 40. - - -ON THE CHEMISTRY OF ORGANIC 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 vegetable and animal kingdoms, although -they cannot themselves 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 substances. - -The class of organic bodies embraces a great variety of products; which, -like inorganic Oxides, may be divided into neutral, acid, and basic. - -The organic _acids_ are numerous, including Acetic Acid, Tartaric, Citric, -and a variety of others. - -The _neutral substances_ cannot easily be assimilated to any class of -inorganic compounds; as examples, take Starch, Sugar, Lignine, etc. - -The _bases_ are also a large class. They are mostly rare substances, not -familiarly known: Morphia, obtained from Opium; Quinia, from Quinine; -Nicotine, from Tobacco, are illustrations. - -_Composition of organic and inorganic bodies contrasted._--There are more -than fifty elementary substances found in the inorganic kingdom, but -only _four_, commonly speaking, in the organic: these four are Carbon, -Hydrogen, Nitrogen, 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, Hydrogen, and Oxygen. The _Nitrogenous bodies_, -so called, containing 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 Nitrogenous 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 formulæ:-- - - Starch C{24}H{20}O{20} - Lignine C{24}H{20}O{20} - Cane Sugar C{24}H{22}O{22} - Grape Sugar C{24}H{28}O{28} - -Inorganic bodies, as already shown, unite _in pairs_,--two elements join -to form a binary compound; two binary compounds produce a salt; two salts -associated together form a double salt. With organic bodies however the -arrangement 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 _Lignine_, or cotton fibre, and Starch,--each -of which contains the three elements united as C{24}H{20}O{20}. - -_Mode of distinguishing between Organic and Inorganic matter._--A simple -means of doing this is as follows:-- place the suspected substance upon -a piece of Platinum-foil, and heat it to redness with a spirit-lamp: if -it first _blackens_, and then burns completely away, it is probably of -organic origin. This test depends upon the fact, that the constituent -elements of organic bodies are all either themselves volatile, or capable -of forming volatile combinations with Oxygen. Inorganic substances, on the -other hand, are often unaffected by heat, or, if volatile, are dissipated -without previous charring. - -The action of heat upon organic matter may further be illustrated by the -combustion of coal or wood in an ordinary 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. - - - - -CHAPTER II. - -VOCABULARY OF PHOTOGRAPHIC CHEMICALS. - - -ACETIC ACID. - -Symbol, C{4}H{3}O{3} + 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 Nitrogenous organic matter of any kind, be added, -it soon acts as a _ferment_, and causes the spirit to unite with oxygen -derived from the atmosphere, and so to become sour from formation of -Acetic Acid, or "vinegar." - -Acetic Acid is also produced on a large scale by heating _wood_ in close -vessels: a substance distils over which is Acetic Acid contaminated with -empyreumatic and tarry matter; it is termed Pyroligneous Acid, and is much -used in commerce. - -The most concentrated Acetic Acid may be obtained by neutralizing common -vinegar with Carbonate of Soda, and crystallizing out the Acetate of Soda -so formed; this Acetate of Soda is then distilled with Sulphuric Acid, -which removes the Soda and liberates Acetic Acid: the Acetic Acid being -volatile, distils over, and may be condensed. - -_Properties of Acetic Acid._--The strongest acid contains only a single -atom of water; it is sold under the name 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 pungent 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 Hydrochloric Acids are also common impurities. They are -injurious in Photographic Processes, from their property of precipitating -Nitrate of Silver. To detect them proceed as follows:--dissolve a small -crystal of Nitrate of Silver in a few drops of water, and add to it about -half a drachm of the Glacial Acid; the mixture should remain quite clear -even when exposed to the light. Hydrochloric and Sulphurous Acid produce -a white deposit of Chloride or Sulphite 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 Pharmacopœia, 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 SILVER. _See_ Silver, Acetate of. - - -ALBUMEN. - -Albumen is an organic principle found both in the animal and vegetable -kingdom. Its properties are best studied in the _white of egg_, which is -a very pure form of Albumen. - -Albumen is capable of existing in two states; in one of 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 temperature 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._--Nitric Acid coagulates Albumen -perfectly without the aid of heat. Acetic Acid however acts differently, -appearing to enter into combination 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. Ammoniacal Oxide of Silver however does not -coagulate Albumen. - -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 Ammonia 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 condition 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 solution of Nitrate -of Silver employed in sensitizing the Albuminized photographic paper is -probably produced by the same compound, although, often referred to the -presence of Sulphuret of Silver. - -Albumen also combines with Lime and Baryta. When Chloride of Barium is -used with Albumen, a white precipitate 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 Phosphorus. - - -ALCOHOL. - -Symbol, C{4}H{6}O{2}. 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 _Quicklime_, 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 becomes 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 is usually -about ·840, and it contains 80 to 83 per cent, of absolute Alcohol. - - -AMMONIA. - -Symbol, NH{3} or NH{4}O. 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 circumstances, and the -result is Ammonia. - -_Properties of Ammonia._--Ammoniacal gas is soluble in water to a large -extent; the solution possessing those properties which are termed alkaline -(see page 308). Ammonia however differs from the other alkalies in one -important particular--it is volatile: hence the original colour of -turmeric-paper affected by Ammonia is restored on the application of heat. -Solution of Ammonia absorbs Carbonic Acid rapidly from the air, and is -converted into Carbonate of Ammonia; it should therefore be preserved in -stoppered bottles. Besides Carbonate, commercial Ammonia often contains -Chloride of Ammonium, recognized by the white precipitate given by Nitrate -of Silver after acidifying with pure Nitric Acid. - -The strength of commercial Ammonia varies greatly; that sold for -pharmaceutical purposes under the name of Liquor Ammoniæ, contains about -10 per cent, of real Ammonia. The sp. gr. of aqueous Ammonia diminishes -with the proportion of Ammonia present, the Liquor Ammoniæ being usually -about ·936. - -Ammonia, although forming a large class of salts, appears at first sight -to contrast strongly in composition with the alkalies proper, such as -Potash and Soda. Mineral bases generally are _protoxides of metals_, as -already shown at page 308, but Ammonia consists simply of Nitrogen and -Hydrogen united without Oxygen. The following remarks may perhaps tend -somewhat to elucidate the difficulty:-- - -_Theory of Ammonium._--This theory supposes the existence of a substance -possessing the properties of a _metal_, but differing from metallic -bodies generally in being _compound_ in structure: the formula assigned -to it is NH{4}, one atom of Nitrogen united with four of Hydrogen. This -hypothetical metal is termed "Ammonium;" and Ammonia, associated with an -atom of water, may be viewed as its _Oxide_, for NH{3} + HO plainly equals -NH{4}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, Sulphate of Ammonia is a Sulphate of -the Oxide of Ammonium; Muriate or Hydrochlorate of Ammonia is a Chloride -of Ammonium, etc. - - -AMMONIO-NITRATE OF SILVER. _See_ Silver, Ammonio-Nitrate of. - - -AQUA-REGIA. _See_ Nitro-Hydrochloric Acid. - - -BARYTA, NITRATE OF. _See_ Nitrate of Baryta. - - -BICHLORIDE OF MERCURY. _See_ Mercury, Bichloride of. - - -BROMINE. - -Symbol, Br. Atomic weight, 78. - -This elementary substance is obtained from the uncrystallizable 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 -Magnesium. - -_Properties._--Bromine is a deep reddish-brown liquid of a disagreeable -odour, and fuming strongly at common temperatures; sparingly soluble -in water (1 part in 23, Löwig), 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 intermediately 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. - - -BROMIDE 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 mixture of Bromide of Potassium and -Bromate of Potash, to redness, in order to drive off the Oxygen from the -latter salt. It crystallizes in anhydrous cubes, like the Chloride and -Iodide of Potassium; it is easily soluble in water, but more sparingly so -in Alcohol; it yields red fumes of Bromine when acted upon by Sulphuric -Acid. - - -BROMIDE OF SILVER. _See_ Silver, Bromide of. - - -CARBONATE OF SODA. - -Symbol, NaO CO{2} + 10 Aq. - -This salt was formerly obtained from the ashes of seaweeds, but is -now more economically manufactured on a large scale from common salt. -The Chloride of Sodium is first converted into Sulphate of Soda, and -afterwards the Sulphate into Carbonate of Soda. - -_Properties._--The perfect crystals contain ten atoms of water, which -are driven off by the application of heat, leaving a white powder--the -anhydrous Carbonate. _Common Washing Soda_ is a neutral Carbonate, -contaminated to a certain extent with Chloride of Sodium and Sulphate of -Soda. The Carbonate used for effervescing draughts is either a Bicarbonate -with 1 atom of water, or a Sesquicarbonate, containing about 40 per cent, -of real alkali; it is therefore nearly double as strong as the washing -Carbonate, which contains about 22 per cent, of Soda. Carbonate 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 substances, 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 Phosphate 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 -digestion in Hydrochloric Acid; but unless very carefully washed it is apt -to retain an acid reaction, and so to liberate free Nitric Acid when added -to solution of Nitrate of Silver. - -_Properties._--Animal Charcoal, when pure, consists, 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 of Charcoal, but is in -great measure peculiar to those derived from the animal kingdom. - - -CHINA CLAY, OR KAOLIN. - -This is prepared, by careful levigation, from mouldering granite and -other disintegrated felspathic rocks. It consists 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 insoluble in water and acids, and produces no decomposition -in solution of Nitrate of Silver. It is employed by Photographers 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 removed by washing the Kaolin in diluted -vinegar and subsequently in water. - - -CHLORINE. - -Symbol, Cl. Atomic weight, 36. - -Chlorine is a chemical element found abundantly in nature, combined with -metallic Sodium in the form of Chloride of Sodium, or Sea-salt. - -_Preparation._--By distilling common Salt with Sulphuric Acid, Sulphate of -Soda and Hydrochloric Acid are formed. Hydrochloric Acid contains Chlorine -combined with Hydrogen; 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 pungent and -suffocating odour; soluble to a considerable extent in water, the solution -possessing the odour and colour of the gas. It is nearly 2-1/2 times as -heavy as a corresponding bulk of atmospheric air. - -_Chemical properties._--Chlorine belongs to a small natural 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 metallic -substances actually undergo _combustion_ when projected into an atmosphere -of Chlorine, the union between the two taking place with extreme violence. -The characteristic bleaching properties of Chlorine gas are explained 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 elements 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 Alkalies in the usual, manner, with -formation of Alkaline Chloride 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. - - -CHLORIDE OF AMMONIUM. - -Symbol, NH{4}Cl. 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 dissolves 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. - - -CHLORIDE OF BARIUM. - -Symbol, BaCl + 2 HO. Atomic weight, 123. - -Barium is a metallic element very closely allied to Calcium, the -elementary basis of Lime. The Chloride of Barium 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 -Photographic 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 Barium._--Chloride of Barium occurs in the -form of white crystals, soluble in about two parts of water, at common -temperature. These crystals contain two atoms of water of crystallization, -which are expelled at 212°, leaving the anhydrous Chloride. - - -CHLORIDE OF GOLD. See Gold, Chloride of. - - -CHLORIDE 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 decomposition at low -redness, but sublimes at higher temperatures; the melted salt concretes -into a hard white mass on cooling. Nearly insoluble in absolute alcohol, -but dissolves in minute quantity in rectified spirit. Soluble in three -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 Calcium, which, being deliquescent, -produce a dampness by absorption of atmospheric moisture: Sulphate of -Soda is also commonly present. The salt may be purified by repeated -re-crystallization, but it is more simple to prepare the pure compound -_directly_, by neutralizing Hydrochloric Acid with Carbonate of Soda. - - -CHLORIDE OF SILVER. _See_ Silver, Chloride of. - - -CITRIC 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 Tartaric Acid. The -adulteration may be discovered by making a concentrated solution of the -acid and adding _Acetate of Potash_; crystals of Bitartrate of Potash will -separate if Tartaric Acid be present. - -Citric Acid is tribasic. It forms with Silver a white insoluble 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 reddening -Citrate of Silver is shown by the Author to be of a similar nature. - - -CYANIDE OF POTASSIUM. - -Symbol, KC{2}N, or KCy. Atomic weight, 66. - -This salt is a compound of Cyanogen gas with the metal Potassium. -Cyanogen is not an elementary body, like Chlorine or Iodine, but consists -of Carbon and Nitrogen united in a peculiar manner. Although a compound -substance, it reacts in the manner of an element, and is therefore -(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 referred. - - -ETHER. - -Symbol, C{4}H{5}O. Atomic weight, 37. - -Ether is obtained by distilling a mixture of Sulphuric Acid and Alcohol. -If the formula of Alcohol (C{4}H{6}O{2}) be compared with that of Ether, -it will be seen to differ from it in the possession of an additional atom -of Hydrogen 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 following 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 exceedingly dense, -and may be seen passing off from the liquid and falling to the ground: -hence the danger of pouring 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 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 gradually react upon and -decompose it. - -The strong alkalies, such as Potash and Soda, also decompose 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. - - -FLUORIDE OF POTASSIUM. - -Symbol, KF. Atomic weight, 59. - -_Preparation._--Fluoride of Potassium is formed by saturating 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 analogous to that involved in the -preparation of Hydrochloric 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. - - -FORMIC ACID. - -Symbol, C{2}HO{3}. Atomic weight, 37. - -This substance was originally discovered in the _red ant_ (_Formica -rufa_), but it is prepared on a large scale by distilling Starch with -Binoxide of Manganese and Sulphuric Acid. - -_Properties._--The strength 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, C{7}H{3}O{5} + H{3}O. Atomic weight, 94. - -The chemistry of Gallic Acid is sufficiently described at page 27, to -which the reader is referred. - - -GELATINE. - -Symbol, C{13}H{10}O{5}N{2}. Atomic weight, 156. - -This is an organic substance somewhat analogous to Albumen, but differing -from it in properties. It is obtained by subjecting bones, hoofs, horns, -calves' feet, etc., to the action of boiling water. The jelly formed -on cooling 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 Russia, from the air-bladders of certain -species of sturgeon. - -_Properties of Gelatine._--Gelatine softens and swells up in cold water, -but does not _dissolve_ until heated: the hot solution, on cooling, forms -a tremulous jelly. One ounce of cold water will retain about three grains -of Isinglass without gelatinizing; but much depends upon the temperature, -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. - - -GLYCERINE. - -Fatty bodies are resolved by treatment with an alkali into an Acid--which -combines with the alkali, forming a _soap_,--and Glycerine, remaining in -solution. - -Pure Glycerine, as obtained by Price's patent process of distillation, -is a viscid liquid of sp. gr. about 1·23; miscible in all proportions -with water and Alcohol. It is peculiarly a neutral substance, exhibiting -no tendency to combine with acids or bases. It has little or no action -upon Nitrate of Silver in the dark, and reduces it very slowly even when -exposed to light. - - -GLYCYRRHIZINE. - -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 exposure -to light. Its preparation is described in the larger works on organic -chemistry. - - -GOLD, CHLORIDE OF. - -Symbol, AuCl{3}. 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 _boil_ the acid, or much of the -Chlorine will be driven off in the form of gas. At the expiration of a few -hours add fresh Aqua-Regia 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 consequently 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-Regia as before; then boil with -excess of Hydrochloric 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-Regia, and evaporate to -dryness 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 Chloride of Gold._--As sold in commerce it usually contains -excess of Hydrochloric Acid, and is then of a bright yellow colour; but -when neutral and somewhat concentrated, it is dark red (_Leo ruber_ of the -alchemists). It gives no precipitate with Carbonate of Soda unless heat -be applied; the free Hydrochloric Acid present forms, with the alkali. -Chloride of Sodium, which unites with the Chloride of Gold, and produces -a double salt, Chloride of Gold and Sodium, soluble in water. - -Chloride of Gold is decomposed with precipitation of metallic Gold by -Charcoal, Sulphurous Acid, and many of the vegetable acids; also by -Protosulphate and Protonitrate of Iron. It tinges the cuticle of an -indelible purple tint. It is soluble in Alcohol and in Ether. - - -GOLD, FULMINATING. - -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 S{2}O{2}. 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 Hyposulphite 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 Hyposulphite of Soda, and -precipitating the resulting salt by Alcohol: the Chloride of Gold must be -added to the Hyposulphite of Soda, and not the Soda salt to the Gold (see -page 250). - -Properties.--Hyposulphite of Gold is unstable and cannot exist in an -isolated state, quickly passing into Sulphur, Sulphuric Acid, and metallic -Gold. When combined with excess of Hyposulphite of Soda in the form of Sel -d'or, it is more permanent. - -Sel d'or occurs crystallized in fine needles, which are very soluble in -water. The commercial article is often impure, containing little else than -Hyposulphite of Soda, with a trace of Gold. It may be analyzed by adding -a few drops of strong Nitric Acid (free from Chlorine), diluting with -water, and afterwards collecting and igniting the yellow powder, which is -metallic Gold. - - -GRAPE SUGAR. - -Symbol, C{24}H{28}O{28}. Atomic weight, 396. - -This modification of Sugar, often termed _Granular Sugar_, or _Glucose_, -exists abundantly in the juice of grapes and in many other varieties of -fruit. It forms the saccharine concretion found in honey, raisins, dried -figs, etc. It may be produced artificially by the action of fermenting -principles and of dilute mineral acids, upon Starch. - -_Properties._--Grape Sugar crystallizes slowly and with difficulty from -a concentrated aqueous solution, in small hemispherical nodules, which -are hard, and feel gritty between the teeth. It is much less sweet to the -taste than Cane Sugar, and not so soluble in water (1 part dissolves in -1-1/2 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 metallic 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. - - -HONEY. - -This substance contains two distinct kinds of Sugar, Grape Sugar, and an -uncrystallizable substance analogous to, or identical with, the Treacle -found associated with common Sugar in the cane-juice. The agreeable -taste of Honey 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 crystalline portion. - -Much of the commercial article is adulterated, and, for Photographic use, -the Virgin Honey should be obtained direct from the comb. - - -HYDROCHLORIC ACID. - -Symbol, HCl. Atomic weight, 37. - -Hydrochloric Acid is a volatile gas, which may be liberated from most of -the salts termed Chlorides by the action of Sulphuric Acid. The acid, by -its superior affinities, removes the base; thus,-- - - NaCl + HO SO{3} = NaO SO{3} + 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. - - -HYDRIODIC 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 solution of -Hydriodic Acid is easily prepared by adding Iodine to water containing -Sulphuretted Hydrogen gas; a decomposition takes place, and Sulphur is set -free: thus, HS + I = HI + s. - -Properties.--Hydriodic Acid is very soluble in water, yielding a strongly -acid liquid. The solution, colourless at first, soon becomes brown from -decomposition, and liberation of free Iodine. It may be restored to its -original condition by adding solution of Sulphuretted Hydrogen. - - -HYDROSULPHURIC ACID. - -Symbol, HS. Atomic weight, 17. - -This substance, also known as Sulphuretted Hydrogen, is a gaseous compound -of Sulphur and Hydrogen, analogous in composition to the Hydrochloric and -Hydriodic Acid. It is usually prepared by the action of dilute Sulphuric -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 SO{3} = FeO SO{3} + HS. - -Properties.--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 -decomposed by Nitric Acid, and also by Chlorine and Iodine. It -precipitates Silver from its solutions in the form of black Sulphuret -of Silver; also Copper, Mercury, Lead, etc.; but Iron and other metals -of that class are not affected, if the liquid contains free acid. -Hydrosulphuric Acid is constantly employed in the chemical laboratory for -these and other purposes. - - -HYDROSULPHATE OF AMMONIA. - -Symbol, NH{4}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 -preparation, 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 Sulphur. Becomes milky on the -addition of any acid. Precipitates, 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 S{2}O{2} + 5 HO. Atomic weight, 125. - -The chemistry of Hyposulphurous Acid and the Hyposulphite 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, Hyposulphite of. - - -HYPOSULPHITE OF SILVER. _See_ Silver, Hyposulphite of. - - -ICELAND MOSS. - -_Cetraria Islandica._--A species of Lichen found in Iceland 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 _kelp_, which is the fused -ash obtained on burning seaweeds. The waters of the ocean contain minute -quantities of the Iodides of Sodium and Magnesium, which are separated and -stored up by the growing tissues of the marine plant. - -In the preparation, the mother-liquor of kelp is evaporated 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 metallic 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. Alcohol and Ether dissolve it more -abundantly, forming dark-brown solutions. Iodine also dissolves freely in -solutions of the alkaline Iodides, such as the Iodide of Potassium, of -Sodium, and of Ammonium. - -_Chemical Properties._--Iodine belongs to the Chlorine group of elements, -characterized by forming acids with Hydrogen, and combining extensively -with the metals (see Chlorine). They are however comparatively indifferent -to Oxygen, and also to each other. The Iodides of the alkalies and -alkaline earths are soluble in water; also those of Iron, Zinc, Cadmium, -etc. The Iodides of Lead, Silver, and Mercury are nearly or quite -insoluble. - -Iodine possesses the property of forming a compound of a deep blue colour -with Starch. In using this as a test, it is necessary first to liberate -the Iodine (if in combination) by means of Chlorine, or Nitric Acids -saturated with Peroxide of Nitrogen. The presence of Alcohol or Ether -interferes to a certain extent with the result. - - -IODIDE OF AMMONIUM. - -Symbol, NH{4}I. 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, CdI. Atomic weight, 182. - -See page 199, for the preparation and properties of this salt. - - -IODIDE OF IRON. - -Symbol, FeI. 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 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 _Perchloride_, 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 solution of Potash -until it begins to acquire a brown colour; a mixture of Iodide of -Potassium and _Iodate of Potash_ (KO IO{5}) is thus formed; but by -evaporation and heating to redness, the latter salt parts with its Oxygen, -and is converted into Iodide of Potassium. - -_Properties._--It forms cubic and prismatic crystals, which should be -hard, and _very slightly or not at all deliquescent_. Soluble in less than -an equal weight of water at 6O°; 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 absolute Alcohol, 1 to 2 grains. The solution of Iodide -of Potassium is instantly coloured brown by free Chlorine; also very -rapidly by Peroxide of Nitrogen (page 86); ordinary acids however act less -quickly, Hydriodic Acid being first formed, and subsequently decomposing -spontaneously. - -The impurities of commercial Iodide of Potassium, with the means to be -adopted for their removal, are fully given at page 197. - - -IODIDE OF SILVER. _See_ Silver, Iodide of. - - -IODOFORM. - -The composition of this substance is analogous to that 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. - - -IRON, PROTOSULPHATE OF. - -Symbol, FeO SO{3} + 7 HO. Atomic weight, 139. - -The properties of this salt, and of the two salifiable Oxides of Iron, are -described at page 29. It dissolves in rather more than an equal weight of -cold water, or in less of boiling water. - -Aqueous solution of Sulphate of Iron absorbs the Binoxide 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 (?). - - -IRON, PROTONITRATE OF. - -Symbol, FeO NO{3} + 7 HO. Atomic weight, 153. - -This salt, by careful evaporation _in vacuo_ over Sulphuric Acid, forms -transparent crystals, of a light green colour, and containing 7 atoms -of water, like the Protosulphate. 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. - - -IRON, PERCHLORIDE OF. - -Symbol, Fe{2}Cl{3}. Atomic weight, 164. - -There are two Chlorides of Iron, corresponding in composition to the -Protoxide and the Sesquioxide respectively. The Protochloride is very -soluble in water, forming a green solution, which precipitates a dirty -white Protoxide 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 solution being acid to test-paper. It is also soluble in -Alcohol, forming the Tinctura Ferri Sesquichloridi of the Pharmacopœia. -Commercial Perchloride of Iron ordinarily 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 porous 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. - - -MERCURY, BICHLORIDE OF. - -Symbol, HgCl{2}. Atomic weight, 274. - -This salt, also called Corrosive Sublimate, and sometimes _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. - -_Properties._--A very corrosive and poisonous salt, usually sold in -semi-transparent, crystalline masses, or in the state of powder. Soluble -in 16 parts of cold, and in 3 of hot water; more abundantly so in Alcohol, -and also in Ether. The solubility in water may be increased by the -addition of free Hydrochloric Acid, or of Chloride of Ammonium. - -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 principal -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 preferred for Photographic use. - -The second constituent, Caseine, is an organic principle somewhat -analogous to Albumen in composition and properties. Its aqueous solution -however does not, like Albumen, _coagulate_ on boiling, unless _an acid_ -be present, which probably removes a small portion of alkali with which -the Caseine was previously combined. The substance termed "rennet," which -is the dried stomach of the 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 remains in a soluble -form in the _whey_; but when the Milk is coagulated by the addition of -acids, the quantity so left is very small, and hence the use of the -rennet is to be preferred, since the presence of Caseine facilitates the -reduction of the sensitive Silver salts. - -Caseine combines with Oxide of Silver in the same manner as Albumen, -forming a white coagulum, which becomes _brick-red_ on exposure to light. - -Sugar of Milk, the third principal constituent, differs from both -cane and grape sugar; it may be obtained by evaporating _whey_ until -crystallization 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 ferment and -form Alcohol on the addition of yeast, like grape sugar, but by the action -of _decomposing animal matter_ is converted into Lactic Acid. - -When skimmed Milk is exposed to the air for some hours, it gradually -becomes _sour_, from Lactic Acid formed in this way; and if then heated to -ebullition, the Caseine coagulates very perfectly. - - -NITRIC ACID. - -Symbol, NO{5}. Atomic weight, 54. - -Nitric Acid, or _Aqua-fortis_, is prepared by adding Sulphuric Acid to -Nitrate of Potash, and distilling the mixture in a retort. Sulphate -of Potash and free Nitric Acid are formed, the latter of which, being -volatile, distils over in combination with one atom of water previously -united with the Sulphuric Acid. - -_Properties._--Anhydrous Nitric Acid is a 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 1·5; if perfectly pure, it is colourless, but usually -it has a slight yellow tint, from partial decomposition into Peroxide of -Nitrogen: it fumes strongly in the air. - -The strength of commercial Nitric Acid is subject to much variation. An -acid of sp. gr. 1·42, containing about 4 atoms of water, is commonly -met with. If the specific gravity is much lower than this (less than -1·36), it will scarcely be adapted for the preparation of 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 high -specific gravity, but this is somewhat deceptive, being caused in part by -the presence of the Peroxide. On mixing with Sulphuric Acid, the colour -disappears, a compound being formed which has been termed a _Sulphate of -Nitrous Acid_. - -In the Appendix a Table is given which exhibits the quantity of real -anhydrous Nitric Acid contained in samples of different densities. - -_Chemical Properties._--Nitric Acid is a powerful oxidizing 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 prolonged -application. Nitric Acid forms a numerous class of salts, _all of which -are soluble in water_. Hence its presence 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 described. Chlorine is detected by diluting the acid -with an 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 testing for Sulphuric Acid, dilute -the Nitric Acid as before, and drop in _a single drop_ of solution of -Chloride of Barium; if Sulphuric Acid be present, an insoluble precipitate -of Sulphate of Baryta will be formed. - - -NITROUS ACID. _See_ Silver, Nitrite of. - - -NITRATE OF POTASH. - -Symbol, KO NO{5}. 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. - - -NITRATE OF BARYTA. - -Symbol, BaO NO{5}. Atomic weight, 131. - -Nitrate of Baryta forms octahedral crystals, which are anhydrous. It is -considerably less soluble than the Chloride of Barium, requiring 12 parts -of cold and 4 of boiling water for solution. It may be substituted for the -Nitrate of Lead in the preparation of Protonitrate of Iron. - - -NITRATE OF LEAD. - -Symbol, PbO NO{5}. 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°. - -Nitrate of Lead forms with Sulphuric Acid, or soluble Sulphates, a white -precipitate, which is the insoluble Sulphate of Lead. The _Iodide_ of Lead -is also very sparingly soluble in water. - - -NITRATE OF SILVER, _See_ Silver, Nitrate or. - - -NITRO-GLUCOSE. - -When 3 fluid ounces of cold Nitro-Sulphuric Acid, consisting 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 formed at first -a thin, transparent, 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 tenacious 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 inferior to -Glycyrrhizine employed for the same purpose. - - -NITRO-HYDROCHLORIC ACID. - -Symbol, NO{4} + Cl. - -This liquid is the Aqua-Regia of the old alchemists. It is produced by -mixing Nitric and Hydrochloric Acids: the Oxygen contained in the former -combines with the Hydrogen of the latter, forming water and liberating -Chlorine, thus:-- - - NO{5} + HCl = NO{4} + HO + Cl. - -The presence of free Chlorine confers on the mixture the power of -dissolving Gold and Platinum, which neither of the two acids possesses -separately. In preparing Aqua-Regia it is usual to mix one part, by -measure, of Nitric Acid with four of Hydrochloric Acid, and to dilute -with an equal bulk of water. The application of a gentle heat assists the -solution of the metal; but if the temperature rises to the boiling point, -a violent effervescence and escape of Chlorine takes place. - - -NITRO-SULPHURIC 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 Potash 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 elementary 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 Nitric Acid and bodies of that class which contain -Oxygen loosely combined. - - -OXYMEL. - -This syrup of Honey and Vinegar is prepared as follows. 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, KO + 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 Carbonate 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 melting 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, in order to prevent them from becoming immovably -fixed by the solvent action of the Potash upon the Silica of the glass. - -The Liquor Potassæ of the London Pharmacopœia has a sp. gr. of 1·063, and -contains about 5 per cent, of real Potash. It is usually contaminated -with _Carbonate_ of Potash, 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, CARBONATE OF. - -Symbol, KO CO{2}. 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 -appear essential to vegetation, and are absorbed and approximated 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. Carbonate 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 196). - - -PYROGALLIC ACID. - -Symbol, C{8}H{4}O{4} (Stenhouse). Atomic weight, 84. - -The chemistry of Pyrogallic Acid has been described at page 28. - - -SEL D'OR. _See_ Gold, Hyposulphite of. - - -SILVER. - -Symbol, Ag. Atomic weight, 108. - -This metal, the _Luna_ or _Diana_ of the alchemists, is found native in -Peru and Mexico; it occurs also in the form of Sulphuret of Silver. - -When pure it has a sp. gr. of 10·5, and is very malleable and ductile; -melts at a bright red heat. Silver does not oxidize in the air, but -when exposed to an impure atmosphere 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 Nitric 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, redissolve 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 resulting compound, which -is white, and consists of 100 parts of the Nitrate + 29·5 of Ammonia. -The compound however which Photographers employ under the name of -Ammonio-Nitrate of Silver may be viewed more simply as a solution of the -Oxide of Silver in Ammonia, without reference to the Nitrate of Ammonia -necessarily produced in the reaction. - -Very strong Ammonia, in acting upon Oxide of Silver, converts it -into a black powder, termed _Fulminating Silver_, which possesses the -most dangerous explosive properties. Its composition is uncertain. In -preparing Ammonio-Nitrate of Silver by the common process, the Oxide -first precipitated occasionally leaves a little black powder behind, on -re-solution; this does not appear however, according to the observations -of the Author, to be Fulminating Silver. - -In sensitizing salted paper by the Ammonio-Nitrate of Silver, _free -Ammonia_ is necessarily formed. Thus-- - - Chloride of Ammonium + Oxide of Silver in Ammonia - = Chloride of Silver + Ammonia + Water. - - -SILVER, OXIDE OF. - -Symbol, AgO. Atomic weight, 116. - -This compound has already been described in Part I., page 17. - - -SILVER, CHLORIDE OF. - -Symbol, AgCl. Atomic weight, 144. - -The preparation and properties of Chloride of Silver are given in Part I. -page 14. - - -SILVER, BROMIDE OF. - -Symbol, AgBr. Atomic weight, 186. - -See Part I. page 17. - - -SILVER, CITRATE OF. _See_ Citric Acid. - - -SILVER, IODIDE OF. - -Symbol, AgI. Atomic weight, 234. - -See Part I. page 16. - - -SILVER, FLUORIDE 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. - - -SILVER, SULPHURET OF. - -Symbol, AgS. Atomic weight, 124. - -This compound is formed by the action of Sulphur upon metallic Silver, -or of Sulphuretted Hydrogen or Hydrosulphate of Ammonia upon the Silver -salts; the decomposition 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.) - - -SILVER, NITRATE OF. - -Symbol, AgO NO{5}. Atomic weight, 170. - -The preparation and properties of this salt have been explained at pages -12 and 362. - - -SILVER, NITRITE OF. - -Symbol, AgO NO{3}. Atomic weight, 154. - -Nitrite of Silver is a compound of Nitrous Acid, or NO{3}, with Oxide of -Silver. It is formed by heating Nitrate of Silver, so as to drive off a -portion of its Oxygen, or more conveniently, by mixing Nitrate of Silver -and 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 Photographic 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, NO{4}, is also -decomposed by water, and yields the same products. - - -SILVER, ACETATE OF. - -Symbol, AgO (C{4}H{3}O{3}). 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 impedes -the decomposition. Its properties have been sufficiently described at page -89. - - -SILVER, HYPOSULPHITE OF. - -Symbol, AgO S{2}O{2}. Atomic weight, 164. - -This salt is fully described in Part I. page 129. For the properties of -the soluble double salt of Hyposulphite of Silver and Hyposulphite of -Soda, see page 43. - - -SUGAR OF MILK. _See_ Milk. - - -SULPHURETTED HYDROGEN. _See_ Hydrosulphuric Acid. - - -SULPHURIC ACID. - -Symbol, SO{3}. Atomic weight, 40. - -Sulphuric Acid may be formed by oxidizing Sulphur with boiling Nitric -Acid; but this plan would be too expensive to be adopted on a large -scale. The commercial process for the manufacture of Sulphuric Acid is -exceedingly ingenious and beautiful, but it involves reactions which are -too complicated to admit of a superficial explanation. The Sulphur is -first burnt into gaseous Sulphurous Acid (SO{2}), 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 SO{2} into SO{3}, or Sulphuric Acid. - -_Properties._--Anhydrous Sulphuric Acid is a white crystalline 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 SO{3}, -is a dense fluid, having a specific gravity of about 1·845; boils at -620°, and distils without decomposition. 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 down even to -zero without solidifying; but a weaker compound, 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 _chars_ organic -substances, by removing the elements of water, and converts Alcohol into -Ether in a similar manner. The _strength_ of a given sample of Sulphuric -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 composition, and seems to be as well -adapted for Photographic use as the _pure_ Sulphuric Acid, which is far -more expensive. 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 precipitate, _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 testing, or a crystalline precipitate -will form, caused by the sparing solubility of the Chloride of Barium -itself in acid solutions. - - -SULPHUROUS ACID. - -Symbol, SO{2}. 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 decomposition of Hyposulphurous Acid, but it -afterwards disappears from the liquid by a secondary reaction, resulting -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. - - -TETRATHIONIC ACID. - -Symbol, S{4}O{5}. Atomic weight, 104. - -The chemistry of the Polythionic Acids and their salts will be found -described in the First Part of this Work, page 157. - - -WATER. - -Symbol, HO. Atomic weight, 9. - -Water is an Oxide of Hydrogen, containing single atoms of each of the -gases. - -_Distilled water_ is water which has been vaporized and again condensed; -by this means it is freed from earthy and saline impurities, which, not -being volatile, are left in the body of the retort. _Pure_ distilled water -leaves no residue on evaporation, and should remain perfectly clear on the -addition of Nitrate of Silver, _even when exposed to the light_; it should -also be neutral to test-paper. - -The condensed water of steam-boilers sold as distilled water is apt to be -contaminated with oily and empyreumatic matter, which discolours Nitrate -of Silver, and is therefore injurious. - -_Rain-water_, having undergone a natural process of distillation, 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 Sodium 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, forming 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 _Photographic 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 Nitrate Bath when nothing -better is at hand. The Chlorides it contains are precipitated by the -Nitrate of Silver, leaving soluble _Nitrates_ in solution, which are not -injurious. The Carbonate of Lime, if any is present, neutralizes free -Nitric Acid, rendering the Bath alkaline in the same manner as Carbonate -of Soda. (See page 89.) Sulphate of Lime, usually present in well water, -is said to exercise a retarding action upon the sensitive Silver Salts, -but on this point the writer is unable to give certain information. - -Hard water is not often sufficiently pure for the developing fluids. The -Chloride of Sodium it contains decomposes the Nitrate of Silver upon -the film, and the image cannot be brought out perfectly. The _New River -water_, however, supplied to many parts of London, is almost free from -Chlorides, and answers very well. In other cases a few drops of Nitrate of -Silver solution may be added, to separate the Chlorine, taking care not to -use a large excess. - - - - -APPENDIX. - - -QUANTITATIVE TESTING OF SOLUTIONS OF NITRATE 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 8-1/2 grains to 6 fluid -ounces. - -In this way, a standard solution of salt is formed, each drachm of -which (containing slightly more than one-sixth of a grain of salt) will -precipitate exactly half a grain of Nitrate of Silver. - -In order to use it, measure out 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 which is to be tested; shake the contents of the -bottle briskly, until the white curds have perfectly separated, and the -supernatant liquid is clear and colourless; then add fresh portions of the -standard solution, by 30 minims at a time, with constant shaking. When the -last addition causes no _milkiness_, read off the total number of drachms -employed (the last half-drachm being subtracted), and multiply that -number by 4 for the weight in grains of the Nitrate of Silver present in -an ounce of the Bath. - -In this manner the strength of the Bath is indicated within two grains to -the ounce, or even to a single grain if the last additions of standard -salt-solution be made in portions of 15, instead of 30 minims. - -Supposing the Bath to be tested is thought to contain about 35 grains of -Nitrate to the ounce, it will be convenient to begin by adding 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. - - -RECOVERY OF SILVER FROM WASTE SOLUTIONS,--FROM THE BLACK DEPOSIT OF -HYPO-BATHS, ETC. - -The manner of separating metallic Silver from waste solutions varies -according to the presence or absence of alkaline Hyposulphites and -Cyanides. - -a. _Separation of metallic Silver from old Nitrate Baths._--The Silver -contained in solutions of the Nitrate, Acetate, etc. may easily be -precipitated by suspending a strip of sheet Copper in the liquid; the -action is completed in two or three days, the whole of the Nitric Acid and -Oxygen passing to the Copper, and forming a blue solution of the Nitrate -of Copper. The metallic Silver however, separated in this manner, always -contains a portion of Copper, and gives a blue solution when dissolved in -Nitric Acid. - -A better process is to commence by precipitating the Silver entirely 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 pouring off the upper clear portion when -the clots have again settled down. The Chloride of Silver thus formed may -afterwards be reduced to metallic Silver by a process which will presently -be described (p. 374). - -b. _Separation of Silver from solutions containing alkaline Hyposulphites, -Cyanides, or Iodides._--In this case the Silver cannot be precipitated by -adding Chloride of Sodium, since the Chloride of Silver 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 1-1/2 fluid -ounce of Oil of Vitriol diluted with 10 ounces of water. The gas is -generated gradually without the application of heat, and must be allowed -to bubble up through the liquid from which the Silver is to be separated. -The smell of Sulphuretted Hydrogen being offensive, 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 decompose -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 convenient, in -operating on a small scale, to proceed in the following manner:--first -convert the Sulphuret into Nitrate of Silver, by boiling with Nitric Acid -diluted with two parts of water; when all evolution of red fumes has -ceased, the liquid may be diluted, allowed to cool, and filtered from -the insoluble portion, which consists principally of Sulphur, but also -contains a mixture of Chloride and Sulphuret of Silver, unless the Nitric -Acid employed was free from Chlorine; this precipitate may be heated, in -order to volatilize the Sulphur, and then digested with Hyposulphite of -Soda, or added to the Hypo-Bath. - -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 unless -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. - - -REDUCTION OF CHLORIDE OF SILVER 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 Carbonate 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 reduction 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 Sulphuric -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 detached. The grey powder -must be repeatedly washed, first with Sulphuric Acid and water (this is -necessary to dissolve a portion of an insoluble Salt of Zinc, probably -an oxychloride) and then with water alone, until the liquid runs away -_neutral_, and gives no precipitate 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 parts 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 -Hyposulphite of Soda, and then again with water. - - -MODE OF TAKING THE SPECIFIC GRAVITY OF LIQUIDS. - -Instruments are sold, termed "Hydrometers," which indicate specific -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 with 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 capillary -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 density of the -liquid. - -Thus, to take examples, supposing the bottle filled with _rectified -Ether_ to require 250 grains to enable it to counterbalance the brass -weight,--then 1· _minus_ ·250, or ·750, is the specific gravity; but in -the case of _Oil of Vitriol_ the bottle, when full, 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 water, -in place of 1000; in this case the number of grains to be added or -subtracted must be multiplied by 2. - -In taking specific gravities, observe that the temperature be within a few -degrees of 60° Fahrenheit (if higher or lower, immerse the bottle in warm -or cold water); and wash out the bottle thoroughly with water each time -after use. - - -ON FILTRATION AND WASHING PRECIPITATES. - -In preparing filters, cut the paper into squares of a sufficient size, -and fold each square neatly upon itself, first into a half-square, and -then again, at right angles, into a quarter-square;--round off the corners -with a pair of scissors, and open out the filter into a conical form, when -it will be found to drop exactly into the funnel, and to be uniformly -supported throughout. - -Before pouring in the liquid, always moisten the filter with distilled -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 returning 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 allowed 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-PAPERS. - -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_-paper. Turmeric is a yellow vegetable substance -which possesses the property of becoming brown when treated 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 containing about one drop of -Liquor Potassæ or Ammoniæ, 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 infinitesimally small, it is essential that -the litmus-paper should be in good condition; and test-papers prepared -with porous paper will be found to show the colour better than those -upon glazed or strongly-sized paper. The mode of employing the paper is -as follows:--Place a small strip in the liquid to be examined: if it -becomes at once _bright red_, a strong acid is present; but if it changes -_slowly to a wine-red_ tint, a weak acid, such as Acetic or Carbonic, is -indicated. In the case of the Photographic Nitrate Bath faintly acidified -with Acetic Acid, a purple colour only may be expected, and a decided red -colour would suggest the presence of Nitric Acid. In the Hypo fixing and -toning Bath which has acquired acidity, the litmus-paper will perhaps -redden in about three or four minutes. - -Blue litmus-papers may be changed to the red papers used for alkalies by -soaking in water acidified with Sulphuric Acid, one drop to half a pint; -or by holding for an instant near the mouth of a bottle containing Glacial -Acetic Acid. In examining a Nitrate Bath for alkalinity by means of the -reddened litmus-paper, at least five or ten minutes should be allowed for -the action, since the change of colour from red to blue takes place very -slowly. - - -REMOVAL OF SILVER STAINS FROM 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 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 SULPHURIC ACID OF -DIFFERENT SPECIFIC GRAVITIES. (URE.) - - +---------+------------++---------+------------++---------+------------+ - | |Real Acid || |Real Acid || |Real Acid | - |Specific | in 100 ||Specific | in 100 ||Specific | in 100 | - |Gravity. |parts of the||Gravity. |parts of the||Gravity. |parts of the| - | | Liquid. || | Liquid. || | 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.) - - +---------+------------++---------+------------++---------+------------+ - | |Real Acid || |Real Acid || |Real Acid | - |Specific | in 100 ||Specific | in 100 ||Specific | in 100 | - |Gravity. |parts of the||Gravity. |parts of the||Gravity. |parts of the| - | | Liquid. || | Liquid. || | Liquid. | - +---------+------------++---------+------------++---------+------------+ - | 1·5000 | 79·700 || 1·4640 | 69·339 || 1·4147 | 58·978 | - | 1·4980 | 78·903 || 1·4600 | 68·542 || 1·4107 | 58·181 | - | 1·4960 | 78·106 || 1·4570 | 67·745 || 1·4065 | 57·384 | - | 1·4940 | 77·309 || 1·4530 | 66·948 || 1·4023 | 56·587 | - | 1·4910 | 76·512 || 1·4500 | 66·155 || 1·3978 | 55·790 | - | 1·4880 | 75·715 || 1·4460 | 65·354 || 1·3945 | 54·993 | - | 1·4850 | 74·918 || 1·4424 | 64·557 || 1·3882 | 54·196 | - | 1·4820 | 74·121 || 1·4385 | 63·760 || 1·3833 | 53·399 | - | 1·4790 | 73·324 || 1·4346 | 62·963 || 1·3783 | 52·602 | - | 1·4760 | 72·527 || 1·4306 | 62·166 || 1·3732 | 51·805 | - | 1·4730 | 71·730 || 1·4269 | 61·369 || 1·3681 | 51·068 | - | 1·4700 | 70·933 || 1·4228 | 60·572 || 1·3630 | 50·211 | - | 1·4670 | 70·136 || 1·4189 | 59·775 || 1·3579 | 49·414 | - +---------+------------++---------+------------++---------+------------+ - - -WEIGHTS AND MEASURES. - - -_Troy, or Apothecaries' Weight._ - - 1 Pound = 12 Ounces. 1 Ounce = 8 Drachms. 1 Drachm - = 3 Scruples. 1 Scruple = 20 Grains. (1 Ounce Troy = 480 - Grains, or 1 Ounce Avoirdupois _plus_ 42·5 grains.) - - -_Avoirdupois Weight._ - - 1 Pound = 16 Ounces. 1 Ounce = 16 Drachms. 1 Drachm - = 27·343 grains. (1 Ounce Avoirdupois = 437·5 grains.) (1 Pound - Avoirdupois = 7000 Grains, or 1 Pound Troy _plus_ 2-1/2 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_ 1-1/4 Pound Avoirdupois. - A fluid Ounce of water _weighs_ 1 Ounce Avoirdupois, or - 437·5 Grains. A Drachm of water _weighs_ 54·7 Grains. - - -_French Measures of Weight._ - - 1 Kilogramme = 1000 Grammes = something less than 2-1/4 Pounds - Avoirdupois. - - 1 Gramme = 10 Decigrammes--100 Centigrammes = 1000 - Milligrammes = 15·433 English Grains. - - A Gramme of water _measures_ 17 English Minims, nearly. - 1000 Grammes of water _measure_ 35-1/4 English fluid Ounces. - - -_French Measures of Volume._ - - 1 Litre = 13 Decilitres = 100 Centilitres = 1000 Millilitres = - 35-1/4 English fluid Ounces. - - 1 Litre = 1 Cubic Decimetre = 1000 Cubic Centimetres. - - 1 Cubic Centimetre = 17 English Minims. - - A Litre of water _weighs_ a Kilogramme, or something less than - 2-1/4 Pounds Avoirdupois. A Cubic Centimetre of water _weighs_ a - Gramme. - - - - -INDEX.[56] - -[56] The preparation and properties of the Chemicals used in Photography -will be found in the Alphabetical List commencing at page 327. - - - Aberration, chromatic, 54; - spherical, 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 favour fogging and spots, 104; - contra-indicated for glass Positives, 111. - Acetic Acid, properties and mode of testing purity of, 327; - useful in preventing fogging, 104; - in rendering the development slow and even, 99; - does not coagulate Albumen, 329; - a good commercial form of acid, 212; - Acetic Acid essential in Calotype, waxed paper, and Albumen - processes, 177; - also in printing paper Positives by development, 260. - Aceto-Nitrate of Silver, term explained, 177. - Achromatic Lenses, their construction explained, 55; - the visual and chemical foci often coincident in, 60. - Acids, nature of, 308. - Actinism, explained, 61; - importance 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 invention, 10; - theory of, 180; - Collodio-Albumen process of M. Taupenot, 294. - Albuminized paper, formula for, 241; - slow in fixing, 131; - not well adapted for toning by Sel d'or, 269; - good for stereoscopic subjects and small portraits, 249; - cannot be sensitized with Ammonio-Nitrate of Silver, 246. - Alcohol, its chemistry, 330; - sometimes too dilute for making Collodion, 84; - mode of rectifying, 196; - must not contain impurities, 96; - effects of adding to Collodion, 84, 96; - to developer, 205. - Alkalies, nature of, 308. - Alkalinity of Nitrate Bath, explained, 88; - the evils it produces, 104; - how to test for it, 377; - how to remove it, 277. - Amber varnish, 226. - Ammonia, preparation and properties, 331; - its use in fixing, 42; - Mr. Shadbolt's formula for, 271; - its action upon Chloride of Gold, 343; - effect of concentrated Ammonia upon Oxide of Silver, 362. - Ammonio-Nitrate of Silver, its chemistry, 262; - used in Positive printing to increase sensitiveness, 125; - to give black tones, 127; - cannot be used with Albumen, 246; - increases permanency of print, 169; - old Nitrate 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 formula, but less sensitive than the last, 258. - Atomic theory explained, 322. - - Bath for fixing and toning Positives. _See_ Fixing and toning Bath. - Bichloride of Mercury, whitening action on glass Positives - explained, 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 inverted, 53; - the term "flatness of field" explained, 54; - best position of the Camera for portraits, 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 process, 276. - Chemical affinity, illustrations of, 312. - Chemical elements, 306. - Chemical focus, directions for finding, 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, Vocabulary of, 327. - Chloride of Silver, its preparation and properties, 14; - more sensitive to white light than Bromide or Iodide, 19; - less sensitive to invisible image, 24; - its blackening by light explained, 20, 141; - accelerated by excess of Nitrate, 19; - by organic matter, 20, 142; - experiments illustrating darkening of papers prepared with, 21; - simple explanation 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 toning, 132, 271; - compounds formed on adding it to Hyposulphite 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 preparing paper with, 246. - Cleaning glass plates, theory of, 39; - details of, 213. - Collodion, its discovery, 10; - chemistry of Pyroxyline, 75; - physical effect of Ether and Alcohol in, 83; - of water in, 85; - glutinosity of, 83; - coloration of iodized, explained, 85; - sensitiveness and intensity affected by the change, 97, 99; - details of manufacture of Collodion, 185; - Positive Collodion, theory of, 108; - formula for, 201; - Negative Collodion, theory of, 113; - formula for, 208; - Collodion for copying engravings, 231; - for keeping processes, 298; - for hot climates, 210; - for working by artificial light, 238; - to remove the brown colour from Collodion, 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 markings on, 281; - conditions which affect its sensitiveness to light, 92; - causes influencing its behaviour 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 photographic action assisted by reflection of white light, 66. - Combination, laws of, 307. - Conjugate foci, explained, 52, 272. - Crookes, Mr., remarks upon chemical spectrum, 63; - upon waxed paper process, 180; - preservative process for Collodion films, 289. - Curvature of luminous image formed by lens, explained, 53. - Cyanide of Potassium, its fixing action explained, 44; - preparation of solution of, 207; - used to remove 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 Positives, 259; - conditions which increase or diminish rapidity of development, 98; - irregularities of development, 103. - Developers, their preparation and properties, 26; - comparative strength of, 98; - theory of, for Positives, 111; - for Negatives, 117; - formulæ for Positive developers, 205; - for Negative, 211. - Diagrams, mode of copying, 232. - Diaphragms for lenses. _See_ Stops. - Double decomposition, illustrated, 14; - explained, 314. - Dry Collodion process, 298. - - Elementary bodies, table of, 306; - combination of, 307. - Engravings, mode of copying, 231; - often yield dark-coloured prints, 255. - Equivalent proportions, 320. - Ether, properties of, 339; - purification of, for Photography, 195; - must be kept in a dark place, 196; - should not be distilled from residues of old Collodion, 96. - Experiments, illustrating 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 photographs, 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. _See_ 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 details of fixing, 225; - fixing paper Positives with Ammonia, 271. - Fixing and toning Bath, its preparation, 250; - conditions which favour or retard its action, 135; - certain states of the Bath injurious to the proofs, 136; - importance of keeping it in an active condition, 168; - must not be employed immediately after mixing, 251; - must not be allowed to become acid by constant use, 168; - theory of the gradual change of properties it undergoes, 156. - Foci, actinic and luminous, 60; - actinic, mode of finding, 229; - variation between them in microscopic objectives, 237. - Focussing the object, 220. - Fogging, theory of, 103; - mode of detecting causes of, 276. - Formulæ for solutions required in Collodion process, 201; - for papers 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; - discolours rapidly when developing dishes are not clean, 179. - Gelatine, its properties, 341; - forms a compound with an Oxide of Silver, 21; - employed in dry Collodion process, 299; - modified form of, 302; - affects the colour in 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 Collodion, 215; - with Albumen, 180. - Glutinous Collodion, explained, 83. - Glycyrrhizine, its nature, 342; - its action in Collodion, 114; - formula for solution of, 209. - Gold, Chloride of. _See_ Chloride of Gold. - Gold salts, their use in Photographic printing explained, 131; - in the Daguerreotype process, 175. - Gradation of tone, in Collodion Photographs, affected by the density of - the film, 109, 113; - by use of Glycyrrhizine, 115. - Gradation of tone, in paper Positives, conditions affecting it in prints - obtained by direct exposure, 123; - in Positives printed by development, 266. - - Hadow, Mr., researches on Collodion, 77; - formula for making Pyroxyline, 187. - Heliography, invented by M. Niépce, 7. - Historical sketch of Photography, 6. - Honey keeping process, 289. - Hunt, Mr., introduces Protosalts of Iron in developing, 111. - Hypo Bath. _See_ Fixing and Toning 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 Nitrate 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 - Perchloride of Iron, 139; - test for presence of, 169. - - Iceland moss, its use in Positive printing, 128; - formula for preparing paper with, 245. - Imperfections in Collodion Negatives, 282; - in Positives, 284; - in paper Positives, 285. - Intensity, explanation of term, 92; - mode of increasing in Negatives, 99, 114; - effect of Acetate of Silver upon, 116; - of Nitrite 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 formation of, 34; - development of, 36; - experiments illustrating, 25. - Iodate, how formed in Collodion film, 94; - produces insensitiveness, 198. - Iodide of Ammonium, preparation of, 198; - not fitted for iodizing Collodion required to be kept long, 210. - Iodide of Iron, an accelerator to Collodion, 116. - Iodide of Potassium and Silver, properties of, 42; - mode of iodizing Calotype papers by, 177. - Iodide of Potassium, tests of purity of, 197; - extent of solubility in Alcohol, 351; - dissolves Iodide of Silver, 42. - Iodide of Silver, its preparation and properties, 16; - unaffected by direct 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 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 solubility 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 Iodate in the Bath, 94; - often useful in Positive Collodion, 110; - in Negative Collodion, if fogging occurs, 105; - mode of removing from Collodion, 86. - Iodized Collodion. See Collodion. - - Kaolin, properties of, 335; - used to decolorize Nitrate Baths, 91; - importance of purifying it before use, 245. - - Landscape Photography, with preserved Collodion plates, 288. - Latent image. See Invisible image. - Laws of substitution explained, 78. - Le Grey, M,, his toning process with Chloride of Gold, 132; - his waxed paper Negative process, 178. - Lenses, various forms of, 51; - foci of, 52; - formation of images by, 53; - use of stops, 58; - portrait, 59; - chemical foci of, 60; - chromatic aberration of, 54; - spherical aberration of, 56; - simple directions 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; - photographic action of coloured light, 60; - refraction of light, 49. - Llewellyn, Mr., his Oxymel process, 291. - - Manipulations of Collodion process, 213; - of Photographic printing, 251; - of toning by 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, substances which should be avoided in, - 155, 164; - details of, 257. - - Negative processes for printing Positives, 259, 263. - Negatives, definition of, 106; - Collodion Negatives, theory of production of, 113; - Calotype, 176; - waxed paper, 178; - Albumen, 180; - Collodio-Albumen (Taupenot), 181; - mode of developing Collodion Negatives, 37, 117, 223; - of converting Positives into, 117; - formula for solutions for Negatives, 208; - the Collodion best adapted for Negatives, 114; - spots and markings upon Negatives, 282; - decomposition of Pyroxyline 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_ Nitrite of Silver), 14; - not acted on by light, 18; - its reduction by Pyrogallic 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 Collodion plate, 92; - dissolves Iodide of Silver, 86; - discoloured by Albumen, 245, 329; - forms a compound with Honey, 289; - with various other organic bodies, 21; - very little acted on by Glycerine, 342; - mode of recovering 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 required to prevent it from yielding foggy pictures, 104; - a caution against the too frequent addition of alkali, 204; - quantitative 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 sensitiveness of Collodion film, 93; - lessens rapidity of development, 98; - tends to prevent fogging, 104; - sometimes usefully employed for glass Positives, 110; - contra-indicated for Negatives, 116; - its accumulation in the Nitrate Bath explained, 94; - mode of removing it, 90; - cannot exist in contact with Acetate of Silver, 116; - produces stains on cloth, 215; - mode of determining the strength of Commercial Nitric Acid, 186; - table of strength of Nitric Acid of different densities, 378. - Nitro-Sulphuric Acid, explained, 77; - process for making by mixed acids, 186; - by Oil of Vitriol and Nitre, 190; - should not be used cold, 83. - Nomenclature, chemical, 315. - Norris, Dr., his dry Collodion process, 298. - Notation, chemical, 318. - - Organic bodies, chemistry of, 324. - Oxide of Silver, preparation and properties, 17; - dissolves in the Nitrate Bath, rendering it alkaline, 88; - properties of its solution in Ammonia, 362; - preparation of ditto, 247; - its solution in Nitrate of Ammonia used in Photography, 249. - Oxymel, keeping process, 291; - preparation of Oxymel, 360. - Paper, Photographic, selection of, 240; - peculiarity of English papers, 241. - Paper, sensitive, for printing. _See_ Sensitive Paper. - Perchloride of Iron, preparation of toning Bath with, 160. - Permanence of Positives, mode of testing, 169. - Photographic image, chemical composition of, 140; - action of destructive 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 construction, 59; - rules for their use, 227; - mode of finding chemical focus, 229. - Portraits, drapery for, 66; - directions for taking, 220; - the position of the Camera, and other points of importance, 228; - the time of exposure, 221. - Positive printing, on Albuminized paper, formulæ for, 241; - on plain paper, formulæ for, 245; - on Ammonio-Nitrate paper, formula for, 246, 258; - by development, formulæ for, 259; - manipulatory details of printing, fixing, toning, washing, and - mounting, 251; - process of toning by Sel d'or, theory of, 134; - practice of, 267; - reasons for the want of correspondence between different formulæ, 257; - use of Chloride of Gold in toning, 132, 271; - theory of the preparation of the sensitive paper for Positives, 122; - theory of the process of fixing, 129; - of toning by Gold, 132; - the Author's researches, 140; - rationale of the printing process, 120; - composition of the image, 140; - fading of Positive prints, 160; - destructive 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; - comparative permanency of prints, 166; - mode of testing permanency, 169. - Positives, definition of, 106; - Collodion Positives, theory of production of, 108; - formulæ for solutions for, 201; - development of, 111, 221; - Collodion and Nitrate Bath best adapted for, 109; - mode of whitening by Bichloride of Mercury, 112; - solution for whitening, 207; - mode of backing up, 226; - spots and markings on, 284; - mode of printing Positives on Collodion, 272. - Positives, enlarged, mode of printing, 272. - Practice of Collodion process, 183. - Preservative processes for Collodion plates, 289. - Printing, Photographic, theory of, 120; - practical details of, 240. - Prism, refraction of light by, 51; - diagram of formation of spectrum 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; - sometimes requires the addition of Nitrate of Silver, 206. - Protosulphate of Iron, its preparation and properties, 29; - its characteristics as a developer for Collodion Positives, 111; - not well adapted for developing Collodion Negatives, 117; - preparation 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 Sulphate of Iron for developing Negatives, 117, 144; - mode of obviating the brown discoloration of developing solutions, 212. - Pyroxyline, its nature and properties, 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 drying, 191; - the glutinous variety produced by cold acids, 83; - recapitulation of the effects of varying the strength of the acid - mixture, 193; - spontaneous decomposition of Pyroxyline, 166. - - Reduction of metallic oxides by developers, theory of, 26; - of Silver salts by developers, theory of, 30; - practical details of reducing Silver compounds to metallic state, 372. - - Salts nature of, 310. - Salts of Silver, their preparation and properties, 12; - their Photographic action, 18; - theory of their reduction by developer, 30; - directions for obtaining metal from, 372. - Sel d'or, toning process by, its theory, 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 preparation of, 22, 122; - its darkening by light described, 123; - preparation of Albuminized paper, 241; - of plain paper, 245; - of Ammonio-Nitrate paper, 246, 258; - of paper for Negative processes, 259; - causes which affect the sensitiveness of Positive paper, 123; - which alter the colour of the image, 126; - spots and markings 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; - estimation 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. - Spherical aberration, 56. - Spirits of Wine, preparation and properties, 330; - not always sufficiently 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; - theory 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 preparation and properties, 15; - decomposed by fixing agents, 141. - Suboxide of Silver, its properties, 18; - forms compounds with organic matters. Citric Acid, Albumen, 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 printing process, 262; - preparation of Serum of Milk for, 355. - Symbols, use of, 318. - Syruped Collodion film, 289. - - Talbot, Mr., his discoveries, 9; - theory of Calotype process, 176. - Taupenot, M., his Collodio-Albumen process, 181; - practical details of, 294. - Temperature, its effect upon development of Collodion film, 102; - upon fogging, 105; - upon keeping Collodion, 210; - upon action of fixing Bath for paper Positives, 130; - upon Hypo toning Bath, 136. - Test-papers, use of, 376. - Toning Bath for Positives, with Sel d'or, 134, 267; - with Hyposulphite and Gold, _see_ Fixing 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 Photographs, 226. - View Lenses, directions for using, 230. - Vocabulary of Photographic chemicals, 327. - - Washing Positive prints, rules for, 162; - details of, 255. - Waxed paper process, theory of, 178. - Weights and Measures, table of, 379. - - -PRINTED BY JOHN EDWARD TAYLOR, LITTLE QUEEN STREET, LINCOLN'S INN FIELDS. - - - * * * * * - - -Transcriber Note - -Minor typos have been corrected. 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