HANDICRAFT SERIES. A Series of Practical Manuals. Edited by PAUL N. HASLUCK, Editor of “Work." Price 50cts. each, post paid. House Decoration. Comprising Whitewashing, Paperhanging, Painting, etc. With 79 Ehigravings and Diagrams. Contents.— Colour and Paints. Pigments, Oils, Driers, Varnishes, etc. Tools used by Painters. How to Mix Oil Paints. Distemper or Tempera Painting, Whitewashing and Decorating a Ceiling. Painting a Room. Papering a Room. Embellishment of Walls and Ceilings. Boot Making; and Mending. Including Repairing, Lasting, and Finishing. With 179 Engravings and Diagrams. Contents . — Repairing Heels and Half-Soling. Patching Boots and Shoes. Re-Welting and Re-Soling. Boot IVjlaking. Lasting the Up^er. Sewing and Stitching. Making the Heel. Knifing and Finishing. Making Riveted Buots and Shoes. How to Write Signs, Tickets, and Posters. With 170 Engravings and Diagrams. Contents . — The Formation of Letters, Stops, and Numerals. The Sign- writer’s Outfit. Making Signboards and Laying Ground Colours. The Simpler Forms of Lettering. Shaded and Fancy Lettering. Painting a Signboard. Ticket-Writing. Poster- Painting. Lettering with Gold, etc. Wood Finishing. Comprising Staining, Varnishing, and Polishing. With Engravings and Diagrams. Contents.— Vrocossos of Finishing Wood. Processes of Staining Wood. French Polishing. Fillers for Wood and Filling In. Bodying In and Spiriting Off. Glazing and Wax Finishing. Oil Polishing and Dry Shining. Re-poli.shing and Reviving. Hard Stopping or Beaumontage. Treatment of Floors Stains. Processes of Varnishing Wood Varnishes. Re-polishing Shop Fronts. Dyna.mos and Electric Motors. With 142 Engravings and Diagrams Contents. — Introduction. Siemens Dynamo. Gramme Dynamo. Manchester Dynamo. Simplex Dynamo, Calculating the Size and Amount of Wire for Small Dynamos. Ailments of Small Dynamo Electric Machines : their Causes and Cures. Small Electro-motors without Castings. ■ How to Determine the Direction of Rotation of a Motor. How to Make a Shuttle-Armature Motor. Undertype 50-Watt Dynamo. Manchester Type 440-Watt D3mamo. Cycle Building and Repairing. With 142 Engravings and Diagrams. Contents. — Introductory, and Tools Used. How to Build a Front Driver, Building a Rear-driving Safety. Building Tandem Safeties. Building Front- driver I'ricycle. Building a Hand Tricycle. Brazing. How to Make and Fit Gear Cases. Fittings and Accessories. Wheel Making. Tyres and Methods of Fixing them. Enamelling. Repairing. Decorative Designs of All Ages for All Purposes. With 277 Engravings and Diagrams. Contents. — Savage Ornament, Egyptian Ornament. Assyrian Ornament Greek Ornament. Roman Ornament. Early Christian Ornament. Arabic Ornament. Celtic and Scandinavian Ornaments. Mediaeval Ornament. Renascence and Modern Ornaments. Chinese Ornament. Persian Ornament. Indian Ornament. Japanese Ornament. Mounting a.nd Framing Pictures. With 240 Engravings, etc. Contents. — Making Picture Frames, Notes on Art Frames. Picture Frame Cramps. Making Oxford Frames. Gilding Picture Frames. Methods of Mounting Pictures. Making Photograph Frames. Frames covered with Plush and Cork. Hanging and Packing Pictures. Smiths’ Work. With 21 1 Engravings and Diagrams. Contents. — Forges and Appliances. Hand Tools. Drawing Down and Up- setting Welding and Punching. Conditions of Work : Principles of Forma- tion. lieading and Ring Making. Miscellaneous Examples of Forged Work. Cranks, Model Work, and Die Forging. Home-made Forges. The Manipula- tion of Steel at the Forge. (Continued on next page.) DAVID McKAY, Publisher. 604-608 South Washington Square, Philadelphia. HANDICRAFT SERIES (continued). Glass Working by Heat and Abrasion. With 300 Engravings and Diagrams. Contents. — Appliances used in Glass Blowing. Manipulating Glass Tubing. Blowing Bulbs and Flasks. Jointing Tubes to Bulbs forming Thistle Funnels, etc. Blowing and Etching Glass Fancy Articles ; Embossing and Gilding Flat Surfaces. Utilising Broken Glass Apparatus ; Boring Holes in, and Riveting Glass. Hand-working of Telescope Specula. Turning, Chipping, and Grinding Glass. The Manufacture of Glass. Building Model Boats. With 168 Engravings and Diagrams. Contents. — Building Model Yachts. Rigging^ and Sailing Model Yachts. Making and Fitting Simple Model Boats. Building a Model Atlantic Liner. Vertical Engine for a Model Launch. Model Launch Engine with Reversing Gear. Making a Show Case for a Model Boat. Electric Bells, How to Make and Fit Them. With 162 En- gravings and Diagrams. Contents. — The Electr c Current and the Laws that Govern it. Current Conductors used in Electric-Bell Work. Wiring for Electric Bells. Elaborated Systems of Wiring ; Burglar Alarms. Batteries for Electric Bells. The Con- struction of Electric Bells, Pushes, and Switches. Indicators for Electric-Bell Systems. Bamboo Work. With 177 Engravings and Diagrams. Contents. — Bamboo ; Its Sources and Uses. How to Work Bamboo. Bamboo Tables. Bamboo Chairs and Seats. Bamboo Bedroom Furniture. Bamboo Hall Racks and Stands. Bamboo Music Racks. Bamboo Cabinets and Book- cases. Bambco Window Blinds. Miscellaneous Articles of Bamboo. Bamboo Mail Cart. Taxidermy. With 108 Engravings and Diagrams. Skinning Birds. Stuffing and Mounting Birds. _ Skinning and Stuffing Mammals. Mounting Animals’ Horned Heads : Polishing and Mount- ing Horns. Skinning, Stuffing, and Casting Fish. Pieserving, Cleaning, and Dyeing Skins. Preserving Insects, and Birds’ Eggs. Cases for Mounting Specimens. Tailoring:. With 180 Engravings and Diagrams. Contents. — Tailors’ Requisites and Methods of Stitching. Simple Repairs and Pressing. Relining, Repocketing, and Recollaring. How to Cut and Make Trousers. How to Cut and Make Vests. Cutting and Making Lounge and Reefer Jackets. Cutting and Making Morning and Frock Coats. Photog:raphic Cameras and Accessories. Comprising How to Make Cameras, Dark Slides, Shutters, and Stands. With 160 Illustrations. Contents. — Photographic Lenses and How to Test them. Modern Half-plate Cameras. Hand and Pocket Cameras. Ferrotype Cameras. Stereoscopic Cameras. Enlarging Cameras. Dark Slides. Cinematograph Management. Optical Lanterns. Comprising The Construction and Management OF Optical Lanterns and the Making of Slides. With 160 Illustrations. Contents. — Single Lanterns. Dissolving View Lanterns. Illuminant for Optical Lanterns. Optical^ Lantern Acces.sories. Conducting a Limelight Lantern Exhibition. Experiments with Optical Lanterns. Painting Lantern Slides. Photographic Lantern Slides. Mechanical Lantern Slides. Cinemato- graph Management. Eng:raving‘ Metals. With Numerous Illustrations. Contents. — Introduction and Terms used, £ngraver^’ Tools and their Uses. Elementary Exercises in Engraving. Engraving Plate and Precious Metals. Engraving Monograms. Transfer Processes of Engraving Metals. Engraving Name Plates. Engraving Coffin Plates. Engraving Steel Plates. Chasing and Embossing Metals. Etching Metals. Basket Work. With 189 Illustrations. Contents. — Tools and Materials. Simple Baskets. Grocer’s Square Baskets. Rouna Baskets. Oval Baskets. Flat Fruit Baskets. Wicker Elbow Chairs. Baske‘ Oottle-casings. Doctors’ and Chemists’ Baskets. Fancy Basket Work. Sussex frug Basket. Miscellaneous Basket Work. Index DAVID McKAY, Publisher, 604-608 South Washington Square, Philadelphia. HANDICRAFT SERIES (continued). Bookbinding . With 125 Engravings and Diagrams. Contents. — Bookbinders* Appliances. Folding Printed Book Sheets. Beat- ing and Sewing. Rounding, Backing, and Cover Cutting. Cutting Book Edges. Covering Books. Cloth-bound Books, Pamphlets, etc. Account Books, Ledgers, etc. Coloring, Sprinkling, and Marbling Book Edges. Marbling Book Papers. Gilding Book Edges. Sprinkling and Tree Marbling Book Covers. Lettering, Gilding, and Finishing Book Covers. Index. Bent Iron Work. Including Elementary Art Metal Work. With 269 Engravings and Diagrams. Contents . — Tools and Materials. Bending and Working Strip Iron. Simple Exercises in Bent Iron. Floral Ornaments for Bent Iron Work. Candlesticks. Hall Lanterns. Screens, Grilles, etc. Table Lamps. Suspended Lamps and Flower Bowls. Photograph Frames. Newspaper Rack. Floor Lamps. Miscellaneous Examples. Index. Photography. With 70 Engravings and Diagrams. Contents . — The Camera and its Accessories. The Studio and Darkroom. Plates. Exposure. Developing and Fixing Negatives. Intensification and Reduction of Negatives. Portraiture and Picture Composition. Flashlight Photography. Retouching Negatives. Processes of Printing from Negatives. Mounting and Finishing Prints. Copying and Enlarging. Stereoscopic Photography. Ferrotype Photography. Index. Upholstery. With 162 Engravings and Diagrams. Contents. — -Upholsterers’ Materials. Upholsterers’ Tools and Appliances. Webbing, Springing, Stuffing, and Tufting. Making Seat Cushions and Squabs. Upholstering an Easy Chair. Upholstering Couches and Sofas. Upholstering Footstools, Fenderettes, etc. Miscellaneous Upholstery. Mattress Making and Repairing. Fancy Upholstery. Renovating and Repairing Upholstered Furniture. Planning and Laying Carpets and Linoleum. Index. Leather Working. With 152 Engravings and Diagrams. Contents . — Qualities and Varieties of Leather. Strap Cutting and Making. Letter Cases and Writing Pads. Hair Brush and Collar Cases. Hat Cases. Banjo and Mandoline Cases. Bags. Portmanteaux and Travelling Trunks. Knapsacks and Satchels. Leather Ornamentation. Footballs. Dyeing Leather. Miscellaneous Examples of Leather Work. Index. Harness Making. With 197 Engravings and Diagrams. Contents . — Harness Makers’ Tools. Harness Makers’ Materials. Simple Exercises in Stitching. Looping. Cart Harness. Cart Collars. Cart Saddles. 1 ^'ore Gear and Leader Harness. Plough Harness. Bits, Spurs, Stirrups, and Harness Furniture. Van and Cab Harness. Index. Sad cilery. With 99 Engravings and Diagrams. Contents. — Gentleman’s Riding Saddle. Panel for Gentleman’s Saddle Ladies’ Side Saddles. Children’s Saddles or Pilches. Saddle Cruppers, Breast- plates, and other Accessories. Riding Bridles. Breaking-down Tackle Head Collars. Horse Clothing. Knee-caps and Miscellaneous Articles. Repairing Harness and Saddlery. Re-lining Collars and Saddles. Riding and Driving Whips. Superior Set of Gig Harness. Index. Knotting and Splicing, Ropes and Cordage. With 208 Engravings and Diagrams. Contents.— Introduction. Rope Formation. Simple and Useful Knots. Eye Knots, Hitches and Bends. Ring Knots and Rope Shortenings, 'lies and Lashings. Fancy Knots. Rope Splicing. Working Coroage. Ham- mock Making. Lashings and Ties for Scaffolding. Splicing and Socketing Wire Ropes. Index. Beehives and Beekeepers’ Appliances. With 155 Engravings and Diagrams. —Introduction. A Bar-Frame Beehive. Temporary Beehive. Tiering Bar-Frame Beehive. The “ W. B. C.” Beehive. Furnishing and Stocking a Beehive. Observatory Beehive for Permanent Use, Observatory Beehive for Temporary Use. Inspection Case for Beehives. Hive for Rear- ing Queen Bees. Super-Clearers, Bee Smoker. Honey Extractors. Wax Extractor.^- Beekeepers' Miscellaneous Appliances. Index. DAVID McKAY, Publisher, 604-608 South Washington Square. Philadelphia. PHOTOGRAPHIC CHEMISTRY With Numerous Engravings and Diagrams EDITED BY PAUL N. HASLUGK PHILADELPHIA DAVID McKAY, Publisher 610 South Washington Square 1916 \ i I HHE GETTY RESEARCH tj, s PUBLISHERS’ NOTE This short treatise on Photographic Chemistry is issued in the confident belief that it is not only thoroughly practical and reliable, but is so simply worded that even inexperienced readers can understand it. Should anyone, however, encounter unexpected difficulty, he has only to address a question to the Editor of Work, La Belle Sauvage, London, E.C., and his query will be answered in the columns of that journal. RESEARCH LIBRARY THE GETTY RESEARCH INSTITUTE OHN MOORE ANDREAS COLOR CHEMISTRY LIBRARY EOUNDATION CONTENTS chapter page I. — Introductory : Relation of Chemistry to Photography 9 II. — Some Fundamental Chemical Laws . , 24 III. — Meaning of Symbols and Equations . . 35 IV. — Water, its Properties and Impurities . . 44 V. — Oxj^gen and Hydrogen Photographically Considered 48 VI. — Theories Concerning the Latent Image . 56 VII. — Chemistry of Development, Toning, In- tensification, etc. 68 VIII. — Nitrogen Compounds Employed in Photo- graphy . 89 IX. — The Halogens and Haloid Salts . . .103 X. — Sulphur and its Compounds . . .114 XI. — Metals, Alkali Metals, etc 122 XII.— Organic or Carbon Compounds Used in Photography 134 XIII. — Pyroxyline, Albumen, Gelatine, etc. . . 143 XIV. — Benzene and the Organic Developers. . 148 Index 157 LIST OF ILLUSTRATIONS Fig. page 1. — Apparatus for Experiment to Prove Indestructibility of Matter 12 2. — Heating Glass Tubing Previous to Bending , , .13 3. — Cork Borers 14 4. — Sharpener for Cork Borers 14 5. — Method of Folding Filter Paper 15 6— Arrangement of Apparatus for Filtration .... 15 7. — Wash Bottle 16 8. — Method of Filtering Acids 16 9. — Evaporating Basin 16 10. — Bunsen Burners 17 11. — Bending Wire for Tripod 17 12. — Apparatus for Evaporation 17 13. — Water Bath with Copper Funnel-holder . . . .18 14. — Liebig’s Condenser 19 15. — Apparatus for Purifying Methylated Spirit . . .20 16. — Apparatus for Sublimation Experiment . . . .22 17. — The Process of Electrolysis 32 18. — Glass Heating Flask . 48 19. — Wide-necked Glass Flask 48 20. — Thistle Funnel . . 49 21. — Stoppered Retort 49 22. — Test Tube and Holder 49 23. — Method of Preparing Oxygen ; 50 24. — Preparation of Hydrogen 53 25. — Diagram Explaining Molecuiar Strain Hypothesis . . 64 26. — Acid Development 74 27. — Alkaline Development 74 28. — Preparation of Nitric Acid . 92 29. — Preparation of Ammonia 95 30. — Preparation of Chlorine 104 31. — Experiment Showing Action of Light on Silver Chloride 106 PHOTOGRAPHIC CHEMISTRY CHAPTER I. INTRODUCTION : RELATION OF CHEMISTRY TO PHOTOGRAPHY. Preliminary . — Photography is essentially a branch of practical chemistry. The materials used and the changes brought about by their agency are all subject to chemical laws. Of course, chemistry does not pretend to explain all the changes taking place in photographic operations, because many of them are still matters for scientifie investiga- tion, such as, for example, the exact composition of the compound produced by exposing silver chloride to the action of light. In this handbook it is intended to give such information as will assist the photographer in gaining a knowledge of the chemical composition and properties of his materials, and of the probable changes brought about by their employment in his profession. Aim of Chemistry . — The photographer is con- scious of innumerable changes continually taking place in his materials and reagents. Developing solutions gradually turn brown ; around the top of the ammonia bottle a white crystalline mass gradually forms; and if the supply of sodium sulphite is exposed to the air for a considerable time, it will lose its transparency and be converted into a white opaque mass, no longer suitable for photographic operations. These incidents, and many others of a similar nature, are of frequent 10 PHOTOGRAPHIC CHEMISTRY. occurrence. The aim of science is to investigate these innumerable transformations and to en- deavour to explain them. As can readily be understood, the domain of science is so vast that, for the sake of convenience, it is divided into cer- tain branches. In this manner have arisen the sciences of chemistry, physics, electricity, etc. Most of these sciences gradually merge into one another, so that it is difficult in some cases to say definitely where one science begins and another ends. This is especially so in the case of physics and chemistry, and it is important to have a very clear idea of the distinction between these two sciences. Distinction between Chemistry and Physics . — If a piece of glass rod is rubbed with a dry cloth it is found that the glass has acquired a remarkable property of attracting light articles, such as pieces of paper. A piece of iron sus- pended vertically for some time gradually ac- quires the property of attracting small pieces of iron, becoming, in fact, a magnet. In both cases the glass and iron have undergone no perceptible change; the glass is still glass and the iron is iron. Again, heat converts water into steam, and on subsequent cooling this passes back again to water ; if it is submitted to a lower temperature it is converted to ice. But during these transi- tions from the gaseous, liquid, and solid condi- tions the matter composing the steam, water, and ice has undergone no change. All such changes in the condition of bodies, unaccompanied by any real alteration in substance, are spoken of as physical phenomena, and their consideration and study belong to the science of physics. It is well known that a piece of bright iron soon tarnishes in a moist atmosphere, and gradually becomes covered with brown scales, a change we term rust- ing. On examination, this iron rust is found to be entirely different from the original bright iron. INTRODtJCTOIlY. 11 and to convert it back again to its former state would be a lengthy chemical process. If some finely divided copper is mixed with powdered sul- phur a greenish-grey powder is produced. If this powder is examined under the microscope the red particles of copper can easily be distinguished from the yellow of the sulphur. If the powder is shaken with water, the sulphur, owing to its lightness, is easily washed away, leaving the heavier particles of copper behind. Or by treat- ing the mixture wdth carbon disulphide, a liquid in which the sulphur is easily soluble, its separa- tion can be readily effected. If, however, this mixture is heated in a test tube, it commences to glow, and on cooling a black mass remains. This black mass, on examination, is found to differ in all respects from the original copper and sulphur. Under the strongest microscope obtainable, not a particle of copper or sulphur can be distin- guished. Washing with Vvater or treatment with carbon disulphide will not effect in any way the separation of the ingredients. Evidently a much deeper change has taken place here than in the case of the physical changes. Such occurrences, therefore, as in the rusting of iron, and the heat- ing of copper and sulphur together, whereby a complete and entire alteration takes place, belong to the science of chemistry. The province of chemistry, then, is the consideration of changes in composition, of a comparatively permanent character, which substances undergo under differ- ent conditions. Special Application to Fhotography . — It is only necessary to consider the foregoing statement for a short time in order to see how intimately the science of chemistry is bound up with that of photography. Consider the numerous changes brought about by the photographer, such as those of development, intensifying, fixing, toning, etc. He is really bringing about a series of chemical 12 PHOTOGRAPHIC CHEMISTRY. reactions, and it is evident that a proper under- standing of these operations can only be arrived at by a knowledge of chemistry. A photographer who understands the why and the wherefore of the presence of each compound in his developer, or toning bath, is evidently better equipped than the person who is lacking in such knowledge and works by mere rule of thumb. In fact, it may Fig. 1. — Apparatus for Experiment to Prove Indestructi- bility of Matter. be said that not only is chemistry necessary for the photographer, but the future advancement of photography will depend upon the proper recog- nition and application of scientific principles. Indestructihility of Matter . — During many chemical operations the material entering into the reaction is, apparently, destroyed. For instance, the night-light or candle, in the ruby lamp, after being ignited, gradually burns away. Has the INTRODUCTORY. 13 matter composing the candle been destroyed during the combustion 1 The answer is in the negative. This statement can be experimentally verified in the following manner An ordinary lamp chimney is procured, and into the bottom is inserted a perforated cork to which a piece of candle is attached. At the top of the chimney is a loose plug of wire gauze containing a few pieces of quicklime and caustic soda. The lamp chimney, with its contents, is Fig. 2. — Heating Glass Tubing Previous to Bending. then attached to one arm of a balance, as in Fig. 1, and carefully counterpoised. The candle is then withdrawn and ignited and is reintro- duced into the chimney. As the candle burns and decreases in size the products of combustion are absorbed by the quick- lime, and the arm of the balance to which the chimney has been suspended gradually descends, thus showing an increase in weight, and not a decrease, as would be the case if the material of the candle were destroyed during combustion. From this experiment, and others of a similar nature, it is proved that matter is indestructible. 14 PHOTOGRAPHIC CHEMISTRY. The methods described below will be useful for making the simple apparatus required in various i experiments illustrating the elementary facts and c principles of chemistry. 1 Bending Glass Tubing . — To do this satisfac- f torily, it is essential that a sufficient length of J tubing should be heated before attempting to bend it. This is readily attained by the use of an ordinary fish-tail burner. The tube is introduced Fig. 3. — Cork Borers. into the top part of the flame, at the place where the bend is desired, and slowly rotated (see Fig. 2). As soon as the requisite pliability has been attained, it is removed from the flame and bent | to the desired angle. | Boring a Cork . — The end of the glass tubing I is gently pressed upon the surface of the cork, so as to leave a slight, but decided, impression. A cork borer is then chosen which just fits this impression ; this is slowly forced into the cork and turned in the opposite direction to that in which ! the cork is being slowly rotated. Fig. 3 shows > a set of cork borers of different sizes, and Fig. 4 I Fig. 4. — Sharpener for Cork Borers. ! I a sharpener for the same. A hole may also be j made through a cork by piercing it with a red- hot needle and then completing the operation with a rat-tail file. Filtration . — The operation of filtration con- sists of the removal of suspended matter from a solution by means of a porous substance such aa blotting paper, flannel, cotton, etc. This is a very important operation to the photographer, as ! INTRODUCTORY. 15 many photographic processes depend for their suc- cess upon the complete removal of solid particles. Developing and “ hypo ” solutions should in all cases be filtered if they contain suspended matter. A piece of blotting paper or other absorbent material is cut into a circle ; this is then folded as indicated in Fig. 5, which shows the three stages. One side is now opened and introduced into the funnel, the filtering apparatus being ar- ranged as illustrated by Fig. 6. The filtered liquid is termed the filtrate. If the residue on the filter paper has to be kept, it is of the greatest Fig-. 6. — Arrangement of Apparatus for Filtration. importance to see that it is thoroughly washed from any adhering filtrate. This washing is con- veniently performed by the aid of a wash bottle (Fig. 7). A wash bottle is constructed in the following manner. A flask, or a bottle with a 16 PHOTOGRAPHIC CHEMISTRY. fairly wide neck, is taken and is fitted with a cork, bored with two holes. Two pieces of glass tubing are then bent as shown, and introduced through the cork. A glass jet is then made by heating a Fig. 8. — Method of Filtering Acids. piece of glass tubing and drawing it out as far as it will go, and connected with the tube reaching to the bottom of the bottle or flask by means of a piece of indiarubber tubing. The filtration of acids or other corrosive liquids can be effected by replacing the filter paper Fig. 9. — Evaporating Basin. by pieces of broken glass or asbestos (see Fig. 8). A glass marble is first introduced into the funnel, and then a few pieces of broken glass are added to a depth of about ^ in INTRODUCTORY. 17 Evaporation . — The operation of evaporation is best carried out in porcelain basins (see Fig. 9), Fig. 10. — Bunsen Burners. which can be bought for a very small outlay. A spirit lamp may be utilised as the source of heat, Fig. 11. — Bending Wire for Tripod. Fig. 12. — Apparatus for Evaporation. care being taken to see that the flame is exactly under the centre of the basin. A small Bunsen B 18 PHOTOGRAPHIC CHEMISTRY. burner (Fig. 10), attached by rubber tubing to a gas supply, is, however, to be preferred, on account of the greater ease with which the heat may be regulated. A support for the basin can readily be made in the following manner. Three pieces of stout iron wire are taken and bent as shown by Fig. 11. The six legs are then bound together with wire so as to form a support for the basin (see Fig. 12). As some compounds, such as gold chloride, readily decompose when evapor- ated over the naked flame, it is better to make Fig. 13. — Water Bath with Copper Funnel-holde’’ use of a water bath. A tin mug or small sauce- pan slightly smaller than the diameter of the basin can be utilised for this purpose. An im- provement on this is the apparatus shown by Fig. 13, which is useful for many photographic purposes. As will be seen, besides its use for evaporation, it will keep solutions warm while filtering, as required in emulsion making, etc. Distillation . — Many volatile substances, such as water, alcohol, benzene, acetone, etc., may be puri- fied by distillation. The liquid is boiled in a glass, or other vessel, and the vapour is con- densed by conducting it into a tube surrounded INTRODUCTORY. 19 by water and known as a condenser. Many forms of apparatus can be used for effecting distillation, a few of which will now be described. A glass retort or flask is a very convenient vessel for boil- ing the liquid in, but more common articles, such 2 to as a tin can or kettle, would in some cases supply their place equally well. In order to condense the vapour given off from the boiler, it is made to pass through a coil of pipe arranged in a bucket of cold water. A very compact apparatus for 20 PHOTOGRAPHIC CHEMISTRY. bringing about condensation is a Liebig’s con- denser. This is essentially a glass tube, sur- rounded by an outer one, which is provided with two small tubes for the inflow and outflow of water (see Fig. 14). If possible, it is better to Ph a: 0) be c3 P. <1 I ,bo have the whole apparatus, boiler, and condenser of glass, as these are then more easily kept in a state of cleanliness. Bistillation of Water . — Ordinary tap water is unsatisfactory as a chemical solvent, since it INTRODUCTORY. 21 invariably contains many impurities, which are harmful from a photographic point of view. The soluble chlorides present react with silver nitrate to form a white insoluble precipitate of silver chloride. The calcium and magnesium salts are precipitated as insoluble oxalates in the presence of any oxalate. These impurities are readily re- moved by submitting the w'ater to distillation. The first pint or so distilling over should be re- jected, as this contains practically all the am- monia. By simply boiling the tap water this ammonia is removed and most of the magnesium and calcium is precipitated as insoluble carbonate. Purification of Methylated Sijirit. — Commer- cial wood spirit is, as a rule, a very complex mixture, consisting of ethyl and methyl alcohols, acetic acid, resin, and water, etc. Impure wood spirit should not be used for washing plates, as the water on the wet plate precipitates the resin from the spirit, which is then deposited on the film. It may be roughly purified in the following manner. It is first of all introduced into a large flask or retort, which is then connected to the con- densing apparatus. The flask or retort is then im- mersed in a water bath — on no account should the spirit be heated, over a naked flame. A large saucepan may be utilised for this purpose, and the spirit distilled (see Fig. 15). To the dis- tillate, that is, the liquid distilling over, is added a few lumps of quicklime, broken into small pieces the size of a walnut, and the whole allowed to stand for some time, being well shaken at inter- vals. The spirit is next decanted off from the quicklime into the distilling flask (any residue from the first distillation being removed) and again distilled. This purified spirit is then suit- able for all photographic purposes, such as varnish making, the drying of plates, etc. Acetone may be purified by a similar method to that described for methylated spirit. Wet photographic plates teOTOGRAriilC CHEMiSTHY. are very quickly dried, by first allowing to drain, and then immersing in methylated spirit. These methylated-spirit washings may be dehydrated {i.e. freed from water) by treating with quick- lime, decanting, and then distilling. Sublimation . — Many solid substances, when heated, are more or less readily converted into vapour without undergoing decomposition, such as iodine, mercuric chloride (corrosive sublimate), ammonium chloride, etc. When the vapour comes Fig. 16. — Apparatus for Sublimation Experiment. in contact with a cold surface it is condensed and reappears in the solid state. As an example of sublimation, the reader can perform the fol- lowing experiment. A small basin is taken and covered with a glass funnel, the shank of which is very loosely packed with a piece of cotton-wool. A small quantity of iodine is introduced into the basin, which is then carefully heated over a small flame (see Fig. 16). The iodine is very soon con- verted into a dark purple vapour which condenses on the inside of the funnel in black lustrous crystals. INTRODUCTORY. 23 C rystallisation . — Many substances exhibit a tendency to crystallise — that is, to assume a regular geometric form. Other substances, such as gelatine, albumen, and the like, are devoid of any such symmetrical arrangement. A third group of substances, such as the starches, exhibit a distinct organised structure, as a rule of a globular nature, but in no sense could it be termed crystalline. Generally speaking, every substance has a definite crystalline form, though there are instances in which two distinct substances have the same cry- stalline shape ; such substances are termed “ iso- morphous.” In certain cases there is a substanee occurring in two crystalline forms; it is then said to be “ dimori3hous.” Crystallisation is im- portant from a photographic standpoint, as it enables a large number of soluble compounds, such as potassium dichromate, washing soda, etc., to be purified in a very simple manner. The sub- stance to be purified by crystallisation is dissolved in hot water so as to form a saturated solution, and filtered if necessary. The hot solution is then cooled as quickly as possible. The object of cool- ing quickly is to get a mass of very fine crystals, because it has been found by experience that small crystals are purer than those of a larger size, obtained by allowing the saturated solution to cool slowly. The supernatant liquor is then poured off from the crystalline mass, which is next washed with a little cold water and allowed to drain. CHAPTER II. SOME FUNDAMENTAL CHEMICAL LAWS. Chemical Theory . — It is advisable at this stage to consider a few definitions and some of the more important theories which attempt to account for chemical phenomena. This portion of the subject is not directly applicable to photography, but as it lies at the root of modern chemistry, and as it is the purpose of this handbook to impart a sim- ple chemical explanation of some of the more common photographic operations, it is of extreme importance that these principles should be thoroughly understood. Definition of an Element . — If a piece of chalk is taken and very carefully analysed, it is found to consist of three distinct substances, calcium (a metal), carbon, and oxygen. If each one of these three substances is taken and submitted to the action of the most powerful chemical reagents, to the most exhaustive operation of analysis, nothing is obtained from the calcium but calcium, from the carbon but carbon, and from the oxj^gen but oxygen. Each of these three substances, then, con- sists of only one kind of matter, and such sub- stances are termed “ elements.’’ Of course, as the science of chemistry advances, and methods of analysis become more powerful, it is quite likely that these so-called elements will be found to be of a complex nature. The number of elements known up to the present time is about seventy- eight. Metals and N on-metals . — If these elements are carefully examined they are found to differ in a very marked manner. In the first place they could be grouped as gases, liquids, and solids ; — SOME EXJXDAMENTAL CHEMICAL LAWS. 25 Gases. Liquids. Solids. Oxygen Mercury Iron Hydrogen Bromine Lead Chlorine Gold Nitrogen Sulphur Iodine Phosphorus, etc. On further examination the elements in the second and third columns in the above table are found to be very different in physical properties. Some are bright and shiny in appearance, and are good conductors of heat and electricity. Other ele- ments are dull in aspect, and are bad conductors of heat and electricity. Two groups of elements then are obtained : — BrirjTit appearance. Good conductor. Dxdl appearance. Beal conductor. Gold Copper Sulphur Iodine Lead Iron Bromine Phosphorus Silver Tin Silicon Boron If these two groups of elements are studied they are found to differ very considerably both in physical and chemical properties. For instance, the first group have the following general pro- perties : — (1) They have a peculiar brilliancy of surface which is knowm as metallic lustre. (2) As a rule, they are good conductors of heat and electricity. (3) They have the power of displacing hydro- gen from acids. 26 PHOTOGEAPHIC CHEMISTRY. These general properties are absent in the second group of elements. The elements in the first group are known as the metals; those in the other group are termed non-metals. Chemical Comiiounds. — When two or more elements unite together to form a new substance, having properties distinct from those of its con- stituent elements, this new body is termed a chemical compound. Water is a chemical compound, and contains the elements hydrogen and oxygen. By com- paring the properties of water with those of hydrogen and oxygen, they are seen to be quite different. The force which unites two or more elements together to form a chemical compound is termed chemical affinity or attraction. Law of Consta7it Froportion. — When chemical elements combine together so as to form a chemical compound, it is found that the proportions in which they combine is always a constant quantity. By making a quantitative analysis of salt it is found to contain 1 part of sodium to 1*54 parts of chlorine. No matter where or how the salt has been obtained — from the North Pole, the Equator, or any other part of the globe, provided it is pure — it always contains these two elements in this proportion, 1 of sodium to 1*54 chlorine. Salt has never been obtained containing either a less or greater quantity of its constituent elements. This has been found to hold true for every chemi- cal compound, whether procured artificially or naturall3^ This fact has been demonstrated so repeatedly by chemists that it is termed a law, and is known as the “ Law of Constant Propor- tion.” This is usually stated as follows : “ Every chemical compound contains the elements of which it is composed in one, and only one, proportion by weight.” Jjaw of Multiple Proportions. — Certain ele- ments combine together to form more than one Some i^undamental ohemical laws. 27 compound. Copper combines with chlorine to form two compounds — cuprous chloride and cupric chloride. Carbon combines with oxygen to form two oxides — carbon monoxide and carbon dioxide. The element nitrogen unites with oxygen to form five distinct oxides — nitrous oxide, nitric oxide, nitrogen trioxide, nitrogen peroxide, and nitro- gen pentoxide. By making a careful analysis and study of each of the above compounds, they are found to con- tain the following weights of each element : — 63 grs. of copper combine with 35 -5 grs. of chlorine. 63 „ 99 710 99 99 12 „ carbon 99 16 99 oxygen. 12 ,, 59 99 32 99 99 14 „ nitrogen 9 9 8 99 99 14 „ 99 16 99 14 „ 99 99 24 99 99 14 „ 99 99 32 99 99 14 „ 99 99 40 99 On examining these numbers it is seen that the relative proportion of : Chlorine combining with the same weight of cop- per is 1 : 2; Oxygen combining with the same weight of carbon is 1 ; 2; Oxygen combining with the same w^eight of nitro- gen is 1 : 2 : 3 : 4 : 5. Hence, when one element combines with another, in more than one proportion by weight, these proportions bear a ratio to one another; the higher proportions are always some simple mul- tiple of the lower proportions. This statement is known as the Law of Multiple Proportions,” and holds true for all elements which form more than one compound with another element. It is important to notice that these laws of constant proportion and multiple proportions are not theories ; they are the outcome of innumerable experiments. 28 PHOTOGRAPHIC CHEMISTRY. Atomic Theory. — The most satisfactory ex- planation, at the present time, of the fact that substances always combine in fixed and definite proportions by weight, or in some simple multiple of these proportions, is to be found in the atomic theory. This generally accepted theory makes the assumption that all elementary matter is com- posed of exceedingly minute particles, so small that they cannot be further subdivided. These indivisible particles are termed “ atoms.’’ Now, according to the atomic theory, when chemical combination takes place, it is due to the action of atom on atom, or groups of atoms on groups of atoms. That is, chemical action means the union or dissociation of these indivisible particles. Atomic Weight. — It follows that if matter is composed of atoms, these possess a definite weight. Also, the atoms of any one element must be sup- posed to have the same weight, while the atoms of different elements have different weights. Owing to the extremely minute character of the atom, so small as to be incapable of further subdivision, it is evident that their absolute weight cannot be determined. All that can be 'done is to obtain their relative weight in terms "of some standard. As the element hydrogen is the lightest known sub- stance, the atom of hydrogen is adopted as the standard of comparison. The number so obtained is termed the atomic weight of the element, and represents how many times heavier an atom of the element is than the atom of hydrogen. The methods employed for the determination of the atomic weights will not be discussed here. The photographer can obtain these from any standard work on chemistry. Explanation^ of the Lairs of Constamt Propor- tion and Multiple Proportion. — If it be granted that elementary matter is made up of atoms, hav- ing definite weights and incapable of further subdivision, it must follow that when two ele- SOME FUNDAMENTAL CHEMICAL LAWS. 29 merits combine, one atom to one atom, the com- bining proportions (representing the weights of the atoms) will always be a constant quantity. For example, an atom of an element A weighing 100 combines with one atom of an element B weigh- ing 200 to form a compound (A and B). Now as atoms are indivisible (atomic theory), it follows that no less quantity than 200 of B can combine with an atom of A. The combining proportions will always be 100 A : 200 B, or 1 : 2. It also follows that if an element combines in more than one proportion by weight with another element, that the higher proportions will always be some simple multiple of the atomic weight of the element or lowest proportion (Law of Multiple Proportions). For example, an element A having the atomic weight of 100, combines with B, another element whose atom weighs 200, to form a series of compounds. Now, according to the theory, the compounds would be : A + B A-I-2B A + 3B,etc. 1 atom 1 atom 1 atom -f 2 atoms 1 atom + 3 atoms 100 : 200 100 : 400 100 : 600, etc. Or the relative proportion of B combining with A would be 1:2:3, which is in agreement with the law of multiple proportions. The atomic theory may or may not be literally true, but it is the best guess that has been made to account for the fundamental laws of chemical action, and it is a very convenient means for interpreting the facts of chemistry. It is a splendid working hypothesis, and has contributed very largely to the advancement of the science. The Molecule . — By making very careful in- vestigations of certain phenomena of light, elec- tricity, of liquid films, and the conduct of gases under varying conditions, physicists have come to the conclusion that these phenomena can only be explained on the assumption that matter is made 30 PHOTOGEAPHTC CHEMISTEY. up of very minute particles, which are termed molecules. A molecule may be defined as the smallest particle of matter which can exist, as such, in a free state. For instance, if a small cube of silver nitrate were taken, it could be cut in half, this half again halved, and so on till a very minute quantity of silver nitrate was obtained. Let it be imagined that this small quantity of substance could be further subdivided and sub- divided, till at last such a minute particle would be obtained, that, if further subdivided, it would break down into its component atoms, silver, nitrogen, and oxygen. :This small particle of matter, representing the limit of subdivision, is termed a molecule. Of course, this is a purely theoretical consideration, as no person has ever seen a molecule. This molecule theory, like the atomic theory, is a good working hypothesis, and is in harmony with most of the observed facts. The molecules of the elements generally consist of two atoms ; the molecules of compounds may consist of any number of atoms, from two up- wards. The molecules of water consist of three atoms, two of hydrogen and one of oxygen. That of silver nitrate contains five atoms, one of silver, one of nitrogen, and three of oxygen. Many facts have to be taken into consideration before we arrive at the number of atoms a compound sub- stance contains, and for further elucidation of this point, the reader is referred to works on chemistry. MolecAilar Weight. — If the atomic weights of the elements, multipled by the number of atoms of each element present in a compound, are added together, the number so obtained is termed the molecular weight. Example : To find the molecular weight of silver nitrate. This substance contains one atom of silver, one atom of nitrogen, and three atoms of oxygen ; SOME FUNDAMENTAL CHEMICAL LAWS. 31 One atom of silver weighs 107*7 = 107*7 One atom of nitrogen weighs 14*0 = 14*0 Three atoms of oxygen weigh 16 x 3 = 48'0 Molecular weight of silver nitrate 1697 Ionic Theory —ThQVQ are many facts in chemis- try which do not receive their complete interpreta- tion by means of the atomic theory alone, and this is especially so in the case of many dilute solutions of metallic salts. The ionic or elec- trolytic dissociation theory of Svante Arrhenius (1887) offers an explanation of a number of ob- scure reactions. As the photographer is, for the most part, working with solutions during develop- ment, toning, fixing, intensifying, reduction, etc., in order to throw some light on these processes, it may not be out of place to give a brief outline of this ionic theory. Electrolysis . — If a rod of zinc is introduced into a solution containing dilute sulphuric acid, hydrogen is evolved and zinc sulphate goes into solution. If now a piece of platinum, a metal which is not attacked by the acid, is introduced into the same liquid, and is connected with the zinc by means of a piece of wire, hydrogen now makes its appearance on the platinum, bubbles of which rise to the surface, the zinc at the same time slowly dissolving without any evolution of gas. Such a combination as described is termed a voltaic cell. A collection of voltaic cells, con- nected together, the platinum to the zinc, zinc to platinum, and so on, is termed a battery. If the last platinum and zinc plates of the battery are connected together with pieces of wire, this wire exhibits electrical properties. From this it is evident that when zinc dissolves in dilute acid in the presence of the platinum, electricity is generated. If the two end wires of the battery are introduced into a solution of copper sulphate the compound undergoes decomposition. The 32 PHOTOGRAPHIC CHEMISTRY. copper is deposited on the wire from the zinc end of the battery, and sulphuric acid and oxygen make their appearance at the wire from the platinum end (see Fig. 17). Such decompositions brought about by the aid of electricity are termed electrolytic decompositions, and the process is spoken of as one of electrolysis. Definition of Terms . — The wire from the zinc is charged with negative electricity and is termed the negative electrode or cathode, and the wire from the platinum is charged with positive elec- tricity, and is termed the positive electrode or anode. The anode is usually represented by the sign + and the cathode by — . The substance undergoing electrolysis is termed an electrolyte. The electrolysis of copper sulphate results in the separation of molecules of copper and molecules of SO 4 , which are deposited at the cathode and anode respectively. These wandering molecules are termed ions. Those making their appearance at the anode are termed anions, and those at the cathode cations. All the metals, with hydrogen. SOME FUNDAMENTAL CHEMICAL LAWS. 33 are cations, and the non-metals anions. The elec- trolysis of copper sulphate might be represented as follows : — Cation Anion + ' =Cu i> SO4 2SO, X 2H2O = H,SO, + O, The ion SO4 is very unstable, and is decomposed by water as indicated above. According to the ionic theory electrolytes do not exist as such in aqueous solution — that is, they undergo ionisation, and break down at once into ions, the anions being charged with negative and the cations with posi- tive electricity. Consequently, in a solution of silver nitrate, instead of having molecules of AgNOg, cations of silver and anions of NO3 are produced. The neutral salts, such as NaCl, KI, NH^Br, AgNOg, etc., are those which undergo ionisation most strongly. The action of silver nitrate on a solution of potassium bromide would be represented on the ionisation theory in the fol- lowing way : — + — -f- — + — Ag + NO3 + K + Br = AgBr + K + NO3 Ions of Ions of Silver bromide Ions of silver and + potassium = non-ionised + potassium NO3 and because it and NO3 bromine. is insoluble in water. Beactions of the Ions , — The reactions used for che detection of substances, according to the ionic theory, depend chiefly on the reactions of the ions. For instance, all those compounds which in aque- ous solution produce the cation Fe (ic) react with ammonia to produce a brown flocculent precipitate of ferric hydrate. If iron is present as a complex ion {i.e. with other elements) this characteristic 0 34 PHOTOGRAPHIC CHEMISTRY. reaction with ammonia is not obtained. For instance, potassium ferrocyanide K4Fe(CN)6, though it contains an atom of iron in the molecule, does not produce a brown precipitate on the ad- dition of ammonia. This is due to the fact that a solution of K^FeCCN)^ undergoes ionisation, forming potassium ions and complex ions of Fc(CISr)r,. It does not produce free ions of Fe. + ”}■ + "t* — K,Fe(CN)e + K -}- K + K - - - Fe(CN)e Similarly with potassium dichromate. This com- pound contains chromium, having the formula KoCroO^, but does not show the ordinary reactions for that metal. This is du3 to the fact that its solution in water results 'in the formation of potassium ions and complex ions of CrgOy. K.Cr^O, — — t>K + K Cr^O, The few points mentioned above will give the photographer some idea of this ionic theory, which of late years has come very much to the front. For a more detailed account reference must be made to some standard work on physical chemistry 35 CHAPTER III. MEANING OF SYMBOLS AND EQUATIONS. Chemical Symbols . — Instead of writing down each time the full name of every element, certain characters are used to represent it, and these characters are known as symbols. In most cases this is either the first or first two letters of the English or Latin name of the element. Thus the symbol for nitrogen is N, for oxygen O. For potassium it is K, being the initial letter of the Latin for potassium, Kalium. As several of the names of the elements commence with the same letter, to prevent confusion, other letters are added to the first letter, such as, for instance : — Chlorine has symbol Cl. Chromium ,, ,, Cr. Cobalt ,, ,, Co. Some of the elements have as symbols two of the most significant letters of the Latin name : — Mercury ... Lat., Hydrargyrum Gold ... „ Aurum Silver ... ,, Argentum Copper ... „ Cuprum A symbol, however, stands for than the mere abbreviation of Symbol, Hg „ Au. » Ag. „ Cu. something more the name of an element. It represents not only some particular element, but denotes at the same time one atom of the substance and a quantity of it equal in amount to the atomic weight. Thus : — The symbol Ag means 1 atom of silver and 107*7 parts by weight; The symbol Cu means 1 atom of copper and 63*5 parts by weight; The symbol O means 1 atom of oxygen and 16 parts by weight. 36 PHOTOGEAPHIC CHEMISTRY. Molecular 'Formulae , — To denote the composi- tion of a compound, the symbols of the elements composing it are written side by side. If more than one atom of each element is present they are denoted by placing a small number, termed the exponent, at the bottom of the symbol on the right-hand side. The expression is termed a formula, and it also denotes that the elements pre- sent in the compound are chemically combined. The formula for silver nitrate is AgNOg ; ,, ,, ,, gold chloride is AuClg ; ,, ,, ,, cane sugar is C12H22O11; ,, ,, ,, silver chloride is AgCl. As a general rule, the molecules of the elements contain two atoms, and they are represented by the following molecular formulae : N2 O2 H2 CI2 Bra CUa. To denote two or more molecules of a certain sub- stance, a large number, termed the coefficient, is placed in front of the molecular formula. Thus four molecules of silver chloride would be repre- sented by 4AgCl. Elements placed in brackets are multiplied by the exponent at the right-hand bottom corner, in order to obtain the total num- ber of atoms present in the molecule : (NHJaSO, or NaH^SO,. Valency . — If the formulae of the chlorides of the elements are examined, they are found to differ, in a large number of cases, in the number of atoms of chlorine contained in the molecules. For instance, the chlorides of silver, copper, gold, and platinum have the following molecular for- mulae : Silver chloride AgCl. Copper chloride CuCE. Gold chloride AuClg. Platinic chloride PtCE. Hence it is seen that atoms of different elements differ in their pov/er of holding other elements in MEANING OF SYMBOLS AND EQUATIONS. 37 combination. The combining power of an element is measured in terms of the combining power oi hydrogen, which is taken as unity. The number so obtained is termed the “ valency ’’ of that par- ticular substance. For examiDle, one atom of hydrogen combines v/ith one atom of chlorine, one atom of bromine, and one atom of iodine to form respectively hydrochloric, hydrobromic, and hydriodic acids. HCl HBr HI It is evident from this that chlorine, bromine, and iodine have the same combining power as hydrogen ; and they have therefore a valency of one. Such elements are said to be monovalent or univalent. If one atom of any other element combines with one atom of chlorine, bromine, or iodine, it follows that they will be also mono- valent, such as the elements potassium, sodium, lithium, etc. If one atom of an element combines with two atoms of chlorine, such as calcium, barium, strontium, cadmium, etc., they will evi- dently have double the combining power of hydro gen. These elements are said to be divalent. Ele- ments combining with three, four, five, six, or more atoms of chlorine are termed tri-, tetra-, quinque- (or penta-), and hexa-valent elements respectively. It is impossible to write the correct formula of a compound or to express a chemical action by means of an equation unless the valency of the elements be known. Bonds to Represent Valency . — In some cases it is found convenient to represent the valency of an element by drawing from the symbol a number of lines or bonds. Thus a monovalent element is said to have one bond, a divalent element two bonds, and so on. By this means, representative diagrams may be drawm which give a much clearer idea of chemical reactions than would otherwise be possible. This is more noticeably the case wdth complex organic comoounds. 38 PHOTOGKAPHIC CHEMISTR-S. I Cl- -- 0 - Au— 1 Cl-H H-O-H Cl I Au— Cl I Cl Of course, this is purely a symbolical representa- tion of the idea that each atom possesses a definite combining power, and it must not be supposed that elements have little arms projecting. Compound Radicals . — So far, mention has only been made of the valency of single elements. In some cases it is found that groups of atoms take the place of mono-, di-, etc., valent elements. These groups are termed “ compound radicals,’’ and are, as a rule, enclosed in brackets when writ- ing the molecular formula of a compound con- taining them. It may be pointed out that a very common com- pound radical is a group containing one atom of nitrogen and four atoms of hydrogen. This is a monovalent group, and behaves in a very similar manner to sodium or potassium. In order to show this analogy it is termed ammonium (NH4). Hydrochloric acid, HCl NaCl KCl NH4CI ; (Sulphuric acid, H2SO4 NagSO. K2SO4 (NH4)2S04 ; Oxalic acid, H2C2O4 Na2C204 K2C2O4 (NH4)2C204 Sodium Potassium Ammonium salts. salts salts. A few other common compound radicals are : (OH), hydroxyl, and (ON) cyanogen. Variable Valency . — Certain elements form two or more classes of compounds, depending upon their degree of oxidation or reduction. For example, mercury and copper each form two oxides with oxygen — mercurous and mercuric oxide, cuprous and cupric oxide. These have the following formulie : — MEANING OF SYMBOLS AND EQUATIONS. 39 Merciirzc oxide, HgO. Cupric oxide, CuO. Mercurow .5 oxide, Hg20. Cuprows oxide, CiuO. It is evident from what has already been said about valency that these metals, mercury and copper, in the “ ic ” condition — that is to say, in the reduced state — are monovalent. In the “ ous ” condition, or oxidised state, they are divalent. Owing to this fact, each metal forms twm classes of salts, the “ ous ” and ic ” compounds. In the table on p. 40 a few of the more common metals, having this variable valency, are given. This variable valency is also met with in the non-metallic elements. It is readily seen if the formulae of sulphuretted hydrogen, sulphur di- oxide, and sulphuric acid are w^ritten out fully : — O II H-S-H 0 = S = 0 H-O-S-0— H II O Sulphuretted hydrogen Sulphur dioxide Sulphuric acid S divalent. S tetravalent. S hexavalent. Chemical Equations. — In dealing with cases of chemical action it is desirable to express the change taking place by means of symbols. A sym- bolical representation of a chemical change is termed an equation. The symbols or formulae of the reacting bodies are placed on the left hand of the sign =, and the resulting substances are represented by formulae and symbols, placed to the right of it. Taking the following equation as an example : — H 2 SO, + Zn = ZnSO, -h H^, this means that one molecule of sulphuric acid acting upon or reacting with (represented by the sign +) zinc, yields, or produces (represented by the sign =), one molecule of zinc sulphate, to- gether with (sign -f) a molecule of hydrogen. Also, it must be noticed that both sides of an squation must balance — that is, if four atoms of 40 Metal in ic or Oxidised Condition a* C c c3 — 1 (N p C3 k Q d r2 o o o g" s W Q pH CC < Ph O o o Oxide. 1 0.0 o 1 bo c = 1 WqPho2<1 cl O ci ^ ffi O r-l I MEANING OE SYMBOLS AND EQUATIONS. 41 a particular element are on the left-hand side, these must be accounted for on the right-hand side. The question of valency must also be considered. For instance, take the following equation : — AgN03 + 3HC1 = AgCl^ + HNO3. This is wrong for these reasons : — (a) Silver should be monovalent, not divalent. {b) Three atoms of hydrogen and three atoms of chlorine are on one side and only two atoms of chlorine and one atom of hydrogen are on the other side. As the silver is a monovalent metal, therefore the formula of the chlorine wmald be AgCl. This one atom of chlorine requires only one molecule of hydrochloric acid, therefore the correct equation wmuld be : — AgN03 + HCl = AgCl 4- HNO3 Silver , Hydrochloric _ Silver , Nitric nitrate acid ~ chloride acid. In words, one molecule of silver nitrate reacting with one molecule of hydrochloric acid produces a molecule of silver chloride and a molecule of nitric acid. Reversible Reactio 7 is. — If steam is passed over finely-divided iron heated to redness, it undergoes decomposition. Its hydrogen is set free, and the oxygen combines with the iron to form oxide of iron, as represented by the following equation ; — SFe + 4H2O = Fe304 + 4H2 Three Four One mole- Four atoms of -f molecules = cule of oxide + molecules iron of water of iron of hydrogen. If this oxide of iron is taken and heated to red- ness in a current of hydrogen it produces metallic iron and water, in accordance with the equation : Fe304 + 4H2 = 3Fe + 4 H.O. This last equation is seen to be the reverse of the first one, and the reaction is said to be a reversi- ble one. A reversible reaction is usually repre- sented by removing the sign of equality from the 42 PHOTOGRAPHIC CHEMISTRY. equation and introducing two arrows pointing in opposite directions, thus : — 3Fe + 4H„0 g "> FegO^^ + 4 H 2 . This docs duty for the two equations given above. Calculation of the Amount of a Compound Produced during a Chemical Reaction . — A chemi- cal equation not only shows the reacting substances and the products obtained, but it also shows the w’^eights of the various bodies taking part and being produced in the reaction. In preparing emulsions for dry plates or making collodion for wet plates, silver chloride, bromide, and iodide are used in varying proportions to suit the subject. Say, for example, it is necessary to as- certain the amount of each of these silver salts obtainable from ten grammes of silver nitrate, and also the weight of sodium chloride, bromide, and iodide necessary to produce them. Taking silver chloride, the equation is first written down : — AgN 03 -f- NaCl - AgCl + NaN 03 — irn-'7 _L Diolee. wt. _ niolec. wt. , molcc. wt. molec. wt. - 1 G 9 7 + ^ - ^,^3.2 + ^55 From the equation it is seen that 169‘7 parts of silver nitrate require 58'5 parts of sodium chloride to produce 143 2 parts of silver chloride. To find the weights of the required substances for ten grammes of silver nitrate is a matter of simple proportion. (1) I69’7 grs. of AgNOs produce 143'2 grs. of AgCl. 10 143-2 „ 169*7 143*2 X 10 169-7 8*4 grs. of silver chloride. (2) 169*7 grs. of AgNO. require 58*5 grs. of NaCI. 1 5 > 10 1 ) 5) 58*5 169-7 ” 58-5 X 10 169*7 = 3*4 grs. of NaCL MEANING OF SYMBOLS AND EQUATIONS. 43 In a similar manner the quantities of silver bromide and iodide, together with the necessary amount of sodium bromide and iodide, can easily be calculated. Water of Crystallisation . — In cases of this kind, where quantities of substances are calcu- lated, it is important to ascertain whether the compounds that are taking part in the reaction contain water of crystallisation. The need of this is seen in the following example. A 5 per cent, solution of sodium carbonate has to be made up from washing soda. Washing soda contains sodium carbonate, which has the formula NaoCOg. But washing soda contains, as well, ten molecules of water of crystallisation, and its complete mole- cular formula is Na 2 C 03 + IOH 2 O. Now a 5 per cent, solution of sodium carbonate means five grammes of anhydrous Na 2 C 03 per 100 parts of solution. Nag = 23 X 2 = 46] r molec. wt. of C = 12 X 1 = 12 1 =106- 3 anhyd r 0 u s 0 ., - 16 X 3 = 48 J 1 sodium car- 10H2O = (2 16) X 10 = 180 (. bonate Molec. wt. of soda = 286 For 106 parts of anhydrous NaaCOg 286 parts of soda „ 1 part „ „ 5 parts ,, From this it is seen that if the sodium carbonate is anhydrous, that is, contains no water of crystal- lisation, all that is necessary is to weigh out 5 grammes and make up to a volume of 100. But as washing soda has to be used, and this contains water of crystallisation, 13*45 grammes must be laken. 286 [are required. 106 286 X 5 106 “ ol soda. 44 CHAPTER IV. WATER, ITS PROPERTIES AND IMPURITIES. Physical Properties . — Water being the chief sol- vent used for preparing photographic solutions, a brief consideration of its chemical and physical properties should be of service to photographers. Water exists in the three physical forms of matter, as a solid (ice), liquid (water), and a gas (steam). Below 0° C. it takes the solid form, and at 100° C., under a pressure of 760 mm. of mercury, it is converted into the gaseous form, steam. Water at different temperatures undergoes some very re- markable changes in volume. If 100 cub. in. of ice is taken and gradually heated to 0° C., it liquefies, and the water produced occupies about 90 cub. in. When the temperature has risen to 4° C., the water undergoes another contraction in volume and now occupies about 89 cub. in. Above 4° C. the water gradually expands till the temperature of 100° C. is reached. Its volume is now roughly 92 cuh. in. If completely converted into steam it increases in volume to about 160,000 cub. in. Water as a Unit of Weight . — From the above it will readily be observed that water is at its maximum density — that is, greatest weight — at a temperature of 4° C. The weight of one cubic centimetre of distilled water, at a temperature of 4° C., is the standard weight of the metric system, and this weight of water is termed the gramme. The Heat Unit, or C alorie.— Owing to the great heat capaeity of water it is taken as the heat unit in comparing heat values. The unit of heat may be defined as the amount of heat necessary to raise a unit weight of water through a unit range of temperature. That is, the amount of heat re- WATER, ITS PROPERTIES AND IMPURITIES. 45 quired to raise one gramme of water from 0° C. to 1° C. This heat unit is termed the calorie. Specific Gravity, The melting and boiling points of water are used for calibrating thermo- metric scales. Water is also taken as the stan- dard in the determination of specific gravities. The specific gravity of a substance is the weight of the body divided by the weight of an equal volume of water. Solubility of Solids .— extent to which solid substances, under the same conditions, are soluble in water, varies very considerably. For instances, bodies such as chalk, iron, and barium sulphate are practically insoluble in watery calcium sulphate and calcium hydrate are only slightly soluble ; whereas pyrogallol, potassium carbonate, and “ hypo ’’ are readily soluble. In any case, however, there is a limit to the amount of soluble solid dissolved by a given quantity of water. When the water has taken up as much of the solid as it can, we have what is termed a saturated solution. The amount of substance required to produce a satu- rated solution is, as a rule, greater, the higher the temperature employed, though no simple relation is observed between the amount dissolved and the temperature. There are a few substances more soluble in cold water than hot. In the following table a few substances are given, together with their solubility in 100 parts of water : — Suhstance. O^C. 20’C. 50^0. lOO^C. Sodium Chloride 35-5 36-0 37-0 39-6 iMercuric Chloride o'7 7-4 11-3 54-0 Potassium Nitrate 13-3 31-2 85-0 246-0 Water of Crystallisation . — Many metallic sub- stances separate from their aqueous solutions in crystalline form, in union with water. Sodium 46 PHOTOGRAPHIC CHEMISTRY. carbonate separates from its solutions in crystals containing ten molecules of water. This com- bined water is termed ‘‘ water of crystallisation.” On submitting the crystals to heat, this water is driven off. In some cases the water of crystallisa- tion is removed by merely exposing the crystals for a short time to a dry atmosphere; they are then said to effloresce. Eain Water. — Rain water is the purest form of natural water, and will in nearly all cases be quite as suitable for photographic purposes as distilled water, provided, of course, that it is collected in clean receptacles. It is rather inter- esting to notice that rain water obtained near the sea, especially if high winds have been preva- lent, usually contains sodium chloride. River Water. — The composition of river water varies very considerably, and will depend upon the strata over which the water flows. For in- stance, the amount of dissolved solid matter in a river such as the Dee in Scotland is only about 5*6 parts per 100,000. But Thames water, having for the most part a drainage area of chalk, con- tains about 30 parts of dissolved matter per 100,000 parts of water. Hard and Soft Water. — Waters containing much solid matter in solution form a lather with soap only with difflculty, and there is a sense of harshness when rubbed through the fingers. Such waters are said to be hard. On the other hand, waters which lather readily with soap, and are soft to the touch, are termed soft waters. The “ hardness ” of a water is due to the presence of dissolved calcium and magnesium salts, princi- pally the carbonates, sulphates, and chlorides. If the hardness of a water is due to the occurrence of carbonates, it can be rendered soft by simply boiling it. The chalk and magnesium bicarbon- ates undergo decomposition, and the carbonates are deposited in the boiler, and constitute the WATEE, ITS PROPERTIES AND IMPURITIES. 47 “ fur.” Hardness removed by boiling is termed “ temporary ” hardness. The sulphates and chlorides of magnesium and calcium are not removed by boiling, and hardness due to the presence of these salts is termed “ permanent ” hardness. Examination of Water for Photographic Pur- poses. — The photographer can roughly test the water he has to use in the following manner : — Colour: If the water is of a pale brown colour this indicates dissolved organic matter, probably of a “ peaty ” nature. This is bad, as it is apt to stain photographic papers. Calcium and Mag- nesium: A white precipitate obtained by adding a solution of ammonium oxalate shows the pres- ence of calcium and magnesium. These metals will be precipitated as insoluble oxalates when making up the ferrous oxalate developer or any other solution containing a soluble oxalate. Sulphates : A white precipitate, produced by add- ing a solution of barium chloride, insoluble in hydrochloric acid, shows that sulphates are present (permanent hardness). Chlorides: If silver nitrate solution produces a white precipi- tate, insoluble in dilute nitric acid, this shows the presence of soluble chlorides, and they will react in this manner to form an insoluble precipitate with any bath containing silver nitrate. Am- monia, : A brown precipitate is obtained by add- ing a small quantity of Nessler’s reagent. This shows the presence of ammonia. If present in small quantities, it has very little action on most photographic solutions, but in large amounts it precipitates iron from its solutions. As a rule, most of it may be removed by boiling the water. Nessler’s reagent is prepared by adding a solution of potassium iodide to a solution of mercuric chloride till the red precipitate first formed re- dissolves. This clear solution is then made strongly alkaline with caustic potash. 48 CHAPTER V. OXYGEN AND HYDROGEN PHOTOGRAPHICALLY CONSIDERED. The apparatus .required for the experiments described in this chapter is of a very simple kind, and can be bought for a few shillings from any dealer in chemical accessories. The articles neces- sary are : One 4-oz. hard glass flask (Fig. 18), one 4-oz. soft glass flask with wide neck (Fig. 19), one thistle funnel (Fig. 20), one retort with stopper Fig. 18. — Glass Heating Fig. 11>. — Wide-necked Flask. Glass Flask. (Fig. 21), ^-Ib. soft-glass tubing, one dozen test tubes (Fig. 22), and one set of cork borers. A few jam jars, assorted corks, and other articles will also be necessary, and they will be men- tioned as required. The above apparatus having been provided, the study of the subject can be commenced. Bow Oxygen is Obtained . — The element oxygen is of interest to the photographer, as it is this gas which causes his “ pyro to turn brown and his sodium sulphite to become useless. Also it enters very largely into a great many photographic re- .OXYGEN AND HYDROGEN. 49 actions. Oxygen forms roughly one-fifth by volume of the atmosphere, the remaining four-fifths being a very inactive gas called nitrogen (together with Fig. 20. — Thistle Funnel. small quantities of argon, neon, etc.). It is a very tedious and lengthy process to obtain oxygen in a state of purity from the atmosphere. It is. however, readily obtained by strongly heating many substances rich in oxygen, such as potassium chlorate (chlorate of potash), manganese dioxide (pyrolusite), etc. Or, again, it may be obtained by heat- ing certain oxides in which the oxygen is combined only very feebly, such as mercuric oxide or silver oxide. Method of Pre^mring Oxygen . — Into the small hard-glass flask introduce an intimate mixture, bf about equal quantities, of potassium chlorate and manganese dioxide, so as to about half fill it. Next fit it with a tightly-fitting cork and delivery D 50 PHOTOGEAPHIC CHEMISTRY. tube, as in Fig. 23. The end of the delivery tube dips beneath a shallow tin can, having a hole cut in the top and another at the bottom, standing in a basin of water. A small jam-jar is next filled with water and brought, mouth downwards, over the top of this tin. The contents of the flask are now carefully heated and the evolved gas (oxygen) collected. The first few bubbles of gas should not be allowed to enter the jar, as this consists, for the most part, of air. Collect four jars of oxygen and cover their mouths with stout pieces of card- board. Fig. 23. — Method of Preparing Oxygen. Experiments with Oxygen . — Experiment 1 : A glowing splint is introduced into a jar of the gas; this, it will be observed, instantly relights. A piece of wood charcoal is fixed to a piece of stout iron wire, heated to redness, and quickly plunged into the jar; the charcoal burns with great brilliancy, throwing off a large number of sparks. Now pour a little clear limewater into the jar, and notice that it instantly becomes of a milky colour. Experiment 2 : Attach a small piece of stick sulphur to a piece of stout iron wire as in Experi- ment 1. Hold it in a flame till it is well alight, and then introduce it into another jar of oxygen. Observe that the sulphur burns with a very feeble OXYGEN AND HYDROGEN. 61 flame in ordinary air, but with a bright lilac flame in the oxygen. After the sulphur has finished burning, pour a little blue litmus solution into the jar and notice that the liquid turns red. Experiment 3 : Fix a coil of very thin iron wire into a piece of cardboard large enough to cover the top of the jar. Heat the end of the wire till it is red-hot, and quickly introduce into the oxygen. The iron burns with great ease. Ex- amine the bottom of the jar, and notice the black powder produced from the burning iron. Experiment 4 : A clear, strong solution of freshly made pyrogallol (pyrogallic acid) and potash is added to another jar of oxygen and allowed to stand tightly corked. In a compara- tively short time the solution becomes dark brown, and if allowed to remain it turns black. This darkening in colour is due to the fact that alka- line pyrogallol is a powerful absorber of oxygen, and from this experiment we can see how impor- tant it is to keep developing solutions containing “ pyro ” from the action of the oxygen contained in the air. Oxides and Oxidation . — We have seen in the first three experiments that substances burn much more brightly in oxygen than in air ; owing to this fact, oxygen is termed a powerful supporter of combustion. When substances burn in oxygen there is a chemical reaction taking place— that is the oxygen, during the combustion, unites with the burning body to form a new compound, which is called an oxide. When carbon burns in oxygen a new body is produced, namely, oxide of carbon, which is known as carbon dioxide, or carbonic acid. Carbon dioxide forms a white compound (chalk) when added to a solution of lime water, hence this is a useful reagent for detecting the presence of this gas. The combustion of sulphur in oxygen results in the formation of oxide of sulphur, or, as it is termed, sulphur dioxide. 52 PHOTOGRAPHIC CHEMISTRY. This sulphur dioxide, in the presence of a little water, forms another new compound known as sulphurous acid, and its presence is indicated by the change of colour on the addition of the blue litmus solution. Litmus is a vegetable colouring matter, obtained from certain lichens, and has the property of changing colour in the presence of acids and alkalis. With acids it is red, and with alkalis blue. A neutral solution — that is, one in which neither acid nor alkali predominates — is of a violet or purple colour. The black compound obtained from the burning iron is oxide of iron. When substances unite with oxygen the process is termed one of oxidation. By the oxidation, then, of carbon, sulphur, and iron, carbon dioxide, sulphur dioxide, and black oxide of iron are ob- tained respectively. It has also been noticed that alkaline pyrogallol (Experiment 4) readily ab- sorbs oxygen, hence it is said that pyrogallol in the presence of alkali (in this case, the potash) is a substance which readily undergoes oxidation. Photo-oxidation . — The process of oxidation is, in a large number of instances, assisted by the action of light, and the operation is then termed one of photo-oxidation. This photo-oxidation plays a very important part in many photo- graphic operations, but it will suffice here to mention a few typical examples. Dry potassium iodide is a stable substance, but moist potassium iodide undergoes photo-oxidation in the presence of sunlight, and is converted into potassium hydrate and free iodine. Many coloured sub- stances, such as certain of the organic dyes, undergo photo-oxidation, with more or less ease, when exposed to the sunlight and atmospheric oxygen, and are thereby converted into colourless compounds. The dark product obtained by ex- posing silver chloride to the action of light is, according to some investigators, due to the absorp- tion of oxygen. OXYGEN AND HYDROGEN. 53 Preparation of Hydrogen. — This substance is not of direct interest to the photographer ; but as it enters indirectly into a certain number of operations, principally those of reduction, its method of preparation is here given. The small wide-mouthed flask is required, a cork pierced with two holes, a thistle funnel, and delivery tube. The apparatus is arranged as in Fig. 24, care being taken to see that the thistle funnel reaches to the bottom of the flask. In place of the small flask mentioned above, a small bottle, having a wide neck, will do equally well for generating the hydrogen, as the apparatus does not require the aid of external heat. Into the flask or bottle is placed a small quantity of granulated zinc or small iron nails. Dilute sulphuric acid (one part of acid to three of water; note, always add the acid to the water, and not vice versa) is next poured down the thistle funnel equal in amount to about a third the capacity of the flask. The gas should be collected in test tubes; on no account in large jars. Allow the first few bubbles of gas to escape, so that all tlie air may be expelled, as was done in the case of the oxygen. Experiments with Experiment 1 : Fill a test tube with the gas, cover the mouth of the tube with the thumb, remove from the basin, 64 FHOTOGKAPIIIC CHtlMISTliY. hold mouth downwards, and introduce a light. If all the air has been driven out from the generating flask before collecting the hydrogen, the gas will extinguish the taper, but will burn itself with a pale blue flame at the mouth of the tube. Experiment 2 : A test tube is filled to about a third of its capacity with water and brought over the issuing gas. In this manner a mixture of hydrogen and air is obtained (containing, as al- ready mentioned, oxygen and nitrogen). On now introducing a light into the mixed gases a sharp explosion takes place. This explosion is due to the fact that the hydrogen unites with the oxygen to form oxide of hydrogen, or water. Nascent Hydrogen . — The hydrogen produced in the immediate neighbourhood of the zinc (or any other metal) and dilute sulphuric acid is in a very active chemical form, and will bring about chemical changes much more readily than hydro- gen that has once left the generating solution. This active form of hydrogen is spoken of as “ nascent hydrogen. Experiment 3 : Place a mixture of zinc and dilute sulphuric acid in a small flask or bottle and add to it a strong solution of ferric chloride. The deep yellow colour of ferric chloride solution is observed to become paler and paler, till it finally assumes a very weak green tint. Experiment 4 : Into .another bottle evolving hydrogen, place a small quantity of silver chloride or bromide. These compounds can be obtained by adding a solution of salt (sodium chloride) or sodium bromide to a solution of silver nitrate. The resulting precipitate is well agitated with a glass rod, and then collected on a filter paper. In this case it is observed that the white silver chloride, if this compound has been taken for the experiment, gradually becomes grey in colour. This grey compound is found on examination to be metallic silver. OXYGEN AND HYDROGEN. 55 Experiment 5 : Make a strong solution of potassium permanganate (permanganate , of potash), and introduce some of this into an ap- paratus evolving nascent hydrogen, as in Experi- ments 3 and 4. In a very short time the richly- coloured permanganate solution becomes colour- less. What is the explanation of the changes taking place in these last three experiments ? It is as follows : The ferric chloride, as its name implies, is a compound of iron and chlorine. The “ nascent ” hydrogen produced on the surface of ) the zinc attacks the ferric chloride and robs it of some of its chlorine, and breaks it down into a compound of iron containing less chlorine than the original ferric chloride. This new compound is known as ferrous chloride. In the case of the silver chloride the nascent hydrogen removes all the chlorine from the silver chloride, thus convert- ing it into metallic silver. Potassium perman- ganate is a substance very rich in oxygen, and the “ nascent ’’ hydrogen in its immediate vicinity abstracts this oxygen and breaks the salt down into colourless compounds. Chemical Reduction . — When a compound loses oxygen, or a halogen (chlorine, bromine Or iodine) so that it contains an increased metallic, but a lowered non-metallic composition, it is said to be reduced. The operation is termed one of re- duction. Consequently “ nascent/’ hydrogen is termed a reducing agent, because such changes as mentioned above are brought about by its agency. Fhoto-reduction . — Generally speaking, it may be said that the behaviour of light on compounds susceptible to its action is one of reduction. On this photo-reduction depend some very important photographic operations, such as the preparation of blue prints, formation of the latent image, etc. It may be noted also that hydrogen assists the photo decomposition of silver chloride. 66 CHAPTER VI. THEORIES CONCERNING THE LATENT IMAGE. Introductory Remarhs.—l^\iQ change which sub- stances undergo under the influence of light may be divided into two kinds : those of a physical and those of a chemical nature. A few examples of a change in physical properties might be noted. For instance, powdered non-crystalline selenium (an element very similar to sulphur) gradually becomes crystalline when exposed to light. Under ordinary conditions, in the dark, this crystalline variety of selenium is a very poor conductor of electricity, but under the influence of light it becomes a conductor. Again, ordinary yellow phosphorus, a highly inflammable substance, is gradually converted by the prolonged action of light into red phosphorus, having very different properties from the yellow variety. In these two examples no chemical change has taken place, non-crystalline and crystalline selenium are identical in chemical properties, and the same is true of the red and yellow phosphorus. The change in physical properties can only be ex- plained by assuming that the light has influenced the molecular condition of the substance on which it has acted. Hence one important action of light is to bring about a molecular change. Chemical Changes Due to Light . — Eder makes the following generalisations : — (1.) All kinds of light from the infra red to che ultra violet are capable of some sort of photo- chemical action. (2.) Photo-chemica] action is only produced by such rays as the illuminated body absorbs, so that the chemical action of light is closely associ- ated with the optical absorption. THEORIES CONCERNING THE LATENT IMAGE. 5? ■L (3.) The sensitiveness of a body towards rays of a definite refrangibility is increased by the ad- mixture of other substances which absorb the same rays. (4.) A substance is, as a rule, decomposed faster by a given colour when it is mixed with a body which absorbs one of the products resulting from the photo-chemical decompositions, such as oxygen or the halogens. From the first generalisa- tion, it is evident that it is not correct to sup- pose that violet light alone is chemically active. For instance, hydrogen sulphide solution is de- composed more quickly by red light than by the violet rays. The yellow rays of sunlight are the most active in producing the photo-decomposition of carbon dioxide by the green parts of plants. Another point to bo noticed is that light may bring about the union or disruption of two or more elements. A mixture of hydrogen and chlorine exposed to light combines with explosive violence to form hydrochloric acid. Cl^ + H, = 2HC1. But this equation does not correctly explain the reaction, because if the two gases are dry it is extremely difficult to make them combine. Hence the presence of moisture is essential to induce the reaction. Potassium iodide in the dry condition is a stable substance, but if damp and exioosed to light it gradually darkens and becomes alka- line. This change may be represented as follows : 4KI -t- 2H,0 4-02 = 4K0H + 2l,. The action of light is partly oxidising and partly reducing, according to the nature of the substance under its influence. Red light acts mostly as an oxidising agent on metallic compounds, and violet light as a reducing agent. Photo-Chemical Extinction . — Rays which have passed through a medium sensitive to light are 68 PHOTOGRAPHIC CHEMISTRY. weakened in their chemical activity; in fact, if the medium is of sufficient thickness, their chemi- cal activity may be destroyed entirely. This phenomenon is termed “ photo-chemical extinc- tion,’’ and is apparently of very frequent occur- rence. From this it follows that light wffiich is chemically active performs a certain amount of work. Fhoto-Chemistry of the Silver Compounds . — The action of light on the compounds of silver is by far the most important phenomenon in photography. AH the compounds of silver, both organic and inorganic, are affected by light rays. The first recorded observation of the darkening of a silver salt by the action of light was apparently made by Johann Heinrich Schulze, of Halle, in 1727. Silver chloride, when exposed to the light, turns violet, and under continued exposure a brownish violet colour. On prolonged exposure to light this salt slowly undergoes decomposition and evolves chlorine. The Latent Image . — There are various views put forward to explain the change brought about on the surface of an exposed photographic plate. Such a plate shows absolutely no difference in appearance from one which has not received a preliminary light treatment. Nor is any loss of weight detected even by the most sensitive chemi- cal balance ; yet a change has taken place under the influence of light, because a marked difference is observed when the plate is submitted to the action of a developer. The imperceptible yet important change produced is termed the “ latent ” or invisible image. So far, then, the following facts are known with certainty : — 1. Light, in some cases, brings about a mole- cular change of a purely physical nature, and in others chemical decomposition. 2. Silver chloride on prolonged treatment with light (a) darkens, and (6) decomposes with the Theories concerning the latent image, so evolution of chlorine. The composition and for- mula of the residue is not known. It evidently contains a higher percentage of silver than the normal haloid. 3. The latent image is {a) produced by a very short exposure; {h) is invisible; and (c) there is no loss of weight. The Theory of Suh-Salts. — In order to explain the formation of the latent image on an exposed plate, various “ theories ’’ have been proposed from time to time. One which has received a great amount of favour is known as the Theory of Sub-Salts. According to this idea, when light acts upon a compound of silver it removes a por- tion of the non-metallic element, and so increases the percentage of silver, and the new compound produced is termed the “ sub-salt.” Thus the action of light on silver chloride would be repre- sented in this manner : — The greater the intensity of the light, the more is this sub-salt formed ; and on this supposition the latent image would consist of layers of silver sub-chloride of varying thicknesses. This sub- salt theory was originally proposed by Fischer in 1814, and was stated very clearly by Wetzlar in 1834. The theory is in harmony with the observed fact that light liberates chlorine from silver chloride. It also explains why the developer attacks the exposed portion of the plate in preference to the unexposed part, be- cause the sub-chloride is already half reduced to the metallic state. The fact that no change of colour is observed on the exposed plate is ac- counted for, because of the extremely minute amount of sub-salt produced. Also, no loss in weight would be detected, because any liberated + light = 2 Silver chloride. Silver s?(,&-chloride. GO MOTOGEAPHlC CHEMISTRY. halogen is absorbed by the gelatine on the dry plate, or by the silver nitrate on the wet plate. Existence of Sub-Salts Doubtful. — The evidence for and against the idea that the latent image consists of silver sub-haloid may now be examined. According to Wetzlar (Schweig’s “Jahrbuch,” 1828), he obtained small quantities of silver sub- chloride by treating solutions of ferric and cupric chloride with silver leaf. In 1839 Wohler passed a current of hydrogen over silver citrate heated to 100° C. On analysing the residue, a compound having the formula Ag^O was said to be present. This Ag^O would be silver suboxide and would correspond to the sub-chloride AgoCl. Von Bibra (“ Journal fiir Prak. Chem.’’ [2] 12-55) treated Wohler’s suboxide with strong hydrochloric acid for some time, and isolated a body having the formula Ag 4 Cl 3 . If this is the formula for the sub-chloride it contains a greater percentage of silver than that represented by AggCl. Many chemists have repeated the work of Wohler and Von Bibra, and have come to the conclusion that the existence of these salts is extremely doubtful. Giintz says that silver sub-chloride, AggCl, is obtained by treating sub-fluoride, Ag 2 F, with strong hydrochloric acid. He obtained the sub- fluoride by passing a powerful current of elec- tricity through a concentrated solution of silver fluoride, using silver electrodes. Up to the present, the formula of this sub-fluoride has not been established. Otto Vogel (“ Phot. Mitt.” [36] 334) attempted to isolate these silver sub-haloids by allowing cuprous chloride, bromide, and iodide to act upon a solution of silver nitrate. On a cupric salt silver nitrate acts as under : — CuBro + 2AgNOs = 2AgBr + Cu(N 03 ) 2 . With a cuprous salt the reaction is expressed by Vogel as : CU 2 CI 2 -t 4AgN03 = 2Cu(Ag,N03)2 + Cl,. THEORIES CONCERNING THE LATENT IMAGE. 61 The three sub-haloids were stable in the air and only very slightly acted upon by the light. The analytical results agreed with the formulse Ag 4 Cl 2 , Ag^Bro, and AgJ^. Vogel finds that mercury does not extract silver from these sub-haloids. Under the microscope complete homogeneity is observed. Nitric acid attacks the compounds, dissolving silver and leaving the ordinary haloids, AgCl, AgBr, and Agl. Vogel explains the fact that silver chloride and bromide are not decomposed by nitric acid when exposed to light (which should be the case if they are converted into sub-haloids), by assuming that the sub-haloid at once combines with the ordinary haloid of silver, to form a stable substance. Waterhouse (“ Photo. Jour.,” 1900) and Emszl (“ Zeits. Anorg. Chem.”) re- peated Vogel’s experiments, and came to the con- clusion that sub-haloids are not produced in this manner. From their investigations it appears that Vogel’s compounds consist of intimate mix- tures of finely divided silver and unaltered haloid. The latent image cannot consist of reduced silver and unaltered haloid, as some have suggested, since silver chloride darkens under nitric acid, and on examining the acid no appreciable amount of silver is found. If the metal were produced by the light action, it would be removed as fast as formed, and the acid would contain silver nitrate. 2’Ae ^‘Fhoto Salts ” of Carey Lea. — By treating ammoniacal solutions of silver chloride with fer- rous sulphate, washing the resulting precipitate, and then treating with hydrochloric acid, various coloured compounds are obtained, containing less halogen than the original chloride. These sub- stances were termed “ photo salts ” by their dis- coverer, Carey Lea (“ Amer. Jour. Science,” 1887), as he considered them identical with the com- pounds produced by the action of light on the silver haloids. Their method of formation sug- gests the possibility of sub-salts existing in 62 PHOTOGRAPHIC CHEMISTRY. admixture with the ordinary haloid. So far no chemical formula can be assigned with certainty to these “ photo salts.” The Oxy-Ghloride Theory . — Many chemists who have turned their attention to photography hold the view that the balance of evidence is against the idea that the latent image is due to the forma- tion of sub-salts. In their opinion, the latent image consists of varying thicknesses of silver oxy-chloride, oxy-bromide, or oxy-iodide. The silver haloid, according to this view, in the pres- ence of light or oxygen, or water vapour, slowly loses a portion of its halogen and absorbs oxygen, to produce what is termed an oxy-haloid. Dr. Hodgkinson examined the darkened product, and as the result of his analysis gave it the formula 'Ag40Ck (“ Chem. of Photog.,” page 56). Baker also found that the darkened silver chloride con- tained oxygen, and assigned to it the formula Ag20Cl. Representing the action of light on silver chloride, according to this view of the change, the equations would be : — 8AgCl -f O. = 2Ag40CL + 2CI2 or 8AgCl 2H2O = 2Ag40Cl3 -f 4HCI Silver Oxychloriile (Hodgkinson). 4AgCl -8 0.= 2Ag.0Cl -8 Cl. or 4AgCl -8 2H2O = 2AgfiCl + 2HC1. Silver Oxvehloride (Baker). Facts Bearing on Oxychloride Theory . — With regard to the formation of this oxychloride the following facts are interesting : Abney found that silver chloride did not darken in a vacuum, even after the expiration of several months, provided it was thoroughly dry. From this experiment it appears that oxygen and water vapour are necessary. Carey Lea mentions that the chloride does not darken in thoroughly dry air or oxygen. This would make it appear that it is not the THEOETES CONCEENING THE LATENT IMAGE. 63 oxygen which causes the compound to darken, but the presence of moisture. He also states, how- ever, that silver chloride darkens under perfectly dry petroleum. As no moisture or air is present in this case, it is difficult to see how any oxy- chloride is formed. The same may be said of an experiment of Baker, who found that silver chloride darkened under dry benzene, and showed that the dark substance was metallic silver. These last two experiments, however, are not incom- patible with the idea of sub-salts. Molecular Strain Theory . — Many are of the opinion that when light acts upon a silver haloid to produce the invisible or latent image, no chemi- cal change takes place. They do not deny that silver chloride on prolonged treatment undergoes chemi- cal decomposition ; but their contention is that the duration of the action of the light in an ordinary photographic exposure is insufficient to decompose the silver haloid. _ For instance, Chap- man Jones (“ Science and Pract. of Phot.,’’ 1895) believes that all the facts agree with the sup- position that the developable image, that is, latent image, consists of particles of silver salt rendered less stable, but not decomposed. Again, Liippo Cramer (“ Brit. Jour. Phot.,” 1902) is of the opinion that, in the present position of our know- ledge, there is a complete absence of proof that normal photographic exposure produces any chemical change in silver bromide. To account for the formation of the latent image, it is sup- posed that when a silver haloid is affected by light, an internal strain is set up in the mole- cules, and that the amount of strain is propor- tional to the light intensity. A similar strain would also be produced in the sensitiser present. The effect of the molecular strain is to render the compound less stable, so that in the presence of a reducing agent (a developer), molecules under the greatest strain are the first to be sundered. 64 PHOTOGRAPHIC CHEMISTRY and so produce metallic silver. If the action of the light is prolonged, the strain becomes so great that the molecule is broken down with the libera- tion of the halogen, and the production of the sub-haloid or oxy-chloride, or some other reduc- tion product. (See Fig, 25 for rough diagram- matic idea of theory.) Relay 86 of Image . — This molecular strain theory also accounts for the relapse of the latent image. The recovery of all latent images is only a question of time. With some substances there is an immediate recovery as soon as the light is THEORIES CONCERNING THE LATENT IMAGE. 65 removed; with others it takes longer, as in the Daguerreotype, where the latent image disap- pears after the expiration of some hours. In the ordinary photographic plates recovery only hap- pens after the lapse of several years. One of the chief functions of the so-called sensitisers may be to prevent the self-recovery of the molecule under strain, and to make the effect permanent. The fol- dowing interesting facts may be noted in connec- tion with this idea of molecular strain : Prof. Dewar (“ Proc. Roy. Inst.,” vol. 13, p. 695) found that chemical activity gradually decreased as the temperature was lowered. At a temperature of — 180° C. a highly active substance like potassium does not show any appreciable action when im- mersed in liquid oxygen. By reducing the tem- perature to — 200° C. it was found that an or- dinary photographic plate was still fairly sensi- tive to light. Now it is rather difficult to see why light should produce a chemical change, such as a sub-salt or oxy-chloride, on the relatively chemically inactive silver haloid, and yet at a temperature of twenty degrees higher no chemical change is obtained with the highly chemically active potassium and oxygen. It seems more reasonable to suppose that the light’s action was a purely physical one. Further Experimental Eesearches. — Major- General Waterhouse Proc. Roy. Soc., 1900 ”) confirmed Moser’s experiments, that invisible images are formed on pure silver or copper plates when exposed to the light. A plate of pure silver was exposed under a masked pattern to the action of sunlight for two hours. It was then held over mercury vapour, and a distinct image of the mask I was obtained. Copper behaved in a similar man- 1 ner. Consequently, these experiments indicate that most of the phenomena produced in the ex- I posure of an ordinary photographic plate, con- ) taining on its surface silver haloids, can be 66 PHOTOaHAPHIC CHEMISTRY. observed upon a plain plate of silver exposed to the light in the air under ordinary conditions. A printed-out image on a thin film of silver was treated with a solution of potassium cyanide. The unexposed parts wrinkled, but not the ex- posed parts. These experiments appear to be best explained by assuming that a physical and not a chemical change has taken place under the light’s action. The uncertainty with regard to the correct composition of the latent image is due to the great experimental difficulty met with, in studying the problem, of isolating the ex- tremely minute amount of changed haloid from the large amount of unchanged salt. Ripening of Emulsio7is . — The sensitiveness of the silver haloids to light depends very much upon their physical condition.' When silver bromide is precipitated in the collodion emulsion, a certain amount of time elapses before it reaches its maximum degree of sensitiveness, and in order to attain this it is allowed to stand for some time; this process is termed the “ripening” of the emulsion. The haloids in the gelatine emul- sion, when just precipitated, are in about the same degree of sensitiveness as those present in the collodion emulsion. But if the gelatine emul- sion is heated in a water bath, it then becomes much more sensitive than the collodion emulsion. It has also been found that the emulsion may be “ ripened ” by heating it with ammonia for a short time. All these ripening processes bring about an increased size of the particles of silver haloid. Freshly precipitated silver bromide par- ticles were measured by Fder, and found to be from ‘0008 to ‘0015 of a millimetre in diameter. After being “ ripened ” they had increased to '003 and '004 of a millimetre in diameter. There is, however, a limit to all ripening processes, as it is found that the silver haloids attack the gela- tine after a certain time, and become partially THEORIES CONCERNING THE LATENT IMAGE. 67 reduced. Plates covered with an over-ripened gelatine emulsion exhibit general fog when im- mersed in the developer. Sensitisers . — Another important matter to be considered in connection with the action of light on the silver haloids, and other compounds sus- ceptible to its influence, is the function of sen- sitisers. It is found that the light’s effect is greatly accelerated by the presence of another body capable of absorbing the halogens. Any sub- stance which behaves in this way, so as To increase sensitiveness, is termed a “ sensitiser.” In the wet collodion process it is the silver nitrate which acts in this manner, and in the gelatine emulsion it is the gelatine. Theory of Sensitisers . — According to the sub- salt and oxy-haloid theories, accounting for the production of the latent image, it is supposed that an infinitesimal amount of halogen is liberated from the silver haloid. Now, on this theory the sensitiser present absorbs this halogen, and re- moves it from the sphere of action as fast as it is formed. There is a good deal of evidence for this, from purely chemical reactions, because it is found that the velocity of a reaction is in- creased by removing the products of the decom- position. But of course this will not account for the behaviour of sensitisers if the light does not decompose the silver haloid. As already men- tioned, on the molecular strain line of reasoning, the supposition is put forward that the action of the sensitiser is to render the molecular strain set up in the silver haloid permanent. In other words, the molecular strain, or stress, produced in the sensitiser, may retard the self-recovery of the haloid, or may actually produce, or excite, a greater strain in the molecule of the haloid than that obtained in the absence of the sensitiser. ( C8 CHAPTER VII. CHEMISTRY OP DEVELOPMENT, TONING, INTENSIFICA- TION, ETC. The Latent Image and Development. — Compounds which render the latent image visible are termed developers. Intimately bound up with the pro- cess of development is the constitution of this latent image, and, as already noticed, the views put forward with regard to this problem are, up to the present, simply conjectural. It follows from this that no complete account of the mechanism of development can be given with cer- tainty. But though a connected chain of proof cannot be put forward, many of the accepted explanations are highly probable. The latent image, according to the various theories, consists of layers of varying thickness of either {a) sub- haloid, ijj) oxyhaloid, or (c) haloid, under vary- ing degrees of molecular strain. Development by Mercury Vapour. — In the old Daguerreotype process, the latent image is ren- dered visible by submitting it to the action of mercury vapour; the amount of the metal de- posited being proportional to the original light intensity. The composition of the compound formed by the mercury, and that constituting the latent image, whether subhaloid, oxyhaloid, etc., is not known, consequently no opinion can be put forward to account for the partiality of the mer- cury for the latent image, in preference to the unaltered haloid. Acid Development {Ferrous Sulphate). — This developer is used in the wet collodion and other processes, in conjunction with acetic acid. Fer- rous sulphate is a very important reducing agent, and in the presence of substances rich in oxygen, CHEMISTKY OF DEVELOPMENT, ETC. 69 such as silver nitrate, it reduces them to a lower state, being itself oxidised to ferric sulphate. This is readily shown by adding a small quantity to a solution of silver nitrate, a black precipitate of metallic silver being instantly produced. The ' reaction may be expressed as follows : — ) 6AgN03 + 6FeSO, = 2Fe2(SO,)3 + Silver Nitrate. Ferrous sulphate. Ferric sulphate. Feo(N 03 )e + 3Ag2. Ferric nitrate. Silver. Now try the effect of adding the ferrous sulphate to a small quantity of silver chloride, free from silver nitrate. Under ordinary conditions no reduction is observed in this case. liestrainers . — On an exposed wet collodion plate there are present the latent image, un- altered haloid, and the sensitiser, silver nitrate. At first sight, a ferrous sulphate developer would appear to be out of place in view of the above reaction, because this silver nitrate should be in- stantly reduced, all over the plate, to the metallic condition, and so cause general fog. Such would be the case in the absence of the acetic acid. This acid prevents the developer from acting too rapidly on the silver nitrate. Reagents which bring about this retarding action are termed restrainers. Many substances behave in this manner ; for example, soluble organic acids, im organic acids, and various viscous compounds, such as glycerine, sugar solutions, etc. ' The acid restrainers exercise this property, by virtue of the fact that they form stable compounds with the silver, and so keep back, for a certain time, the reducing action of the developer. ' Thus far, then, it will have been noticed that the developer re- duces the silver nitrate, the acetic acid preventing this reduction from becoming too rapid, and is without action on the unaltered haloid. Appar- ently the ferrous sulphate does not reduce the 70 PHOTOGRAPHIC CHEMISTRY. silver composing the latent image (see Meldola, “ Chem. of Phot.,” p. 162). Where, then, does the silver which is deposited on the latent image come from ? Evidently, if the silver of the latent image, and of the unaltered haloid, does not suffer reduction, it must be from the sensitiser, silver nitrate, that the image receives its deposit of metal. The Ferrous Oxalate Developer . — The ferrous salts of certain organic acids, as would be ex- pected, can be utilised for purposes of develop- ment. One very important salt of this group, still largely employed, is ferrous oxalate. This must properly be considered as an acid developer, for, although it may be used in a neutral con- dition, it works best in an acid state, and the formulae given for this developer almost invariably recommend the addition of an acid, generally sulphuric, citric, or acetic. As ferrous oxalate is practically insoluble in water, it is brought into solution in the form of a double oxalate of iron and potassium. This compound is readily obtained by adding a solution of potassium oxa- late to one of ferrous sulphate in the proportions required by the following equation : — 2K,C,0, -h FeSO, = K.FeCCA)^ + K,SO,. Potassium Ferrous Potassio-ferrous Potassium oxalate. sulphate. oxalate. sulphate. The potassium sulphate produced apparently plays no part in the development. The first action of the developer is on the latent image, which it reduces to the metallic state, being itself at the same time oxidised to ferric oxalate. This latent image silver, then, in the presence of the developer, decomposes the haloid immediately be- neath it as before described. Reactions of Ferrous Oxalate Developer . — The complete equations representing the reaction tak- ing place will necessarily depend upon the com- CHEMISTRY OF DEVELOPMENT, ETC. 71 position of the invisible image. On the sub-salt hypothesis the first equation would be— 3Ag,Br + 3FeC,0, = Fe,(C,0,)3 + FeBr3 Silver sub- Ferrous Ferric oxalate. Ferric, bromide. oxalate. bromide. + SAgj. Silver from latent image. Secondly 2FeBr3 + 3K,C,04 = Fe,(CA).3 + 6KBr. This latent image silver, %>lus the haloid beneath, plus more developer, produces more silver till sufficient density has been obtained on the nega- tive. r On the oxy-haloid hypothesis the first action of the developer would be — 3Ag,OBro Silver ox \ bromide. + 6FeC,0, = 2Fe,(CA)3 + 6Ag,. -f 2FeBi*3 The ferric bromide would then react with the potassium oxalate as above. On the molecular strain view of the invisible image, the first re- action would be — 6AgBr -f eFeC^O, - 2Fe,(C30j3 -f 2FeBi*3 3Ag,. Action of Thiosulphate in Ferrous Oxalate Developer . — Although it has been known for a considerable time that the presence of a small quantity of sodium thiosulphate increases the activity of a ferrous oxalate developer, and that developer only, the mechanism of the reaction is still very obscure. The thiosulphate not only assists development, but actually decreases the time of exposure. According to Abney, the period of exposure can be reduced one-third by the use of thiosulphate. The favourable action of this compound is only noticed when it is em- ployed after exposure. If a plate is treated to a preliminary bath of the thiosulphate solu- tion, then exposed and developed with ferrous oxalate, it shows general fog and under exposure. Other compounds besides thiosulphate increase n tHOTOGEAPHIO CHEMISTRY. the activity of the ferrous oxalate developer. Sodium sulphite, Na 2 S 03 , is weaker in its effect, and liver of sulphur stronger, than sodium thiosulphate. One explanation of the action of the thiosulphate is that it probably exercises a solvent effect on the silver haloid, and so brings the developer more into action. According to Meldola (“ Chem. of Phot.,’’ p. 188) the increased activity of the ferrous oxalate developer, in the presence of this compound, is due to its reducing action on the ferric oxalate produced during the course of the development. Alkaline Development . — The so-called organic developers, working in alkaline solution, form another important class. Perhaps the most generally useful of this group is an alkaline solution of pyrogallol, tri-hydroxy-benzeno, which is so largely used for developing gelatino- bromide or “ dry ” plates. Pyrogallol not only reduces silver nitrate solutions, but also the haloids of silver. In order to moderate this powerful reducing action, restrainers have to be used, and the most suitable is found to be a solu- ble bromide ; potassium bromide being, as a rule, employed. The restraining action of this salt is probably due to the formation of a stable com- pound with the silver bromide, which is not so readily acted upon by the developer. If an ex- posed gelatino-bromide plate is treated with an alkaline solution of potassium pyrogallate, a negative is obtained in varying thicknesses of metallic silver. Origin of the Beduced Silver . — Attention must be directed to the source of this silver. In the dry plate no silver nitrate is present, as in the wet collodion process ; consequently the metal must come either from the unaltered haloid or from the latent image. If its source is the un- altered haloid, the only manner in which it can be deposited on the latent image is by the former' CHEMISTRY OF DEVELOPMENT, ETC. 73 dissolving in the alkali and then undergoing reduction by the developer, as described in the case of silver nitrate. If, however, unexposed silver bromide is repeatedly washed with a soliu "tion of ammonia, potash, or soda, of the same strength as that used in development, it is found that only the merest trace of the haloid is dis- solved, quite insufficient to account for the density of the silver deposit on the negative. Evidently, therefore, this deposit must have its origin in the invisible image. Explanation of Density . — It has been proved that the change brought about by the action of light on a sensitive film is of an extremely minute character, and its equivalent in metallic silver would likewise be exceedingly small. To account for the density of a negative, then, further in- vestigation has to be made. Towards this end, the following experiments appear to offer a clear explanation. Abney exposed a dry plate, and then covered half of it with a collodio-bromid.e emulsion. The plate was next developed and the two films separated. On examining these an image was found on each. The only way of ac- counting for the image on the collodion film, which had not been exposed to light, is by as- suming that the exposed haloid on the gelatine film is first reduced by the developer, then this liberated silver, together with more developer, sets up a decomposition of the silver haloid im- mediately above it. This second layer of metal, jilus more developer, then reduces another layer of haloid, and so on, till no more haloid is avail- able. The same action takes place under the reduced image on the gelatine plate, and extends downwards. In an experiment of Dr. Eder a very thick gelatino-bromide emulsion was ex- posed and then developed. In this case the metallic silver of the image extended right through the thickness of the film, and is clearly 74 PHOTOGRAPHIC CHEMLSTRY. formed from the haloid immediately below the latent image. Difference Between Acid and Alkaline Development. — Aeid development, such as that described under ferrous sulphate, differs in a marked manner from alkaline development, es- pecially in the way in which the image on the plate is obtained. In the first method the image receives its silver from the sensitiser, silver nitrate, which is deposited on the plate and grows upw going dec( developme the haloid i mrd, without the haloid beneath under- imposition (see Fig. 26). In alkaline nt no extra silver is added to the plate, is the source of silver, and the image 1 .J } { A Fig. 26.-— Acid Development, \ 1 mmmm rnmimm i ..,7': ' A Fig. 27. — Alkaline Development. grows downward. This difference in the nature of the image is shown by treating negatives ob- tained by the two methods with dilute nitric acid. The negative from the acid developer is simply restored to its previous condition, whilst that from the alkaline developer has its gelatine sur- face pitted in places where the image originally rested (see Fig. 27). Grovjth of the Silver Image. — It is very re- markable that the silver from the latent image, under the influence of the developer, should re- duce the silver haloid just beneath or the silver nitrate above. It is extremely probable that the action referred to is one of electrolysis. In fact, the experiments of Abney and Eder quoted above CHEMISTRY OF DEVELOPMENT, ETC. T5 almost demonstrate electrolytic action. Each minute particle of reduced silver from the latent image can be looked upon as constituting the electrodes of an enormous number of minute cells, each electrolytically decomposing the haloid, or silver nitrate, in its immediate neighbourhood, and depositing the silver. Lermontoff’s experi- ment practically demonstrates this electrolytic behaviour of the silver. A solution of ferrous sul- phate is separated by a porous diaphragm from a solution of silver nitrate. A thin piece of silver is then bent so that one end dips in the iron and the other in the silver solution (Mel- dola, “ Chem. of Phot.’’). In a very short time a crystalline growth of silver makes its appear- ance on that part of the metal in the silver nitrate. Neutral Development . — Besides the two classes of developers already considered — those which are used in an acid state, and those employed with an alkali or alkaline salt — there is a third class which cannot be included under either heading. Ami- dol, dianine (diamido-resorcin hydrochlorate), and tri-amido-phenol, for instance, may be used with sodium sulphite as developers without any alkali. Adurol may be used with water only, although it then becomes inconveniently slow. Synthol can be employed, with sodium sulphite, either with or without alkali. In addition to these, there are several other less known develop- ing agents which are capable of successful employ- ment in the absence of alkali, and must conse- quently be considered as neutral. The potassium oxalate solution employed for the reduction of the image in the platinotype process may also be regarded as a neutral developer. It is possible to use eikonogen without an alkali, but this is seldom or never done. Belapse and Destruction of Latent Image . — It is a curious and rather perplexing fact that after the expiration of a certain length of time — some 76 PHOTOGRAPHIC CHEMISTRY. years in the case of a gelatine film — the invisible image disappears. This phenomenon is explained, on the sub-haloid and oxyhaloid hypotheses, by assuming that the liberated halogen, resulting from the formation of these compounds, is ab- sorbed by the sensitiser, and is then slowly re- absorbed by the latent image, in this manner reverting to its original state. Acording to the molecular strain theory, the relapse or recovery of the invisible image is of a purely physical nature; the energy absorbed by the molecules of haloid and sensitiser from the original light action gradually disappears. The molecules may be compared to minute secondary batteries, con- sisting of stored energy, which, when completely run down, are then in the same condition as un- exposed molecules of the silver haloid. Now it has been found that not only does the latent image return to its original molecular condition by itself, but that oxidising agents and the halo- gens cause a like change. Tlalogeji Absorijtio7i.—OnQ explanation of the cause of the destruetive action of these compounds on the latent image is that it is due to the gradual oxidation and re-halogenisation (or halogen ab- sorption) of the sub-haloid or oxy-chloride. If this is so, it is simply a good illustration of a reversible reaction. From a chemical point of view this is an extremely probable explanation, if the aetion of the light is to decompose the silver haloid, because practically all chemical changes under the proper conditions can be reversed. Molecular Disturbance Theory . — If the change on an exposed film is merely a case of energy ab- sorption for the time being, this rehalogenisation or oxidation hypothesis is inadequate. On the physical view of the invisible image, the addition of the destructive agent may result in a mole- cular disturbance, or proceed further, and be accompanied by a chemical change of the sensi- CHEMISTRY OF DEVELOPMENT, ETC. 77 tiser. For instance, in the case of the latent image on a Daguerreotype plate this is instantly destroyed, if treated to the vapours of a halogen. Mn this instance the excess of halogen simply dis- turbs the molecular condition of the altered haloid, and discharges its absorbed energy, thereby converting it to the original haloid. An exposed gelatino-bromide plate loses its invisible image on treatment with oxidising agents such as nitric acid, chromates, permanganates, etc. Now it is a well-known fact that gelatine is susceptible to the action of oxidising agents, such as, for ex- ample, potassium bichromate. Hence it is very probable that the oxidising agent first attacks the sensitiser, that is, the gelatine, producing a micro-chemical change, and the molecular change engendered thereby sets up a corresponding dis- turbance, of an opposite kind to that produced by the light originally, thus causing the altered haloid to revert to its former condition. Reversal by Light Action . — The continued ac- tion of the light on a sensitive film also results in the partial or complete destruction of the latent image. This phenomenon is known under the name of solarisation or reversal. For in- stance, a greatly over-exposed film, on develop- ment, produces a positive instead of a negative. It has been shown by numerous investigators that the latent image behaves in a most peculiar man- ner under prolonged exposure. Up to a certain point it gradually gains in intensity, and then slowly disappears. It again reaches a certain degree of intensity, gradually diminishing a second time, and so on. In this connection, the following facts, due for the most part to Abney, are interesting : Solarisation is facilitated by a preliminary exposure to diffused daylight, by the action of powerful developing solutions, and by treating the plate with a solution of some oxidising agent before exposure. According to 78 PHOTOGEAPHIC CHEMISTRY. Abney, atmospheric oxidation is essential in pro- ducing solarisation. Difficulty of Explaining Reversal . — It must be confessed that it is extremely difficult to attempt to explain, from either a chemical or physical point of view, the various facts underlying re- versal. It is very probable that the prolonged exposure necessary to produce solarisation is more photo-chemical in its behaviour than physical. It is only necessary to consider the possibility of having on a plate, in less than microscopic .quan- tities, unaltered silver haloid, reduced silver haloid (subhaloid or oxyhaloid, or a combination of the two), silver haloid under molecular tension, gelatine partly oxidised, partly halogenised, and partly under molecular tension, to see how very complicated the subject becomes. In the present state of photo-chemical knowledge the so-called ex- planations are of a purely speculative character. Experiment with Mercuric Chloride . — In con- nection with this subject of the destruction of the latent image, the following experiment of Reiss (“ Chem. Zeit.,” 26 [40])O-is interesting. He utilised the well-known destructive action of mer- curic chloride on the invisible image to render exposed plates fit for a second exposure. The ex- posed plate, containing the image to be destroyed, is first treated with a solution of 5 per cent, mer- curic chloride and then well washed. It is next quickly immersed in an amidol developer, which seems to facilitate the action of the light in the second exposure, dipped in water, and then ex- posed while wet. The exposure takes from about 100 to 150 times that of the first, and a much longer development, to produce the second latent image. It is rather curious that no fog results. The negatives obtained are well covered in the lights, but are perfectly clear in the shadows. Apparently the action of the mercuric chloride on the latent image induces a far greater change CHEMISTRY OF DEVELOPMENT, ETC. 79 than that involved in merely converting it to its original condition. Reduction . — In some cases the negative, after development, is too dense, and takes a very long ^ time to print. This defect can be remedied by submitting it to reagents which will remove some of its silver, the process being termed one of photographic reduction. Ammonium persulphate is a substance capable of acting as a reducer, and will presently be noticed in the chapter dealing with sulphur and its compounds. Ferric^ Chloride Reducer . — By immersing a negative in a solution of ferric chloride the iron is converted into the ferrous condition, and some of the silver, on the negative, into chloride, the equation being as follows : — 2FeCl3 + Ag2 = 2FeCl2 + 2AgCl. Ferric Ferrous chloride. cliloride. If a solvent for silver chloride, such as sodium thiosulphate, be now added, it dissolves, and thereby weakens the original deposit. This method of reduction is not a good one for two reasons. In the first place the operator is unable to follow the course of the reaction, as the amount of reduction is only observable after the silver has been removed by the thiosulphate. Secondly, the ferric chloride is continually oxidising the thiosulphate, which is also a disturbing factor. Na^S^Oa + SFeCla + 5H.O = SFeCL + 2'MaKSO^ + 8HC1. EdeFs Process . — In Eder’s process the silver is removed in the form of oxalate. A solution of potassio-ferric oxalate and sodium thiosulphate is added to the negative. The metallic silver re- duces the ferric oxalate to ferrous oxalate, form- ing at the same time silver oxalate, which dis- solves at once in the thiosulphate. In this way the photographer actually sees how much silver 80 PHOTOGRAPHIC CHEMISTRY. is being removed from the negative undergoing reduction while the reaction is going on. Thus (1) 2FeOl3 + + 6 KC 1 . Ferric Potassium Ferric Potassium cliloride. oxalate. oxalate. chloride. (2) Feo(CoOj3 + Ago = 2 FeC 204 + Ag^CaO^. Ferrous Silver oxalate. oxalate. ( 3 ) AgoC.04 + 2 NaoSo 03 = 2 AgNaS 203 + Soluble silver sodium thiosulphate FTa2C204. Sodium oxalate. The Ferricyanide Eeducer. — In Howard Farmer’s process a freshly-made solution of potas- sium ferricyanide and sodium thiosulphate is used for reduction. By this method the silver is slowly converted into ferrocyanide, and it is very prob- able that small quantities of the ferricyanide are formed at the same time. Both compounds, how- ever, dissolve at once in the sodium thiosulphate, thus allowing the amount of reduction to be ob- served. The following reactions take place : — (1) 4K3Fe(CN), -1- Ag4 = 3K4Fe(CN)3 + Potassium Potassium ferricyanide. ferrocyanide. Ag4Fe(CN), Silver ferrocyanide. (2) 8K3Fe(CN)e + 3 Ag. = 6K,Fe(CN)e F 2Ag3Fe(CN), Silver ferricyanide. ( 3 ) Ag4Fe(CN)e -f 4 Na.S .03 = 4 AgNaS .03 + Na4Fe(CN), Sodium ferrocyanide. (4) 2 Ag 3 Fe(CN)„ + CNa.S.O, = 6AgNaSA + 2Na3Fe(CN)3 Sodium ferricyanide. Removal of Green Fog. — Henderson has shown that green fog can be removed, or an overdense negative reduced, by placing it over a fairly strong solution of potassium cyanide for several CPIEMISTRY OF DEVELOPMENT, ETC. 81 hours. The action in this case is probably due to the carbon dioxide decomposing the easily ionised cyanide, thus liberating HCN, which forms easily soluble AgH(CN)o. Intensification . — Intensification is, of course, the opposite to reduction. Extra material is added to a weak negative, in order to increase its density. In most cases the silver image is first bleached by immersion in mercuric chloride solution. This bleaching is due to the formation of a mixture of silver and mercurous chloride, thus : — Ag3 + 2HgCl3 = 2AgCl + Hg,Cl3 ]\Iercuric Mercurons chloride. cliloride. As this whitened image is hardly opaque enough for printing, it is darkened either by treating with ammonia, ammonium sulphide, sodium sulphide, or with a ferrous oxalate de- veloper. The Blachening Action of Ammonia . — A dilute solution of ammonia blackens the image, probably by the formation of complicated mercurous and silver derivatives of ammonium chloride ; the two chief compounds being NHoHg 2 Cl and NHAg- Hg,Cl. II Hg Hg 1 Cl-N^H 1 Cl-N-Hg 1 Cl— N— Hg 1 H Vh H H Am inonium Di mercurous Dimercurous chloride. ammonium silver ammouiuna chloride. chloride. From some recent investigations of M. F. Leteur the black compound, on analysis, contains silver, and these experiments confirm the formula NHAgHg 2 Cl proposed by Chapman Jones. The equation showing the change would then be — F 82 PHOTOGEAPHIC CHEMISTRY. AgCl + Hg,Cl, + 3 NH 3 = NHAgHg^Cl + 2NH,C1 Diinerciiroiis silver ammonium chloride. In the case of using ammonium sulphide the metals forming the bleached image are converted into sulphides. 2 AgCl + 2 Hg 2 Cl 2 + 3(NH4)2S = AgaS + 2 Hg 2 S + 6NH,C1 Intensification with Sodium Sulphite. — Inten- sification by means of sodium sulphite results in the partial dissolving of the chlorides on the film as complicated sulphites, the rest being reduced, for the most part, to the metallic state. 4AgCl + 2Hg2Cl2 + TNaoSOa = Ag, + Hg + Ag 2 S 03 + 3 HgNa 2 ( 803)2 + 8 NaCl. According to G. Hauberrisser (“ Phot. Runds- chau,’’ 1902) sodium thiosulphate solutions remove from the intensified negative silver and mer- cury, leaving a black residue. On treatment with acid, large amounts of sulphuretted hydrogen were produced. He concludes by saying that the negative intensified by sulphite and mercuric chloride probably consists of mercury and silver in union with a small quantity of sulphur. But according to E. Valenta (“ Phot. Corr.,” 1902) the blackened image contains no compound of silver or mercury in combination with sulphur. If a sufficient amount of sulphite is used, it consists entirely of metallic silver and mercury. With a weak solution of sulphite, or if a concentrated solution of sulphite is used for a short time, the blackened image consists of varying amounts of silver chloride, plus metallic silver and mercury. Intensification with Ferrous Oxalate. — Accord- ing to Chapman Jones (“ Roy. Phot. Soc. Jour.,” 1897) the most reliable method of intensification is by the use of the ferrous oxalate developer on the bleached image. The developer reduces the CHEMISTRY OF DEVELOPMENT, ETC. 83 chlorides to the metallic state, the increased den- sity being due to the deposition of mercury. (1) Hg.Cl^ + 2FeC.O, + K,C.O, = Hg^ + Fe,(CA)a + 2KC1. (2) 2AgCl + 2FeCo04 + K.CoO,, = Ag^ + Fe2(C20,)3 + 2KC1. If the negative has not sufficiently gained in intensity, it can be put through the bleaching and developing process again, so as to deposit more mercury on the film. This metal apparently forms a stable amalgam with the silver, owing to the protective action of the gelatine. Lead and Uranium Intensifiers. — Another method of intensification consists in the use of certain metallic ferricyanides, the more common being those of lead and uranium. The principle of these intensifiers is the formation of an insolu- ble ferrocyanide of the metal, by the reducing action of the silver on its ferricyanide. If the negative is treated with lead ferricyanide, obtained by adding lead nitrate to potassium ferricyanide, it becomes coated with a greyish deposit consisting of silver and lead ferrocyanides. The changes taking place may be represented as follows : — (1) 2K.3Fe(CN), + = Potassium Lead ferricyanide. nitrate. Pb,[Fe(CN)J, + 6KNO3 Lead Potassium ferricyanide. nitrate. (2) 2Ag, + 2Pb[Fe(CN),], = Ag,Fe(CN), + SPbjFeCCNie Silver Lead ferrocyanide. ferrocyanide. The mixture of ferrocyanides is then treated with ammonium sulphide or potassium chromate, forming the sulphides and chromates of silver and 84 PHOTOGEAPHIC CHEMISTRY. lead respectively, in order to render the negative more suitable for printing purposes. ( 1 ) Ag,Fe(CN), + Pb,Fe(CN), + 4(NH,),S = 2Ag,S + 2PbS + 2(NHJ,Fe(CN)« Silver Lead Ammonium Suli)liide. Sulphide. ferrocyanide. ( 2 ) Ag.Fe(CN), + Pb,Fe(CN)„ + 2K,CrO. = Potassium chromate. 2 Ag 2 CrO, + PbCrO, + 2K,Fe(CN)e. Silver Lead Potassium chromate. chromate. ferrocyanide. In the case of the uranium intensifier, the mix- ture of silver and uranium ferrocyanides, being dark brown in colour, is sufficiently opaque. A probable equation to express the change is as follows : — 2Ag, + 2(UO,)3[Fe(CN)J, = Ag,Fe(CN), + Uranium ferricyanide. (?) 3(UO,)3Fe(CN), Uranium ferrocyanide. (?) Album, enised Paper. — This is prepared by coat- ing paper with albumen containing ammonium chloride. The salted paper is sensitised as re- quired by floating on a solution of silver nitrate. * Evidently the paper now contains silver chloride, intimately mixed with the albumen, due to the interaction of the ammonium chloride and silver nitrate : AgN03 + NH,C1 = AgCl + NH,N03 Another reaction also takes place between the albumen and the silver nitrate, to produce an insoluble compound of silver and albumen, whose nature is not known. This is usually termed silver albuminate.’^ That this is the case is easily shown by adding silver nitrate solution to a solution of albumen in water. The compound ) CHEMISTRY OF DEVELOPMENT, ETC. 85 is thrown down as a white curdy precipitate. The sensitive surface of albumen paper consists of silver chloride and silver albuminate, together with a small quantity of silver nitrate. In the ready-sensitised paper citric acid is present as well. Action of TAglit on Alhumenised Taper . — Now this is a truly formidable list of compounds to have together, and in the present state of know- ledge very little is known of the changes brought about in them by the action of light. In the first place, the constitution of albumen is unknown, and it is very questionable whether the formula given in the text books is true. Secondly, the composition of the albuminate is unknown, and it is idle to speculate about its photo-decomposi- tion. Thirdly, the light decomposition of silver chloride is extremely vague, especially in the presence of organic matter. The reddish-brown reduction compounds, produced by the light’s action on the albumen paper, may be identical with the photo-salts of Carey Lea (see p. 61). They would thus consist of a series of reduced compounds, from metallic silver to the unaltered product. Perhaps the first action of the light is to set up an internal strain on the molecules composing the sensitive surface. Action of Light on Collodio- and Gelatino- cliloride Papers. — In the “ printing-out ” papers, collodio and gelatino emulsions of silver chloride and citrate are used. The action of light on these, as in the case of albumen paper, is allowed to continue to the period of visible decomposition. The remarks made above, in connection with the chemical changes taking place when printing with albumen paper, apply also to these various print- ing-out papers ; that is to say, practically nothing is known. Chemistry of Bromide Printing. — Bromide paper is a paper covered with a gelatino-bromide 86 PHOTOaUAPHlC CHEMISTRY. of silver emulsion, similar to that used for coat- ing dry plates, but much slower. In this case an exposure is made behind the negative, but the image is invisible. This is then developed with hydroquinone, ferrous oxalate, or almost any other developer. The chemistry underlying the process is identical with ordinary development, and it follows that the picture is composed of metallic silver. Printing in Platinum . — Platinum prints are not secured by the direct photo-reduction of plat- inum salts. Instead, they are obtained indirectly through the photo-reduction of ferric oxalate. It has been noticed already that ferric oxalate, in the presence of light, is converted into ferrous oxalate. In the preparation of “ blue ” prints, this ferrous compound is treated with potassium ferricyanide, thus producing Turnbull’s blue. In the platinotype process, the ferrous oxalate ic made to reduce a platinum salt to the metallic state. In the actual process a paper is covered with ferric oxalate and potassium chloroplatinite. It is then exposed, whereby the ferric compound is reduced to ferrous oxalate, the platinum com- pound remaining unaltered. It is next treated with a warm solution of potassium oxalate. As soon as the ferrous oxalate dissolves in the potas- sium oxalate, it reacts with the potassium chloro- platinite, and reduces it to metallic platinum. The reaction taking place may be written : — SK^PtCl, Polassiimi chloroplatinite. eFeC.O^ = 3Pt -h 2Feo(CoOj3 2FeCL -h 6KC1. + By sensitising the platinum paper with mercuric citrate, obtained by adding mercuric oxide to citric acid, tones varying from yellow, black, brown, to red-brown are obtained with a cold developer (Hiibl, “ Chem. Zeit.,” 25). Toning . — To remove the objectionable red- brown colour of the freshly printed albumen or Chemistry of development, etc. 87 gelatino-chloride paper, it undergoes the opera- tion of toning. As a rule, this consists of the deposition of some metal on the silver and reduc- tion products obtained in the printing. Gold Toning . — In gold toning, a solution of gold chloride, AuCla, or sodium chlor-aurate, N'aAuCl4, is used in a neutral solution, or in the presence of some mild organic acid. The sub- stances usually employed are ammonium sulpho- cyanide, sodium carbonate, acetate, borate (borax), phosphate or tungstate. The silver and the brown-red reduction products on the paper first of all reduce the auric chloride to aurous chloride, and then this compound to the metallic state. The colour of the precipitated gold de- pends upon the rate of deposition, the strength of the toning bath, and the temperature of the solutions. The operation of toning is probably electrolytic in its action, if much silver is present in the print. (1) AUCI3 4 - Ag, - AuCl + 2AgCl. Auric Aurous chloride. chloride. (2) 2A11CI + Aga = AUo + 2AgCl. In the fixing bath the thiosulphate removes this silver chloride and any not affected by the light originally. The precise action of the thiosulphate is explained in Chapter X. Toning with Platinum and Lead.—\n platinum and lead toning an exactly similar set of reactions are produced as in the case of toning with gold. For instance, the platinum is reduced from the platinic to the platinous state, and then deposited as the metal PtCl,. Platinic chloride. 2PtCl, + 2Ag, = Pt, + 4AgCl. Platinous chloride. or 2KjPtCl. + 2Ag, = Ptj + 4 Ag 01 + 4 KC 1 . 88 PHOTOGRAPHIC CHEMISTRY. In the case of a lead compound, metallic lead takes the place of the silver. Uranium Toning . — Bromide prints are some- times toned with uranium compounds. A choco- late deposit is produced, consisting of silver and uranium ferrocyanides. From what has been said already, many other processes could also be used for changing the colour of a bromide print. Gaedecke considers that the permanence of bromide prints, toned by the formation of certain metallic ferrocyanides, is of a very doubtful nature. The red and blue tones obtained by uranium nitrate and ferric oxalate respectively are not to be trusted as regards permanency. For further details concerning uranium toning see p. 130. 89 CHAPTER VIII. NITEOGEN COMPOUNDS EMPLOYED IN PHOTOGRAPHY. Simple Compounds Containing Nitrogen . — It is proposed to consider in this chapter a few common compounds containing the element nitrogen, some of which are of great use in photography. The compounds to be dealt with are enumerated below : — • N Nitrogen. N3O Nitrous oxide- NHo Ammonia. NO Nitric oxide. HNOo Nitric Acid. Nitrogen . — This element occurs in a free state in the atmosphere, along with oxygen and small quantities of argon and neon. These last two gases are very similar in their properties to nitrogen. Nitrogen is readily obtained by care- fully heating a solution, in water, of potassium nitrite and ammonium chloride. It is produced in accordance with the following equations : — (1) KNO3 + NH,C 1 - NH.NO^ -f KCL Potassium Ammonium _ Ammonium , Potassiiim nitrite. chloride. ~ nitrite. chloride. (2) NH,NO„ = N3 -P 2H3O Ammonium nitrite = Nitrogen -i- Water. Nitrogen is characterised by its great inactivity towards chemical reagents. It has no action on blue or red litmus, is a non-supporter of combus- tion, is non-combustible, and has neither colour nor odour. Oxides of Nitrogen . — The compounds nitric and nitrous oxides are produced when metals are dissolved in nitric acid ; consequently, they are formed when silver is treated with this acid, in 90 PHOTOGRAPHIC CHEMISTRY. order to convert it into silver nitrate. The amount of each oxide of nitrogen produced, how- ever, varies with the metal undergoing solution, and the strength of the nitric acid used. Nitric Oxide . — This oxide is best obtained by allowing dilute nitric acid to act upon metallic copper. The apparatus to be used is illustrated by Fig. 24 (p. 53). The equation representing the change is as follows : — • 3Cu -!- 8HNO3 = 3 Cu(N 03)3 + 2H,0 + 2NO Copper. Nitric acid. Copper nitrate. Water. Nitric oxide. If the copper is replaced by metallic silver a simi- lar change takes place, only, of course, with the formation of silver nitrate in place of the copper nitrate. 3Ag + 4 HNO 2 = 3AgN03 + 2 H 2 O + NO Silver nitrate. Collect a jar of the gas, and notice that it is colourless. Cautiously remove the cover of the jar, and observe that as soon as the air is intro- duced it turns a fine brown colour. This brown gas is due to the formation of another oxide of nitrogen, known as nitrogen dioxide or nitrogen peroxide. 2NO + 20 = N,0, Nitric oxide - — {> Nitrogen peroxide (colourless). (brown gas). Nitrous Oxide . — This oxide is produced, along with other oxides of nitrogen, when zinc is dis- solved in dilute nitric acid. It is best obtained by heating ammonium nitrate. A quantity of solid ammonium nitrate is introduced into the hard-glass flask so as to half fill it. The flask is then fitted with a cork and delivery tube, and arranged as in Fig. 23, p. 50. The ammonium nitrate is then cautiously heated and the gas NITROGEN COMPOUNDS EMPLOYED. 91 collected over water. The production of the gas is thus represented symbolically : — NH 4 NO 3 = N,0 + H,0 Ammonium nitrate. J> Nitroi SO. Totassium , Sulphuric nitrate. acid. K ^>SO. + HNO3 , *■. Potassium _ wii. hydrogen + Nitric sulphate, acid. The acid in the receiver, which is in a very con- centrated form if the materials in the retort were dry, is divided into two parts. To one part is added twice its bulk of water. Experiments v'ith Nitric Acid.~{\) Into a portion of the strong acid introduce a piece of metallic lead. Notice that the lead is instantly attacked, but soon becomes covered with a white compound, and the action ceases. NITKOGEN COMPOUNDS EMPLOYED. 93 (2) Introduce a piece of lead into a small quantity of the diluted acid. Observe that the lead is soon attacked by the acid, and if left for a short time completely disappears. There is evidently here a marked difference in the be- haviour of metals towards the dilute and concen- trated acids. In the strong acid the lead is con- verted into lead nitrate, and this forms a protective covering over the surface of the metal, so that the action soon ceases. In the case of the dilute acid, lead nitrate is also produced, but as fast as it forms it is dissolved by the water, so that the surface of the lead is continually kept clean, and of course is attacked by the acid as long as any remains. This is a very important point to notice when dissolving metals in acids, that water must be present in sufficient quantity to dissolve the salt produced. (3) Pour a few drops of the concentrated acid on a piece of white paper or wood. Observe the yellow stain produced, which is not removable by washing with water. (4) To a portion of the dilute acid add some blue litmus solution. Observe* that it turns red. This is an important test for acids. (5) Add a small quantity of washing soda to another portion of the dilute acid. Notice that a brisk effervescence occurs. This is due to the liberation of carbon dioxide. (6) Introduce a small quantity of the dilute acid into a cool solution of ferrous sulphate. Notice the ferrous sulphate turns brown. This is another important test for nitric acid. Precautions in Preparing Nitric Acid. — (1) After removing the receiver from the basin of water, be careful to see that the end of the retort does not dip beneath the surface of the water. (2) To clean the retort, allow the contents to cool somewhat and then carefully pour in a very thin stream into water. 94 PHOTOGRAPHIC CHEMISTRY. (3) Take great care that the acid does not get on the hands, as the skin is stained yellow. With large quantities bad wounds are produced. Impurities in Commercial Nitric Acid. — The impurities usually met with are sulphuric acid and hydrochloric acid. The last acid is the worst offender, as it reacts with silver nitrate. These two acids may be recognised by the following tests : A portion of the nitric acid is diluted with about three times its bulk of distilled water. To a portion of this is added a solution of barium chloride. If sulphuric acid is present a white precipitate is produced of barium sulphate, in- soluble in hydrochloric acid. To another portion of the diluted commercial sample, a solution of silver nitrate is added. If a white precipitate is obtained — silver chloride, soluble in ammonia — this shows the presence of hydrochloric acid. Tests for Nitric Acid and Nitrates. — Nitrates and nitric acid may be recognised as follows : — (1) A nitrate treated with a drop or two of con- centrated sulphuric acid gives a pungent odour of nitric acid. On dropping in a few pieces of metallic copper, brown fumes of nitrogen per- oxide are evolved. (2) A solution of the nitrate is mixed with a solution of ferrous sulphate in the cold, in a test tube. A drop of concentrated sulphuric acid is now very cautiouly added down the side of the tube. The acid sinks to the bottom., and where it meets the solution of the nitrate and sulphate a brown ring is produced. This forms a very delicate test for a nitrate, and is usually spoken of as the “ brown ring test.’^ Ammonia. — Ammonia is a gaseous compound, containing three atoms of hydrogen and one atom of nitrogen in the molecule, consequently its formula is NH3. It may be prepared by heating any compound containing ammonia, such as am- monium sulphate, oxalate, chloride, etc., with NITROGEN COMPOUNDS EMPLOYED. 95 quicklime or caustic potash. It is conveniently prepared in the following manner : A hard-glass flask is taken, fitted with a cork and delivery tube, and arranged as in Fig. 29. A mixture of any ammonium salt and pow- dered quicklime is then introduced into the flask and heated. Reaction. — Fig. 29. — Preparation of Aramonia, CaO -f 2NH,C1 = CaCF -f H^O -f 2 NH 3 Lime. + Ammonium = Calcium Water, -f Ammonia chloride. cliloride. gas. Notice that the gas is invisible and has a pungent odour. To a jar of the gas add a little red lit- mus solution. Observe that it is instantly turned blue, showing that the ammonia is an alkali. Place a jar of ammonia gas, mouth downwards, in a basin of water. The water instantly rushes up the jar and completely fills it. This solution of ammonia gas in water is termed ammonium hydroxide, and it is in this form that ammonia is used. 96 PHOTOGEAPHIC CHEMISTEY. NH3 + H3O = NH,OH Ammonia gas. Liquid ammonia. Photographic Uses of Ammonia. — Ammonia solution is one of the alkalis used in development with pyrogallic acid. Owing to the readiness with which ammonia combines with silver nitrate, it is used in the preparation of gelatino-bromide emulsion by the ammonia process. Ammonia also has the property of dissolving silver chloride and bromide, but not the iodide. AgCl AgBr Agl Very soluble j. Not so Insoluble in in ammonia. ^ soluble. ^ ammonia. Ammonium Nitrate (NH4NO3). — If this com- pound is added to water, a considerable lowering of temperature is obtained, as the salt dissolves. This has been suggested for preventing frilling during development in hot countries. The de- veloping dish is placed in a somewhat larger dish containing water, and to this is added, from time to time, solid ammonium nitrate. The ammonium nitrate can be recovered by simply evaporating the solution to dryness on a water bath. Acids, Alkalis or Bases, and Salts. — It will be convenient here to have a few definitions of what is understood by the terms “ acids,^^ “ bases or alkalis,” and “ salts.” Acids. — These are bodies which possess in a more or less marked degree the following proper- ties : — (1) They have a sharp, sour taste. (2) Blue vegetable colouring matters, such as litmus, are turned red. (3) With sodium carbonate they cause effer- vescence due to liberation of CO2. (4) They neutralise alkalis forming salts. Acids in all cases contain hydrogen, and they may be grouped according to the number of atoms of this element present in the molecule. Acids NITEOGEN COMPOUNDS EMPLOYED. 97 containing one replaceable hydrogen atom are termed monobasic acids. For example, HCl, HBr, HI, HNO3. Those containing two replace- able hydrogen atoms are termed dibasic acids, as H2SO4, H2C0O4 (oxalic acid), H2SO3 (sulphurous acid), H2CO3 (carbonic acid). Acids containing three or four replaceable hydrogen atoms are termed tri-, tetra-, basic acids, etc. Acid Anhydrides. — If a molecule of water is abstracted from an acid, a substance is obtained which is termed the anhydride of that acid. For example : — HNO3 _ JJ Q ^ Q HNO3 ^ ^ 2 molecules of nitric acid. Nitric anhydride. H2SO4 - H2O = SO3 Sulphuric acid. Sulphuric anhydride. H2SO3 - H2O = SO2 Sulphurous acid. = Sulphurous anhydride. Of course, acids like hydrochloric acid, having the formula HCl, do not form anhydrides. An- hydride formation is only possible when the acid contains oxygen. Alkalis or Bases. — These substances have pro- perties opposed to those of an acid : — (1) They have, as a rule, a “ soapy ” taste. (2) Reddened vegetable colouring matters are turned blue. ( 3 ) They absorb carbon dioxide. ( 4 ) They neutralise acids partially or entirely, forming salts. These compounds are usually divided into three classes : — {a) Metallic oxides, such as CaO, Na20, K2O. {h) Metallic hydroxides or hydrates. These are compounds containing a metal united with one or more hydroxyl groups. Hydroxyl is the name applied to a monovalent grou]i containing one atom of hydrogen and one of oxygen (OH), a 98 PHOTOGRA.PHTC CHEMISTRY. For example, sodium hydrate, NaOH; barium hydrate, Ba(OH)2; ferric hydrate, Fe(OH)g. (c) Certain compounds containing hydrogen, the chief of which is NHg, ammonia. Salts . — These are substances which result by treating an alkali or base with an acid. They are obtained by replacing one or more atoms of hydrogen in the acid by a metal or some group of atoms playing the part of a metal, such as the radical ammonium' (NH^). Salts of monobasic acids will be of one kind only, because they contain only one replaceable hydrogen atom. For example : — Acid. Silver Salt. Sodium Salt. Ammonium Salt. HNO3, Nitric acid HCl, Hydrochloric ) acid ) AgN03 AgCl NaNOg NaCl NH4NO3 NH4CI HBr,Hydrobromic \ acid 1 HI, Hydriodicacid AgBr NaBr NH^Br Agl Nal NH4T Dibasic acids, containing, as they do, two replace- able hydrogen atoms, form two classes of salts Sulplmric acid. >SO, >so. h/ Na/ Sodium hydrogen Di-sodium sulpliate. sulphate. Acid Salts . — Salts obtained by replacing only part of the hydrogen of the acid are termed acid salts, because they still retain some of their acid character. They are also designated by the pre- fix “ hi ” ; thus the first compound sodium hydro- gen sulphate is also known as sodium bisulphate. Compounds obtained by replacing all the hydro- gen atoms in the acid are termed normal or neutral salts, because, speaking generally, they NITROGEN COMPOUNDS EMPLOYED. 99 ' are neither acid nor alkaline in their behaviour. The potassium salts of oxalic acid are : — Oxalic acid. Potassium Normal or neutral binoxalate. potassium oxalate. Tribasic acids, containing three replaceable hydrogen atoms, form three classes of salts. These are termed primary, secondary, and ter- tiary salts. For instance, citric acid is a tri- basic acid, and forms three ammonium salts : — H\ nh^\ H—yCeOyHs II NH^-^CjjOyHs NH^-^CeO^Hr H / II / H / Citric acid. Primary Secondary ammonium ammonium citrate. citrate. V , ^ Acid salts. Preparation of Salts . — It will be convenient h(3re to consider a few general equations repre- senting the formation of salts : — (1) Action of acid on metal. Zn -P H0SO4 = ZnSO^ -1- H2 Zinc. Dilutesulphuric _ Zinc sulphate. Hydrogen, acid. (2) Action of acid on oxide of metal. CuO -f H2SO4 = CuSO, -t- H.O Copper Sulphuric _ Copper Water, oxide. acid. sulphate. (3) Action of acid on carbonate of metal. Na,CO, + H,SO. = Na,SO. + H,0 + CO, Sodium , Sulphuric _ SodiuTn , Water. Carbon carbonate. acid. sulphate. dioxide (4) Action of acid on the hydroxide. H,C,0. + 2NH,OH = (NH.),CA + 2H,0 Oxalic , Ammonium _ Ammonium Water, acid. hydroxide. oxalate. (5) Many salts are obtained by allowing one salt to act upon another, whereby the acid part of N1I4/ Tertiary ammoniun citrate. Neutral S salt. 100 PHOTOGRAPHfO CHEMISTRY. each compound is mutually exchanged. This is termed double decomposition. AgNOa + NaBr = AgBr +NaNOa Silver Sodium _ Silver , Sodium nitrate. bromitle. bromide. ^ nitrate. CdBr^ + 2AgN03 = CclCNOj)^ + 2AgBr Cadmimu , Silver _ Ca CO3 Aramoniuin bicarbonate, AH these compounds are obtained when carbon dioxide acts upon ammonia gas. Conversion of Bicarbonate and Carbamate of Ammonia to Normal Carbonate . — If a little liquid ammonia is added to the bicarbonate and car- hamate they pass ir *;0 the normal carbonate aa follows : — ™.>C03 NH3 NHy + H ,0 1 ! 0 0 M Ammonium carbonate should be preserved in well-stoppered bottles, otherwise it loses am- monia and passes into the bicarbonate. NH4 NH, > CO, Normal ammonium carbonate. NH, NH.. Ammonium carbamate. > CO, 103 CHAPTER IX. THE HALOGENS AND HALOID SALTS. Explanation of Term, “ HalogensE — The four elements chlorine, bromine, iodine, and fluorine are very much alike in their chemical behaviour, and as a group they are spoken of as the halo- gens or haloid elements. The word halogen means literally “ salt former,” and was applied to the four elements mentioned because they form salts having similar properties. They are mono- valent, and their compounds with hydrogen and the metals are named in the following manner : — HCl Hydrochloric acid or hydrogen chloride. HBr Hydrobromic acid or hydrogen bromide. HI Hydriodic acid or hydrogen iodide. HE Hydrofluoric acid or hydrogen fluoride. Metal + chlorine is termed a chloride. „ + bromine „ „ bromide. „ + iodine „ „ iodide. „ + fluorine „ „ fluoride. Chlorine. — As this element plays a very im- portant part in photography its preparation and properties are given below. This gas is best pre- pared in the open air, and great care must be taken to avoid breathing it. A small, wide- mouthed flask, of about 4-oz. capacity, is taken, having a tightly-fitting cork pierced with two holes. Through one hole passes a thistle funnel, and through the other a delivery tube bent as in Fig. 30. About half fill the flask with concen- trated hydrochloric acid, and then add about half this quantity of manganese dioxide. Cautiously heat the flask and collect the gas. As it has a slight 104 PHOTOGRAPHIC CHEMISTRY. greenish-yellow colour it is easy to see when a jar is full. Equation : MnO, 4HC1 = MnCL + 2H,0 -f- Cl^ Manganese Manganese dioxide. cldoride. JE xijeriments with Chlorine . — Experiment 1 : Introduce a lighted taper into a jar of the gas. Notice that the taper burns, but with a very smoky flame. Experiment 2 ; Pour a few drops of turpen- tine on to a piece of tissue paper and drop this Fig. 30. — Preparation of Chlorine. into another jar of the gas. If a fair amount of chlorine is present, the paper instantly bursts into flame. The turpentine and the wax of the taper contain the elements of hydrogen and car- bon. The chlorine abstracts this hydrogen, set- ting free the carbon, and this takes place so rapidly in the case of the turpentine that it is ignited. Experiment 3 : Damp a piece of coloured cloth or paper, and allow it to remain for some time in a jar of chlorine. The colour will be found to be destroyed. Hence chlorine is a bleaching agent. All the organic colouring matters used in photography are bleached by chlorine. The bleaching action of chlorine is largely influenced THE HALOGENS AND HALOID SALTS. 105 by the presence of moisture. Dry chlorine has practicably no bleaching properties. Experiment 4 : Make a little starch paste and add to it a solution of potassium iodide. Dip a piece of blotting paper into this solution and introduce into a jar of the gas. Observe that it is instantly turned blue. This forms a delicate test for the presence of chlorine. The chlorine acts upon the potassium iodide solution, forming potassium chloride and free iodine. This iodine then forms a blue-coloured compound with the starch : 2KI -f Cl, = 2KC1 -f I,. Chlorine also possesses the property of displacing bromine from its solutions. This is readily seen by passing some of the gas into a dilute solution of potassium or ammonium bromide. A brownish- red colour is instantly produced. 2KI -f CL = 2KC1 + Br„ 2NH,Br + CL = 2NH,C1 -h Br,. Influence of TAght on Chlorine in Solution . — The chemical action of chlorine is materially in- creased by the action of sunlight. For instance, a mixture of equal volumes of chlorine and hydro- gen combine in direct sunlight with a violent explosion to produce hydrochloric acid. H, -h CL = 2HC1 In diffused daylight the action is a gradual one, unaccompanied by an exposion. In the dark,, especially if the gases are dry, no action takes place at all. This mixture of chlorine and hydro- gen constituted one of the first actinometers, as the amount of hydrochloric acid produced is pro- portional to the light intensity. It has already been stated that chlorine has a great affinity for the hydrogen in compounds containing that ele- ment. Thus water is decomposed into hydro- chloric acid and oxygen, and the greatest decom- position is effected in the presence of direct 106 PHOTOGKA.PHIC CHEMISTRY. sunlight. If a glass jar containing, and standing over chlorine water, be exposed to the sunlight, a gas is evolved which collects at the top of the jar, and on examination it is found to be oxygen. In diffused daylight the action is slower, and in the dark it almost ceases. The main course of the reaction may be repre- sented as follows : — 2 H 2 O + 2 CI 2 = 4HC1 + O 2 Small quantities, however, of compounds contain- Fig’. 31. — Experiment showing Action of Light on Silver Chloride. ing chlorine, hydrogen, and oxygen are produced at the same time. Action of Light on Metallic Chlorides. — Many metallic chlorides lose chlorine on exposure to bright light, especially in the presence of mois- ture. For instance, if a quantity of moist silver chloride is exposed to bright sunlight for about a week, in a glass tube, as shown in Fig. 31, chlorine is found to be present in the water con- tained in the beaker. Mercuric chloride passes to mercurous chloride if its solution is exposed to bright sunlight for a short period. 4HgCl2 + 2 H 2 O = 2HgCl + 4HC1 + O, THE HALOGENS AND HALOID SALTS. 107 The chlorine, instead of being liberated, attacks the water, as described above, and liberates the oxygen. Bromine is a heavy, reddish-brown liquid, with an exceedingly penetrating odour (hence the name bromine, from bromos = a stench). Bromine can be obtained by passing chlorine into a solution of a bromide. It is ob- tained commercially by passing chlorine into magnesium bromide. MgBra + CI2 = MgCla + Bl'2 Magnesium bromide. In its properties bromine is a perfect analogue of chlorine, though weaker than that element in chemical activity. In contact with organic matter containing hydrogen, like chlorine, it abstracts it, forming, however, hydrobromic acid. It is interesting to notice that bromine does not combine with hydrogen in the presence of sun- 'i!ight. With starch paste, bromine produces a yellow colour. Iodine . — This element is a blackish solid. Of late years large quantities have been obtained from Chili saltpetre, as this substance contains sodium iodate. Iodine has a characteristic odour; it stains the skin and organic matter generally a brown colour. It may be displaced from its solutions by passing chlorine through them. 2KI + Cl, = 2KC1 + I, The deep blue colour imparted to starch solution is characteristic of iodine. It does not combine with hydrogen in the sunlight, and has not the property, like chlorine and bromine, of removing hydrogen from its compounds. Iodine may be purified by submitting it to sublimation. Fluor ine.~T\i\^ element is not so important as the other haloids, as only one or two of its compounds are used in photographic work. It 108 PHOTOGRAPHIC CHEMISTRY. will be sufficient here to mention that it is the most active of all elements, and very difficult to isolate in the free condition. Hydrogen Compounds of the Halogens — Hydrogen Chloride. — This compound, as already mentioned, is formed by the direct union of hj^dro- gen and chlorine in the presence of sunlight. It is interesting to notice that this union is also brought about by the light given out by burning magnesium. This acid is readily prepared by treating any chloride with strong sulphuric acid. Na I Cl + H H Salt. / so. Sulphuric acid. Sodium + bisulpliate. HCl Hydro- chloric acid. This reaction may be carried out with the same apparatus as described under “ Chlorine.’’ It is best to work out of doors, taking care not to breathe the gas. Experiments with Hydrogen Chloride. — Ex- periment 1 : Place a jar of the gas, mouth down- w'ards, in a vessel containing water. Observe that the water very quickly dissolves the gas, and rises to the top of the jar. Experiment 2 : Make a solution of the gas in water, and treat successive portions with : — {a) Blue litmus paper or solution. (6) Sodium carbonate solution. (c) Silver nitrate solution. Observe that {a) turns red and {h) gives off carbon dioxide. These experiments show the acid character of hydrogen chloride. In {c) a white flocculent precipitate of silver chloride is ob- tained. H Cl + Ag I NO3 = AgCl + HNO3 In photographic work, hydrochloric acid — that is, a solution of the gas in w^ater — is used for dissolving out ferrous oxalate from platinotype prints, for the preparation of certain chlorides, THE HALOGENS AND HALOID SALTS. 109 and for a large number of minor uses. The com- mercial acid is really a strong solution of the gas in water, and the strong acid contains about 33 per cent, of HCl. This acid is also known under the name of muriatic acid. The muriates are the chlorides. As ordinarily obtained the commercial acid contains iron and small quan- tities of sulphuric acid. Tests for Impurities in Hydrochloric Acid . — Impurities may be detected as follows : — Iron. — A strong yellow colour denotes the pres- ence of iron. A portion of the acid is diluted, and to this is added a solution of potassium ferrocyanide (yellow prussiate). The presence of iron is shown by the formation of a blue precipi- tate of Prussian blue. Sulphuric Acid. — A portion of the commercial acid is diluted with distilled water, and then a solution of barium chloride added. White pre- cipitate, insoluble on warming, shows the pres- ence of sulphuric acid. If the acid is submitted to distillation these impurities are removed. Aqua lieyia. — A mixture of one volume of nitric acid and three volumes of concentrated hydrochloric acid is known as aqua regia, as such a mixture has the power of dissolving gold and platinum, which neither of the acids alone is capable of doing. The powerful solvent action of the aqua regia is probably due to the presence of free chlorine, and a body known as nitrosyl chloride NOCl. 3HC1 + HNO3 = 2H3O + NOCl + CI3 A mixture of sodium nitrite solution and hydro- chloric acid has also the property of dissolving gold. The equation representing the change is probably as follows : — 3NaN02 + 6HC1 + O. = 3NaCl 4- NOCl + 3H3O -F N3O, + CL Chlorides Used in Fhotoyraphy. — Sodium 110 PHOTOGRAPHIC CHEMISTRY. Chloride^ or common salt (NaCl). This was one of the earliest of the fixing agents, as it has the power of dissolving silver chloride, but cannot be compared to “ hypo.” It was at one time largely used for “ salting ” albumenised paper, to bring about the formation of silver chloride. NaCl + AgN 03 = AgCl + NaN 03 It has also many minor uses in photography. Ammonium Chloride (NH^Cl). — This com- pound is used for “ salting ” albumenised paper, and the reaction taking place is : NH,C1 + AgN03 = AgCl + NH,N03 It may also be used for preparing strong solu- tions of mercuric chloride. This mercuric chloride is not very soluble in water, but it is much more soluble in solutions of ammonium chloride. The strongest solution is obtained when the salts are one molecule of mercuric chloride to six molecules of ammonium chloride. Ferric Chloride (FeClg or FesClg). — This com- pound is obtained by dissolving iron in hydro- chloric acid, and then passing chlorine gas through the ferrous chloride so formed. Fe + 2HC1 = FeCl^ Feri'ous chloride. 2FeCl, + CI 3 = 2 FeCl 3 Ferric cliloride. If the ferrous chloride is exposed to the air , or has air blown through it, it slowly oxidises to ferric chloride. Ferric chloride nearly always contains free hydrochloric acid, but the presence of the acid does not render it harmful; in fact, in certain photo-mechanical processes it is an advantage. Of course, it must not contain too much acid. If too acid, this may be neutralised in the following manner : A quantity of the ferric chloride is taken and treated with ammonia. This produces a gelatinous precipitate of ferric hydrate. THE HALOGENS AND HALOID SALTS. Ill Cl NH,. Cl 4- NH. Cl NH, OH / OH OH = Fe/OH + SNH^CI. OH \ OH This precipitate is well washed by decantation, and then carefully added to the ferric chloride to be neutralised till the right degree of neutralisa- tion has been reached. Cl Cl = FeCls + 3H2O. Cl Acid present in the ferric chloride. Gold Chloride (AuClg, Auric chloride). — This is obtained by dissolving gold in aqua regia, or by treating the metal with chlorine. (1) Au + HNO3 3HC1 = AUCI3 4- 2H3O -F NO, etc. (2) 2Au 4- 3CL = 2AUCI3. On evaporating its solutions in the presence of hydrochloric acid it crystallises in yellow-coloured needles containing one molecule of hydrochloric acid and four molecules of water of crystallisa- tion. It is in this form that it is bought. HAuCl^ 4H0O. Chlorauric acid. Fe^ OH H OH 4- H OH H As it is important that the gold toning bath should bo neutral, this hydrochloric acid is removed by adding a small quantity of chalk to the gold solution. By evaporating a solution of gold and sodium chlorides to the crystallising point, a double neutral chloride of sodium and gold separates, which also has the property of not being deliquescent — that is, of absorbing water. ^-^01 + AuCf = NaAuCl, Scdiutn cliloraurate. Platinic Chloride (PtCl^). — This compound is obtained by dissolving platinum in aqua regia ; it is essentially the nascent chlorine which attacks the metal. 112 PHOTOGRAPHIC CHEMISTRY. Pt + 2CL = PtCl, When heated carefully to 200° C. it breaks down into platinous chloride, PtCl 2 . Platinous chloride forms a double chloride with potassium, and this compound is used in platinotype printing. 2KC1 + PtCL = K^PtCl,. Calcium Chloride (CaCla). — This compound is used as a drying agent to prevent damp from reaching platinum paper. It is obtained by treating chalk with hydrochloric acid, evaporat- ing to dryness, and then fusing the resulting com- pound. CaC03 + 2HC1 = CaCl^ + H,0 + CO^ Chalk. Tlydrohromic Acid (HBr) and Hydriodic Acid (HI). — These compounds are both gases, and fume strongly in the air. Both are soluble in water, and it is in this form that they are met with in practice. They are not obtained by treating a bromide or an iodide with strong sulphuric acid. If sulphuric acid is added to an iodide, hydriodic acid is first formed, but it instantly attacks the sulphuric acid and breaks it down to sulphur dioxide. (1) KI + g> SO, = g>SO, + HI (2) H,SO, + 2HI = 2H,0 + SO, + I, Hydrobromic acid behaves in a similar manner; hence it is important to remember that both these acids are powerful reducing agents, owing to the readiness with which they give up their hydrogen. Bromides and Iodides Used in Photography . — These compounds may be obtained by adding the halogen acid, HBr or HI, to the hydrate or car- bonate of the metal. THE HALOGENS AND HALOID SALTS. 113 Sodium bromide and iodide Potassium bromide and iodide .. Ammonium bromide and iodide.. Cadmium bromide and iodide .. Zinc bromide and iodide NaBr and Nal. KBr and KL NH^Br and NH J. CdBr^ and Cdl,. ZnBrg and Znla. NaOH + HI = Nal + H,0 K2CO3 + 2HBr = 2KBr + CO^ + H.O They are largely used in photography for two main purposes— (a) collodion and gelatine emul- sion making, and (b) as restrainers in develop- ment. In the emulsions the silver nitrate present reacts with a mixture of various bromides and iodides to form the corresponding bromides and iodides of silver. 2AgN03 + CdBr, = 2AgI + Cd(N03)2 2AgN03 + Znl^ = 2AgI + Zn(N03)3 They are used as restrainers in development be- cause they prevent the too rapid reduction of the silver haloid on the plate. The silver and mer- cury haloids are dealt with in Chapter XI. Hydrofluoric Acid (HF). — This compound is a gas having a very sharp, disagreeable odour. It has a most powerful solvent action on glass, and consequently it cannot be prepared or kept in glass vessels. In practice it is made by decom- posing calcium fluoride with strong sulphuric acid in leaden retorts, and passing the gas into water. The aqueous solution is then put oa the market in rubber bottles. Preparation of Hydrofluoric Acid . — Procure a small quantity of calcium fluoride and introduce into a leaden basin. (This is easily made from a small piece of sheet lead.) Cover the fluoride with a little strong sulphuric acid and bring over the top of the basin a glass plate covered with wax, on which a design has been scratched. On melting the wax and removing it from the plate the design will be found to be etched in. H m CHAPTER X. SULPHUR AND ITS COMPOUNDS. Classification of Sulphur Compounds. — Some of these compounds, such as the sulphites, thiosul- phates, persulphates, etc., are used in photo- graphic work continually, and a clear idea of these compounds in photography can only be ob- tained by understanding their chemical behavioi*/ under varying conditions. In the following scheme the names of these compounds, together with their formulae, are given, and their relation- ship to sulphur (see p. 115). Compounds in which the oxygen has been re- placed by equivalent quantities of sulphur are termed thio compounds. For example : — NaaSOaO {i.e. NasSO^) = Sodium sulphate. Na^SOgS {i.e. Na2S20g) = Sodium thiosulphate. NH^CNO = Ammonium cyanate. NH^CNS = Ammonium thiocyanaJte. 'This nomenclature is not systematically adhered to in all cases, but sufficient has been said to give the photographer some idea of the method chemists use in naming these more complicated compounds. Sulphur . — This element is not employed as such in photography, but is produced in many bye operations; for example, when the fixing bath becomes acid. A very remarkable fact to be noticed about sulphur is that it exists in four different physical forms. One variety occurs in rhombic prisms, a second in monoclinic prisms. (NH2)oCO (NH2>,CS — Urea. = Thiourea. SULPHUR AND ITS COMPOUNDS. 115 O QJ cJ ^ o "9 •g .2 ^00 A ■§1 ’§ ® Si, S' I II o ” w M rP S W A A o a m ^ § I rri ’-^ p ^ -u 3 Pi Ph o p< w d a- j;! s :3 Q> GQ I " Pii<' U1 cp ■pi 'S P< o I II .2 3 So i> 3 O) P il O « Salts = Mwcyan«2!(?5 or sulphocyanides. 116 PHOTOGRAPPIIC CHEMISTRY. Another variety is plastic, and can be drawn out in a similar manner to india-rubber. The fourth variety is non-crystalline and non-plastic. It is in this last modification, known as “ milk of sul- phur,’^ that it is met with in photography. Add a small quantity of hydrochloric acid to a solu- tion of hypo, and notice the white deposit of sulphur produced. Allotropism . — When an element occurs in more than one physical form, which are each identical chemically, the phenomenon is termed one of “ allotropism,’’ and the physical varieties are termed allotropic modifications. As will be seen later, metallic silver exists in various allotropic forms. Sulphur Dioxide . — When sulphur is burnt in the air or oxygen it undergoes oxidation, forming sulphur dioxide. S + O, = SO,. It is better prepared in the following manner ; Fit up the apparatus as described under “ Chlorine ” (p. 103). Into the small flask place some sodium sulphite, and cover wfith a little concentrated hydrochloric acid. Warm cautiously and collect the gas by downward displacement. The operation is best carried out in the open air, care being taken not to breathe the gas. Equa- tion : — Na,S 03 + 2HC1 = 2NaCl + SO, + H,0 Sodium sulphite. Note that the gas has the odour of burnt sul- phur. Pass some of the gas into water, and then treat as under with successive portions : — {a) Add a little blue litmus solution. Note that it is turned red. This is due to the presence of sulphurous acid, which is produced in accord- ance with the following equation : — H,0 + SO, = H,S03. {h) Introduce a piece of red paper into some fetTLPHtJR AND ITS COMPOUNDS. 11 ? of the solution, or pass the gas through some old brown pyro solution. Notice that the red or brown colour, as the case may be, gradually dis- appears. This shows that sulphur dioxide is a reducing agent. The salts of sulphurous acid are the sulphites. Sulphurous acid. Sodium bisulphite. Sodium sulphite. Tteducing Action of Sulphites . — These salts are reducing agents like sulphurous acid. They gradually oxidise on exposure to air, forming the corresponding sulphates : — NaHS 03 + O = NaHSO, ISsiSO, + O = Na,SO, When developing with pyrogallol, dark-coloured oxidation products are obtained which stain the gelatine. The longer the development the deeper is the colour. To a great extent the presence of a sulphite in the developing solution prevents the formation of this staining. There is, however, a limit to the amount of sulphite which can be used, as a large excess of this substance acts as a restrainer and practically stops development. Sulphur Dioxide from Hypo.— Add to a little strong solution of hypo some hydrochloric acid. Notice the presence of sulphur dioxide, and ob- serve the deposit of sulphur (milk of sulphur). The change taking place in this case is this : — Na,S 203 + 2HC1 - 2NaCl + H,0 + SO^ + S Sodium tliiosulpbfite. (Hypo.) Note especially that : — (1) Any sulphite treated with acid produces sulphur dioxide. (2) Any thiosulphate treated with acid pro- duces sulphur dioxide and sulphur. Thiosulphates . — These compounds may be re- garded as sulphates in which an atom of oxygen 118 PHOTOGRAPHIC CHEMISTRY. has been replaced by an atom of sulphur. They are derivatives of a very unstable acid known as thiosulphuric acid (H2S0O3). Sodium thiosul- phate, or hyposulphite, commonly termed hypo, is obtained by boiling an aqueous solution of neutral sodium sulphite with flowers of sulphur. Na2S03 -t- S = Na2S203 Sodium sulphite. P- Sodium thiosulphate. It might be noted here that the term hyposulphite for this compound is, strictly speaking, wrong. Sodium hyposulphite is the sodium salt of hypo- sulphurous acid, H2SO2, and would have the formula Na2S02. It has very different properties from sodium thiosulphate, Na2S20a. Sodium thiosulphate crystallises with five molecules of water in large colourless prisms. It is somewhat deliquescent if exposed to the air. An iodine solution is instantly decolorised by a solution of sodium thiosulphate, with the formation of a compound termed sodium tetra-thionate and sodium iodide. :>S.O, Na Na'/ Na.SjOe + 2NaI. Sodium tetra-thionate. Two molecules of sodium thiosulphate. Fixing by Means of Thiosulphate . — Sodium thiosulphate has the property of dissolving the silver haloids, and this is taken advantage of in removing the unaltered silver salt from the ex- posed negative or paper. They are then said to l3e “ fixed.’’ If a concentrated solution of sodium thiosulphate is added to silver chloride it dis- solves and goes into solution as sodium silver thiosulphate. This may be represented as follows : — (1) Ag Ag Silver thiosulphate. Cl + Na Cl + Na \ Ag >S,03= >S,03 + 2NaCl Aar SULPHUR AND ITS COMPOUNDS. 119 (2) Ag,SA + Na,SA = Na,Ag,(SA) Sodium silver tliiosuipiiate. In the combined toning and fixing bath it is prob- able that the thiosulphate plays an important part in the toning by forming lead and gold thio- sulphates, as well as doing its duty as a fixing agent. The combined bath is acid in its reaction, and great care must be taken to see that the prints are thoroughly washed ; it has already been noted that thiosulphates in the presence of acid gradu- ally undergo decomposition, producing sulphur dioxide and free sulphur. These substances, of course, bleach, and gradually destroy the print. Old baths containing thiosulphate gradually undergo oxidation, with the liberation of sulphur, etc., and should not be used for fixing, as they will bleach or discolour the print or negative. Removal of Thiosulphate. — Various substances have been suggested in order to remove the thio- sulphate from the print or negative in a quicker manner than by washing. Some of these are : iodine, bromine water, bichromates, perman- ganate, etc. These should be used with caution, as they more or less attack the film or image. Lumiere Bros, and Seyewitz recommend a neutral or alkaline solution of ammonium persulphate. It is rendered neutral, or alkaline, by adding to it the alkaline bicarbonates, carbonates, acetates, tungstates, etc. The F er sulphates. —Hh.Q free acid, persulphuric acid (H2S2O8), has not been isolated in a pure condition. A solution of the acid is obtained at the anode when an electric current is made to pass through a well-cooled solution of sulphuric acid. If an electric current is passed through a well-cooled, concentrated solution of the sulphates, the persulphates are formed at the anode. Employment of Persulphates in Photography. — These persulphates are used in certain photo- 120 PHOTOGBAPHIC CHEMISTBY. mechanical processes where etching is necessary and as a photographic “ reducer.’^ They etch metals by converting them into sulphates. For instance, the action of ammonium persulphate on metallic zinc is as follows : — (NHJ.S.Os + Zn = (NHJ^SO, + ZnSO, Ammonium Ammonium ■, Zinc persulphate. sulphate. ^ sulphate. Ammonium Persulphate in Photographic Re- duction . — It sometimes happens that a negative after development is too dense, and has to be reduced. To bring this about, “ reducers are employed. The density of an image is due to the amount of metallic silver on the plate, and the principle of reduction is the gradual conversion of this silver into some soluble salt so that it can be removed. Ferric chloride in the presence of metallic silver is converted into ferrous chloride, silver chloride being produced at the same time, which is removed by sodium thiosulphate. FeClg + Ag = FeCL + AgCl. Potassium ferricyanide and sodium thiosulphate also bring about reduction owing to the formation of silver ferricyanide, which is soluble in the thiosulphate. Lately ammonium persulphate has come into use towards this end. This compound, when used as a reducing agent, diminishes con- trast — that is, it attacks the high lights, repre- sented by the greatest deposit of silver, in prefer- ence to the shade. The persulphate attacks the gelatine film, containing the most silver, convert- ing it into silver sulphate, and is itself reduced to the ordinary sulphate, accompanied by the formation of sulphuric acid and oxygen. Ammonium " Thiocyanate or Sulpho cyanide . — ■ This compound may be obtained by heating a solu- tion of ammonium cyanide with sulphur. It is most readily procured by heating carbon disul- phide (an inflammable liquid obtained by passing SULPHUR AND ITS COMPOUNDS. 121 sulphur vapour over heated carbon) with alco- holic ammonia. 2NH,CN + 2S = 2NH,CNS. Ammonium cyanide. = Ammonium thiocyanate. CB, + 4NH3 = NH,CNS I- (NH,),S. Carbon Ammonium Ammonium disulphide. thiocyanate. sulphide. It may be purified by recrystallisation from water or alcohol. It is used in photography in con- junction with gold chloride in the operation of toning. Thiocarh amide or Thio-urea (CS(NHo)n). — When ammonium thiocyanate is heated to about 110-180° C. it undergoes a peculiar intra-molecular change and passes into thio-urea. NH.CNS 1 > CS Ammonium NH thiocyanate. Thfocarkmiide. This compound, thio-urea, has the property, in the presence of a developer, of inducing reversal — that is, a positive is obtained instead of a negative. 122 CHAPTER XL METALS, ALKALI METALS, ETC. Group of the Alkali Metals . — So far only the chemistry of the more important non-metals tak- ing part in photographic operations has been considered, very little mention being made of the metals. It is now proposed to consider, in an ele- mentary manner, a few important metals used, in various forms, in photographic work. By making a detailed study of the metals it is found that they divide themselves into certain well- characterised groups. One important group is known as the alkali metals. This group includes potassium, sodium, lithium, rubidium, and caesium. All these are used, with the exception, perhaps, of rubidium, in photography. As a rule, the non-metal combined with them is of more importance than the actual metal. Photographic Uses of Alkali Metals . — The bro- mides, iodides, and chlorides of the alkali metals are used in emulsion making and as restrainers. Their carbonates and sulphites are used in de- velopment, and thiosulphates and cyanides in fixing. With regard to the metals themselves they are all very similar in chemical properties. They are soft, easily fusible metals, decomposing water wdth great care, being at the same time converted into the corresponding hydroxide or hydrate. Na^ + = 2XaOH + Sodium. Caustic soda. Sodium hydroxide. All the metals are monovalent. Although the alkali metals show a great similarity in chemical behaviour, there are one or two points to be METALS, ALKALI METALS, ETC 123 noticed by the photographer. In the first place, potassium salts are more reactive, or stronger in their chemical behaviour than the sodium com- pounds, and secondly the potassium compounds in most cases are more soluble in water than the corresponding salts of sodium. The Metals of the Alkaline Earths. — Three com- mon metals constitute this group, barium, stron- tium, and calcium, and they manifest a very similar chemical behaviour. The metals them- selves are isolated with difficulty, and, like the alkali metals, decompose water readily with the formation of the hydrate. 20a + 4H,0 = 2Ca + 2H, Calcium hydrate. Here, again, in most cases it is the acid or non- metal in combination with the barium, strontium, or calcium, which is important in photography. Calcium G onvpoands. — Calcium carbonate or chalk (CaCOg) is often used for neutralising acids. 2HC1 + CaC 03 = CaCL + H,0 -f CO^ The calcium haloids are used in the preparation of certain emulsions. They react with the silver nitrate in the same manner as the potassium or sodium haloids. SAgNOg + Cal2 = 2AgI -f- Ca(N0s)2 Calcium iodide. Calcium nitrate. The fused calcium chloride, owing to its pro- perty of absorbing water, is used as a drying agent in the storing of platinum paper. Bleach- ing powder, having the formula CaOCL (calcium chlorohypochlorite) is used in photographic re- duction. Barium and Strontium C ompounds. — The haloid compounds of these metals are also used in emulsion making. SrCl^ + 2AgN03 - 2AgCl + Sr(N03)2 Strontium chloride. Strontium nitrate. 124 MoTOGRAPmC chemistry. Magnesium Group . — Four metals are met with here — magnesium, zinc, cadmium, and mercury. They are very similar in most of their chemical reactions, but do not show such a complete anal- ogy as the alkali metals. Magnesium is exten- sively used in the preparatioij of flash-light mix- tures. These, as a rule, consist of magnesium and finely-divided aluminium, with potassium chlor- ate, or some other body rich in oxygen. The “ flash ” or light given out by the burning mag- nesium is rich in chemically active rays. The equation is : — 2Mg -f- O 2 = 2MgO Magnesium. Magnesium oxide. Great care should be exercised when working with these “ flash-light ” mixtures. The zinc and cad- mium haloids are used in emulsion making. Znlj + 2AgN03 = 2AgI -f Zn(N03)2 Zinc iodide. Zinc nitrate. CdBr 2 + 2 AgN 03 = 2AgBr + Cadmium Cadmium bromide. nitrate. Mercury . — This is the only metal which is a liquid at ordinary temperatures. The vapours of this metal were used for developing the Daguer- reotype image. Silver plates previously treated with the vapours of a halogen were exposed to light. On treatment with mercury vapour, the vapour condensed on the parts exposed to the light and produced a visible image. Mercury dissolves almost all metals (not iron), and the resulting products are termed amalgams. In the intensification process, using mercuric chloride and then ferrous oxalate, an amalgam of silver is produced in the final reaction. Mercury Haloids . — Mention has already been made of the fact that mercury forms two chlorides — the mercurous and mercuric compounds. These are the chief mercury salts used in photography. METALS, ALKALI METALS, ETC. 125 Mercuric Chloride (HgCL). Corrosive sub- limate. — This compound is obtained on the large scale by subliming a mixture of mercuric sulphate and sodium chloride. HgSO, + 2NaCl = HgCl, + NaSO, Mercuric sulphate. Mercuric chloride. It is not very soluble in water, but is more soluble in alcohol. Mercuric chloride also forms double chlorides with many metallic chlorides. HgCl^ + NaCl - HgNaCl 3 With ammonia a bulky white precipitate is pro- duced of varying composition. Action of Silver on Mercuric Chloride . — Metallic silver, when treated with mercuric chloride solutions, abstracts an atom of chlorine with the production of argentic mercurous chloride : Ag -f HgCL = AgHgCl^ Argentic mercurous chloride. This fact is taken advantage of in intensification, the AgHgCla compound being referred to as the “ bleached image.” Action of Light on IlgCU^. — A solution of mer- curic chloride undergoes photo-reduction on ex- posure to light, with the separation of mercurous chloride. This change may be represented in the following manner : — H H >0 = 2HgCl Mercurous ^°\C1 Mercuric chloride. chloride. Copper, Silver, and Gold. — This group, con- taining, as it does, silver and gold, is the most important one to the photographer, and is there- fore dealt with more fully than the preceding groups. 126 PHOTOGRAPHIC CHEMISTRY. Copper . — This metal is not of great import- ance in photographic operations. Cupric salts undergo photo-reduction in the presence of light, the action being assisted by the presence of oxidis- able compounds, as in the case of mercuric salts. Thus cupric chloride is reduced to cuprot4S chloride. /Cl Nci ^ H Ici + H Cl Cupric chloride. O = 2CuCl + 2HC1 + O Cuproiii- chloride. This cupric chloride, in conjunction with common salt, was proposed by J. Spiller in 1883 as a means of reducing dense negatives. The silver on the plate reacts with the cupric chloride as fol- lows : — CuCla + Ag = CuCl -1- AgCl On plate. The salt then removes the silver chloride as fast as it is formed. Silver . — This is an extremely lustrous metal, crystallising in cubes or eight-sided crystals. When obtained from its compounds by means of nascent hydrogen it appears as a grey powder. In very thin sheets it is blue when viewed by transmitted light. Molten silver absorbs about twenty times its volume of oxygen. On cooling, this gas is liberated almost completely; the silver still retains a small quantity of oxygen even at ordinary temperatures. Silver is harder than gold, softer than copper, and in its chemical be- haviour it is very similar to these metals. Allotropic Modifications of Silver . — In addi- tion to the ordinary form of white metallic silver there are several other varieties, which differ in a very marked manner as regards appearance, their action as regards light, and solubility in water. They were discovered by Carey Lea, who METALS, ALKALI METALS, ETC. 127 carried out a series of experiments on the proper- ties of silver precipitated from its solutions by means of reducing agents, such as ferrous citrate, ferrous tartrate, and dextrin, in the presence of an alkali. The precipitates of silver show almost every shade of colour, from blue to red, green, purple, and golden. Action of Nitric Acid on Silver. — Silver readily dissolves in dilute nitric acid, with the formation of silver nitrate and the evolution of oxides of nitrogen. It crystallises in rhombic tables isomorphous with potassium nitrate. When perfectly pure, it is unaffected by light. Organic substances turn it black owing to the production of silver. Action of Other Acids on Silver. — Dilute sul- phuric acid has practically no action on silver. Hot concentrated sulphuric acid produces silver sulphate. 2H,S04 + 2Ag = Ag,SO, + SO, 4- 2H,0 Silver sulphate. Hydrochloric acid has a very slight action on the metal. Hydrobromic and hydriodic acids attack sil- ver, with the production of the corresponding silver haloids accompanied by the evolution of hydrogen. 2HI + Ago = 2AgI + H 2 2HBr + Ago = 2AgBr + H 2 Action of Potassium Iodide and Cyanide on Silver. — A solution of potassium iodide in the presence of air slowly dissolves silver with the formation of potassio-silver-iodide (AgKIo). This may be represented by an equation as follows : — 4KI H- O + H 2 O + 2Ag = 2KOH + 2AgKl2 Hot potassium cyanide solution acts on metallic silver to produce a solution of potassio-silver- cyanide as follows : — 128 PHOTOGRAPHIC CHEMISTRY. 4KCN + 2HOH + 2Ag = 2KOH + 2AgK(CN>2 + H,. Silver Haloids. — Silver Chloride {AgCl ). — This compound is produced when any soluble chloride, such as those of sodium, cadmium, am- monium, zinc, lithium, etc., is added to a solu- tion of silver nitrate. Attention has been already drawn to this in the preceding articles. Stas dis- tinguishes four allotropic varieties, two common forms being the flocculent and the powdery. It is soluble to a slight extent in boiling water and concentrated hydrochloric acid. It is also solu- ble, to a far greater extent, in concentrated solu- tions of the chlorides of Ba, Sr, Ca, Mg, Na, and K. One litre of aqueous ammonia of sp. gr. 0’924 dissolves 69*5 grammes of silver chloride. 2NH,0H + AgCl = AgCl2NH3 Soluble. Dry silver chloride absorbs ammonia gas to form a white compound having the formula 2AgCl3NH3. Potassium cyanide readily dissolves silver chloride, forming potassio-silver-cyanide. AgCl + 2KCN = AgK(CN)2 + KOI. The best solvent for silver chloride is undoubtedly sodium thiosulphate. Mercuric nitrate and silver nitrate in concentrated solution also dissolve sil- ver chloride. Silver Bromide (AgBr). — This compound is produced by treating silver nitrate solution with any soluble bromide. Stas describes six allotropic modifications. Silver bromide dissolves in most of the sol- vents mentioned under silver chloride. It dis- solves, however, with more difficulty in ammonia than in the chloride. In other respects it is per- fectly similar to the latter. Silver Iodide (Agl). — Like the chloride and bromide, this compound is obtained by adding any J^IETALS, ALKALI METALS, ETC. 129 soluble iodide to a solution of silver nitrate. Two forms of the iodide apparently exist. One variety, obtained by precipitating an alkaline iodide with an excess of silver nitrate solution, is of a curdy yellow appearance, and the other is produced as a yellow powder by adding excess of alkaline iodide solution to silver nitrate. This last is more sensitive to light than the curdy yellow modification. Silver iodide is distinguished from the chloride and bromide by its yellowish-green colour and its insolubility in ammonia. In other respects it behaves like the chloride. It is soluble in silver nitrate solution, provided it contains more than 3 per cent. AgNOg. On the addition of water the iodide is reprecipitated. Gold. — This metal, in the form of chloride or in combination with chlorine and sodium chloride as sodium-chloraurate, is used in toning. It ap- parently exists in three allotropic modifications. One variety is ordinary gold of a bright yellow lustre, and if beaten out into thin sheets (gold leaf) is green by transmitted light. The second variety is obtained as a dark brown powder when many reducing agents, such as ferrous sulphate, oxalic acid, etc., are added to solutions of gold. The third variety is soluble in water, and is ob- tained in a rather complicated manner hy what is known as Heinrich’s method. Gold salts may easily he recognised by the hrown precipitate of metallic gold which is thrown down when ferrous salts are added. Two Classes of Gold Salts. — Gold exists in two conditions in its comi3ounds; as aurous, in which it is monovalent, and as auric, in which it is trivalent. The auric compounds are used in photography. The metal is soluble in aqua regia, and solutions containing free chlorine (see “ Gold Chloride,” p. 111). The chloride has an acid re- action, and has the composition HAuCl^, hence it is termed chlorauric acid[. I 130 PHOTOGRAPHIC CHEMISTRY. H K NH^ Na II I I Au Au + 2H2O Au + 2IH2O Au + 2H2O II I i cu Cl, Cl, Cl, y . " f ' Y ' Chlor- Potassium Ammonium Sodium auric acid. cliloiaurate. cliloraurate. chloraurate. Aurates . — Auric acid (HAuOa). By heating auric chloride (A11CI3) with magnesia, and then treating with dilute nitric acid to remove excess of Mg, auric oxide is obtained as follows : — 2AUCI3 + 3MgO = AU2O3 + SMgCla This auric oxide dissolves in caustic potash to form potassium aurate, which crystallises in bright yellow needles. AU0O3 + 2KOH = 2KAUO2 + H2O Potassium aurate. On the addition of silver nitrate solution to this potassium aurate a precipitate of silver aurate is obtained. KAuOa + AglSrOg = AgAu02 + KNO3 Silver aurate. Uranium . — Uranium is used in photographic work for toning broihide prints and in the uranium intensifying process. With regard to the chemistry of this metal, all that the photographer need notice is that, in the first place, it has a valency of four in uranous compounds and a val- ency of six in uranic compounds. Secondly, the oxide UO2, termed uranyl, takes the place of the simple metal uranium in its salts. Thus : U02(N03)2 UO2SO, Urauyl nitrate. Uranyl sulphate. Platinum - — This metal, like gold, is not at- tacked by acids; it is only soluble in liquids generating free chlorine, such as aqua regia. It forms two classes of salts; in the platinous com- pounds it is divalent, and in the platinic tetra- valent. METALS, ALKALI METALS, ETC. 131 PtCl, PtCl, Platinous chlor'de. Platinic chloride. Platinic Chloride (PtCl^) is obtained when platinum is dissolved in aqua regia. On evapora- tion the chloroplatinic acid separates in brownish- red deliquescent crystals, containing six molecules of water. It forms characteristic double chlorides with ammonium and potassium, which are only soluble in water' with difficulty. PtCl, + 2HC1 - H^PtCle Cliloroplatinic acid. PtCl, + 2KC1 = K.PtCl^ Potassium chloroi>latinate. Platinous Chloride (PtCL). — By cautiously heating platinic chloride to 200° to 250° C., it loses a molecule of chlorine and is converted into solu- ble platinous chloride, a greenish powder. PtCl, = PtCl^ -f CL Platinic chloride. Platinous chloride. With hydrochloric acid the platinous chloride forms easily soluble chloroplatinous acid. PtCL + 2HC1 = H,PtCl, Chloroplatinous acid. With the alkaline chlorides it forms easily solu- ble chloroplatinites : PtCL + 2KC1 = K.PtCL Potassium chloroplatinite. The photographer will see the connection better, perhaps, between the various compounds men- tioned above if they are compared as under : PtCL — ^ H.PtCl, — 1> K.PtCL Platinic {> Chhjroplatinic p Potassium chloride. acid. chloroplatinate. V PtCl, — > H.PtCl, — > KjPtCI. Platinous P Chloro]datinous Potassium chloride. acid. chloroplatinite. Platinotype . — The salt used in platinotype printing is potassium chloroplatinite. In prac- 132 PHOTOGKAPHIC CHEMISTRY. tice it is conveniently prepared by reducing the chloroplatin«^6 with freshly precipitated cuprous chloride. K^PtCle + CU2CI2 = K^PtCl, + CuCl^ Potassium + Cuprows > Potassium + Cupric chloroplatinate. cliloride. chloroplatinite. chloride. It is essential to the proper working of platinum paper that it should be kept dry, and towards this end fused calcium chloride is employed in the tins used for storing it. The Alums . — These compounds, of which several are used in photographic work, are obtained by crystallising together the sulphates of aluminium, chromium, and iron with those of the alkali metals, Na, K, etc. The ordinary, or potash, alum, has the follow- ing formula : K,SO, + AIJSOJ3 + 24H3O Potassium , Aluminium , Water of sulphate. sulphate. cryst illisation. It is important to remember that this substance has an acid reaction. It is principally used for hardening the surface of gelatine films. The connection between potash alum and the other alums is readily seen by examining the following formulae : — Aluminium Alums : KoSO^ + A1 o(S 04)3 + 24H2O Potash alum. Na2 SO4 + ALCSO^^a + 24H2O Soda alum. (NH4)2S04 + ALCSO^)^ + 24H2O Ammonia alum. Chromium Alums : K,SO. + Cr,(SO,), + 24 H,OP— Na,SO, + Cr,(SO,>3 + 24H3O „ (NHJ^SO, + Cr3(SOj3 + 24H30^™omralum. Iron Alums : K 3 SO. + Fe3(S03>3 + 24H30>^?“Z. Na^SO, + Fe3(S03>3 + 24H3O (NHJ 3 SO. + Fe3(S03>3 + 24H30^SKum.' ^METALS, ALKALI METALS, ETC. 133 The Cyanides . — Compounds containing the group (CN) are numerous and important. The chief compounds, from a photographic point of view, are potassium cyanide and the ferro- and ferri-cyanides. . Potassium Cyanide (KCN).— This substance is the potassium salt of prussic or hydrocyanic acid (HCN). The cyanide forms double salts with metals, which are, as a rule, very soluble in water. Owing to this fact, potassium cyanide is used as a fixing agent. It readily dissolves the silver haloids forming potassio-silver-cyanide. The equations representing the change are : 2KCN + AgCl = AgK(CN)2 + KOI. 2KCN H- AgBr = AgK(CN). + KBr. 2KCN + Agl = AgK(CN)^ + KI. Potassio-silver cyanide. Ferro cyanides and Ferricyanides . — These com- pounds are employed in certain processes for bringing about the reduction or intensification of negatives, and for the preparations of blue prints. Potassium ferrocyanide or yellow prussiate of potash has the formula K 4 Fe(CN) 6 . If added to a solution containing a ferric salt a dark blue precipitate of Prussian blue is ob- tained. 3 K 4 Fe"(CN )3 + 4 FeCl 3 = Fe/[Fe'"(CN) J 3 -t- Potassinm Ferric Prussian blue 12KC1 ferrocyanide. chloride. or ^ Ferric ferrocyanide. In the presence of an oxidising agent, such as chlorine, potassium ferrocyanide is converted into potassium ferricyanide or red prussiate of potash. Conversely, potassium ferricyanide in the presence of organic matter, especially under the influence of light, is reduced to the ferrocyanide. 134 CHAPTER XII. ORGANIC OR CARBON COMPOUNDS USED IN PHOTOGRAPHY. Meaning of “ Organic^ — Perhaps the most im- portant compounds used in photography are the so-called “ organic ” developers, such as pyro- gallol, amidol, eikonogen, etc. Before discussing these substances it would be as well to have some idea of the meaning of the term “ organic.” In the early days of chemistry two classes of chemi- cal compounds were recognised, those obtained from mineral substances and those of animal or vegetable origin. It was held, at that time, that substances of the latter character could not be obtained artificially. Organic Compounds Obtained Artificially . — In 1826, however, Hennell obtained ethyl alcohol artificially, and in 1828 Wohler synthesised an essentially organic substance, urea. Neither of these investigators utilised the living organism for the production of those compounds. The terms ‘‘ organic ” and “ inorganic ” are, however, still retained, simply for the sake of convenience. All organic compounds contain the element carbon, consequently organic chemistry is often defined as the chemistry of the carbon compounds. Classification of Carbon C ompounds. — As the result of a detailed study of organic compounds, they are found to fall naturally into two groups. One division contains those whose properties and reactions can only be explained by assuming that the carbon atoms in the molecule are arranged in the following manner : — ORGANIC OR CARBON COMPOUNDS. lo5 ! I I i _c— C— C— C— etc. I I I I As some of the most important members of this group were found in various fats and oils, they were termed “ fatty compounds.” A better way of referring to them, however, is to call them “ open chain ” compounds, owing to the manner in which their carbon atomiS are arranged in the molecule. The second group behave in a different way towards chemical reagents, and this differ- ence of behaviour can only be accounted for on the assumption that the carbon atoms in the mole- cule are arranged in the form of a closed ring, or cycle, as below : -C~C-^ I I C-C- I I \ / \ c / \ / ^ c / \ Hence they are termed “ closed chain ” or cyclic bodies. They are also known by the name of “ aromatic ” compounds, because at one time their most characteristic substances were obtained from the various aromatic gums and balsams. Molecular and Constitutional Formula :. — By making a qualitative examination of the organic compounds their component elements are ascer- tained, and those usually present are found to be carbon, hydrogen, oxygen, nitrogen, and sulphur. If the compound is submitted to a quantitative analysis, the percentage of each element is ob- tained; and if its physical properties in the state of vapour are examined, the chemist is then in a position to state the number of atoms of each element present in the molecule. [For a detailed 138 PHOTOGRAPHIC CHEMISTRY. explanation of these processes the photographer is referred to any text-book on organic chemistry.] By writing the symbols of these elements, together with their proper exponents, the molecular for- mula is obtained. Thus : Ethyl alcohol contains the elements carbon, hydrogen, and oxygen. From a quantitative and physical examination of the substance it is found to contain in the mole- cule two atoms of carbon, six atoms of hydrogen, and one atom of oxygen ; its molecular formula is, therefore, CaHgO. The constitutional formula of ethyl alcohol will be found on p. 139. Isomerism. — It so happens that a large number of organic compounds have the same molecular formulae, but have different chemical and physical properties. This peculiarity is termed isomerism, and the compounds are said to be isomeric. Thus the formula CgHgOa represents three important compounds, pyrocatechol, resorcinol, and hydro- quinone. The formula CaHgO represents ethyl alcohol and methyl ether. By making a careful study of the reactions and transpositions of iso- meric substances they are found to differ in chemi- cal deportment, which leads to the assumption that their molecules are differently arranged or constituted. A formula which shows this arrange- ment of the atoms in the molecule is termed a constitutional formula, and a knowledge of the latter is obviously highly important in dealing with organic compounds. “ Ox>en Chain G ompounds. — It is now pro- posed to consider a few “ open chain ’’ compounds used in photographic work. With regard to the hydrocarbons, the only one, perhaps, that the photographer will have to deal with is acetylene, which is used for illuminating purposes. This hydrocarbon belongs to a series of compounds having the general formula CnH 2 n — 2, wherein is the number of carbon atoms. It is obtained by treating calcium carbide with water. ORGANIC OR CARBON COMPOUNDS. 137 CaCs + 2H2O = Ca(OH )2 + Calcium Calcium Acetylene, carbide, hydroxide. It will suffice to mention here that it is poison- ous, and forms a highly explosive mixture with air. The Alcohol Family . — Methyl alcohol, CH3OH. This compound is used as a solvent for varnish making, etc. In the presence of an oxidising agent it undergoes oxidation, producing in the first place formaldehyde and then formic acid. H I H— C-O’H + 0; I i i :h i ■ H I H-C = 0 + O = H H-C-0 + H,0 Formaldehyde, OH I H— 0 = 0 or HCOOH Formic acid. Ethyl Alcohol, C 2 H 5 OH. — This is the next al- cohol in the series and has a variety of uses in photography, principally as a solvent. When mixed with methyl alcohol (commercially known as wood spirit) it is termed methylated spirit. Submitted to oxidising agents it passes first to acetaldehyde, the next aldehyde to formaldehyde, and then to acetic acid, the next acid in the series to formic acid. If ethyl alcohol is treated with strong sulphuric acid and then heated to about 147° C., a molecule of water is abstracted from two molecules of the alcohol, and ether produced. Aldehyde Family. — Formaldehyde, or formalin HCHO. Formaldehyde is obtained by oxidising methyl alcohol, as already mentioned. It is used in photography for hardening the gelatine films, so as to prevent frilling in hot weather, and also as a preservative of mountants, as it destroys bacteria. At ordinary temperature the formalde- 138 PHOTOGRAPHIC CHEMISTRY. hyde is a gas, and it is usually met with in prac- tice as a 40 per cent, solution under the name of formalin. Polymerism . — x\fter standing, the formalin undergoes a very remarkable change, producing a variety of compounds which, on analysis, are found to be multiples of HCHO. HCHO [HCHO], [HCHO], Formaldehyde. Paraformaldehyde. Metaformaldehyde. Compounds which condense with themselves to produce new compounds, as in the case of form- aldehyde, are said to undergo polymerisation, and the new compounds formed are termed polymers of the original substance. If exposed to the air it undergoes oxidation, producing formic acid. H OH I I ,HC = 0 -f ,0 = ,H-C = 0 Formic acid. Owing to this fact, formaldehyde and the alde^ hydes as a class are powerful reducing agents. This reducing action is readily seen by adding formalin to an ammoniacal solution of silver nitrate. After a short time silver separates on the sides of the vessel as a brilliant mirror. Acetaldehyde, CH,CHO. — This is the aldehyde of acetic acid, and may be obtained by adding an oxidising agent to ethyl alcohol. C,H,OH + O = CH 3 CHO -I- H,0 Ethyl alcohol. ^ Acetaldehyde. The aldehyde combines directly with prussic acid. HCN, and the alkaline bisulphites; with HCN they produce compounds termed cyanhydrins. Family of Organic Acids . — The organic acids form a very large group of compounds, and for purposes of study they are divided into various sub-groups, according to the radicals present. For instance, acids containing one carboxyl group are termed monocarboxylic acids, those containing two are called dicarboxylic acids, and so on. If ORGANIC OR CARBON COMPOUNDS. 139 the acid contains hydroxyl groups as well, they are said to be hydroxy-carboxylic acids, mono, di, or tri, etc., as the case may be. The most common organic acids used in photography are probably acetic, oxalic, and citric acids, together with their salts. Mono-Carhoxylic Acid Family . — Acetic acid, CH 3 COOH. This is a very common acid, and in a dilute solution, together with colouring matter, it constitutes vinegar. Ordinary brown vinegar is obtained by allowing sour beer, etc., to undergo bacterial oxidation. If spirits or white wines are used in place of the beer, white vinegar is obtained. As is well known, beer and spirits contain ethyl alcohol, and when this compound undergoes oxidation it produces acetic acid. It it due to this acid that vinegar has a sharp taste. CHg CHg CH3 ' ITTIT —> H-C -f- O — {> C— OH II II H 0 0 C 2 H 5 OH — 1 > CH 3 CHO — i> CH,,COOH Etliyl alcohol. Acetaldehyde. Aeetic acid. The pure, concentrated acetic acid is known under the name of glacial acetic acid, it being this variety that is principally used in photography. Acetic acid is used with ferrous sulphate in de- veloping wet collodion plates. The acid acts as a restrainer, by preventing the too rapid deposi- tion of silver. It dissolves the silver, forming silver acetate. 2CH3COOH + 2Ag - 2CH3COOAg -f H3 Silver acetate. This acid is also employed in the lead and uran- ium intensifiers in order to keep these solutions weakly acid, and for washing bromide prints after development with ferrous oxalate, or toning with uranium salts. It should contain no fur- H-Gi I O 140 PHOTOGRAPHIC CHEMISTRY. furol or formic acid, as these substances are harm> ful for photographic purposes. Test for Furfural . — This is a cyclic body con- taining oxygen, and causes complications by act- ing as a reducing agent. It may be detected in minute quantities by adding a drop of aniline to the suspected acetic acid. If present, a deep red colour is produced, disappearing on standing. Test for Formic Acid . — This acid acts as a powerful reducing agent. It is detected by add- ing a solution of silver nitrate. A brown precipi- tate of reduced silver shows that formic acid is present. In some cases acetic acid is adulterated with the mineral acids, hydrochloric and sul- phuric acids. Their presence may be shown by using the reagents mentioned under nitric acid. These impurities may be removed by distilling the acetic acid from a retort to which a little potassium bisulphate and bichromate have been added. Di-Carhoxylic Acids . — Oxalic acid, H2C2O4 or COOH. I COOH. This acid is obtained by oxidising sawdust, by fusion with caustic potash. It is also obtained when many complex organic compounds, such as sugar, are heated with strong nitric acid. In photographic work the acid is employed in the form of its ferrous salt, in the well-known ferrous oxalate developer. Ferrous oxalate itself is in- soluble in water, but is readily soluble in a solu- tion of potassium oxalate, consequently the de- veloper is so arranged that this potassio-ferrous oxalate is present. This condition is obtained by mixing a solution of ferrous sulphate with potas- sium oxalate, and is produced in accordance with the following equation : — ORGANIC OR CARBON COMPOUND: 141 FeSO, + 2K,C,0, = K,Fe(C,OJ, + K,SO, or FeSO, COOK + 2 I COOK COOK i COOK COO. + 1 >Fe + K,SO, coo-^ Potassio-fei'rous oxalate. Action of Light on Ferric Oxalate . — It is inter- esting to notiee that a ferric oxalate solution, which is the compound produced after the ferrous oxalate developer has done its work, is recon- verted to the ferrous state by exposure to the action of sunlight. Fe,(CA)a = 2FeC,0, + 200^ Ferric oxalate. Ferrous oxalate. Because oxalic acid contains two carboxyl groups, it therefore forms two classes of salts. Thus : — COOH 1 COOH COOK I COOH Acid potassium oxalate. COOH I COOK. Neutral potassium oxalate. Oxalic acid and its salts are powerful reducing agents, and it is owing to this fact that they find employment in photography. If they are added to solutions of gold, silver, or platinum, the metal is precipitated. Oxalic acid crystallises with two molecules of water. H yclroxy-tri-C arhoxylic Acids. — Citric acid. CH2(C00H)— CH(OH)COOH— CHXOOH. This acid forms three classes of salts, because it contains three carboxyl groups. The three potas- sium salts are given below. CH^COOH I OH-C— COOH I CH.,COOH Citric acid. CH,COOK OH-C-COOH i CH.COOH Primary potassium citrate. 142 PHOTOGKAPHIC CHEMISTRY. CH^COOK OH-C-COOK CH^COOH Secondary potassium ^citrate. CH^COOK I OH-C-COOK I CH2COOK Tertiary potassium citrate. Sensitisers . — Citric acid and the citrates, in the presence of the silver haloids, increase the photo-decomposition of the latter, and make them more sensitive to the action of light. Hence they are often spoken of as sensitisers. Family of the Ketones . — Practically the only ketone used in photographic work is the di-methyl ketone, or, as it is usually termed, acetone. This compound is obtained technically from crude wood spirit or by the dry distillation of calcium acetate. On the addition of sodium or potassium bi-sul- phite to acetone a white crystalline compound is obtained known as acetone alkaline bi-sulphite. Amiclo Carboxylic xicids. — Glycin, or glycocoll, NH2 — CH2 — COOH. This compound is acetic acid in which one hydrogen atom of the methyl group has been replaced by an amido group (NH2). H NH2 I I CH2COOH CH2COOH. Acetic acid. Glycocoll or amido acetic acid. The photographic developer known by the namt of “ glycin is glycocoll in which one of the amidic hydrogen atoms has been replaced by the complex group — CfHj (OH) — present in carbolic acid or phenol. This CgH^ (OH) group is cyclic in structure, consequently the developer is both a cyclic and open chain compound. Its formula is CsHgOgNi. 143 CHAPTER XIII. PYROXYLINE, ALBUMEN, GELATINE, ETC. Complex Organic C om-pounds. — There are a large number of organic compounds of a verj^ complex nature whose constitution, up to the present, has not been determined. It so happens that these bodies are indispensable in photography, and the art has been brought to a great state of perfec- tion by their employment. Among them may be mentioned cellulose, albumen, and gelatine. Cellulose (Ci2H2oOio)x. — This is the principal constituent of the cell membranes of all plants. It may be obtained in a pure form by submitting wadding or plant fibre to the action of (1) dilute potash, (2) dilute hydrochloric acid, washing with water in each case. It is then treated with alcohol and ether. So obtained, it is a white amorphous mass. Swedish filter paper, which has been sub- mitted to these reagents, consists almost entirely of pure cellulose. This substance is practically insoluble in all the usual solvents, but dissolves, without undergoing any change, in ammoniacal copper solutions. Acids reprecipitate it as a gelatinous mass. It is soluble in concentrated sulphuric acid, depositing a starch-like compound on the addition of water. Cellulose Nitrates, or “ Nitro ” C elluloses . — It has already been noticed that by replacing the hydrogen in nitric acid by a metal the nitrates are obtained. Organic radicals or groups of radicals can also replace the hydrogen of nitric acid, forming an organic nitrate. Thus : — C.H.O -h HNO3 = C2H3ONO2 + H2O Ethyl nitrate. 144 PHOTOGRAPHIC CHEMISTRY. If cellulose is treated with nitric acid various nitrates are obtained. + HNO3 = C,3H,3(0N0,)0, + H,0 Cellulose mono-nitrate. -1- 2HNO3 = C,,H.,(ONO,)A + 2H,0 Cellulose di-nitrate. It will be noticed that water is produced in the reaction. To remove this, and thus prevent it from diluting the nitric acid, strong sulphuric acid is used. The resulting nitrates exhibit varj"- ing properties depending upon their method of formation. Gun-G otton, or Pyroxyline. — If, for example, pure cotton is immersed two or three times in a cold mixture of one part of nitric and three of sulphuric acid, and then washed with water, it is converted into cellulose hexa-nitrate, which is known as gun-cotton, or pyroxyline. + 6HNO3 = C,,H,,(0]SrO,)A 6H,0 Cellulose hexa-nitrate, or pyroxyline. This hexa-nitrate is insoluble in alcohol and ether. Collodion . — If cotton is exposed to the action of a warm mixture of twenty parts of powdered potassium nitrate and thirty parts of concen- trated sulphuric acid, a mixture of tetra and penta cellulose nitrates is obtained, which dis- solves in ether containing a little alcohol. This is termed soluble pyroxyline, and the ether alcohol solution is termed collodion. + 4HNO3 = + 4 H,o Cellulose tetra-niti ate. 5HNO3 = C„H„(0N0,),0, -F 5 H,0 Cellulose penta-nitrate. On allowing collodion to evaporate the pyroxy- line is left as a uniform transparent film. To render it photographically sensitive, the collo- dion is treated with varying mixtures of some soluble iodide and bromide, usually the am- monium and cadmium compounds. The coated PYROXYLINE, ALBUMEN, GELATINE, ETC. 145 plate is then dipped in a solution of silver nitrate, by which means it is covered with a layer of silver bromide and iodide. The equations representing the changes are NH ,1 + AgNOa = Agl + NH^NOg Ammonium Silver Silver Ammonium iodide. nitrate. iodide. nitrate. Cd Bra + 2Ag Cadmium bromide. NO3 = 2AgBr + Cd(N03)a Silver Cadmium bromide. nitrate. The prepared plate is exposed whilst still wet with the silver nitrate solution, as this is the sensitiser. This is a very important point to be noticed, in connection with the wet collodion process, as the collodion by itself has no halogen absorbing power. Celluloid. — The mono, di, and tri nitro com- j pounds of cellulose are dissolved in special sol- I vents, such as acetone and camphor, thereby pro- ducing plastic masses known under the names of celluloid and xylonite, which can be moulded and cut into various forms. I Albumen. — The molecular magnitude of albu- men is unknown. According to Sabanejeff, the [ number 15,000 was obtained as the molecular [I weight of purified egg albumen. Stohmann and I Langbein have given albumen the molecular formula 0720111134850248X218. This may or may not be the true formula, but it is sufficient to show I that albumen contains a very large number of I atoms in the molecule. For photographic use, I purified egg albumen (or white of egg) is em- I ployed. This is soluble in water, but if heated ! to a temperature of about 70 ° C. becomes insol u- I ble, or, as it is termed, coagulated. Many other j substances also coagulate albumen, such as alum, : J 146 photOgeaphio chemistey. nitric acid, methylated spirit, and many metallic salts. ^ When albumen is treated with silver nitrate, an insoluble compound is precipitated containing silver. This compound is either a salt, or double compound, of the silver and albu- men, and it is generally termed silver albuminate. Under the influence of light it suffers decomposi- tion, forming brown-red reduction compounds. The sensitive surface of albumen paper consists of a mixture of silver chloride, silver albuminate, together with an excess of silver nitrate. Gelatine . — When bones, hoofs, etc., are sub- mitted to the action of superheated steam, various nitrogenous substances are extracted, which separate from their watery solution as a jelly on cooling. In this form it is termed “ size.’' If it is dried a hard mass results, forming the glue of commerce, by the purification of which the gela- tine is obtained. Isinglass is a form of gelatine obtained from the air-bladder of the sturgeon. So far, no molecular formula has been assigned to gelatine, but from what has been ascertained, it appears to contain a great number of atoms in the molecule. Tannic acid precipitates gelatine from its aqueous solutions as gelatine tannate, a brownish-yellow sticky substance. Gelatine swells considerably in water, but does not dissolve till heated. On cooling, it separates as a gelatinous mass. According to Eder, a good specimen should produce a firm jelly when a 4 per cent, solution is cooled down to 20° C. On boiling with dilute acids, or alkalis, gelatine undergoes de- composition, producing complex mixtures of amido fatty acids. Aqueous solutions of gelatine slowly decompose on standing, producing am- monia, and substituted ammonias. Photographic Emulsions . — As already stated, solutions of gelatine have the property of gelatin- ising, i.e. separating as a jelly, on cooling, and it is this property which makes that substance so PYROXYLINE, ALBUMEN, GELATINE, ETC. 147 important in preparing photographic emulsions. A gelatino-haloid emulsion consists of a solution of gelatine in water, of such a degree of viscosity that a finely divided precipitate of silver haloid is kept in a state of suspension. This is secured in practice by heating a solution of gelatine with silver nitrate, potassium iodide, and bromide, and then allowing to set. AgNOg + KBr = AgBr + KNO 3 AgN03 + KI = Agl + KNO 3 A very important point must be noticed here; that is, to have sufficient of the potassium haloids to precipitate all the silver. Unless this is done the silver nitrate combines with some of the gela- tine to form a double insoluble compound, which undergoes some decomposition during the heating. When plates covered with such an emulsion are developed, this silver “ gelatino-nitrate ’’ attacks the developer and causes a general fog. Gelatine and collodion emulsions have to undergo a pro- eess of ripening in order to increase their sensi- tiveness to light. Hardening of Gelatine . — The various alums, such as ordinary potash alum and chrome alum, and formaldehyde or formalin, have the property of rendering gelatine hard, and making it in- soluble in water. Hence the use of these sub- stances for the prevention of “ frilling.” Chromium compounds have also this property, especially in the presence of light, and this is taken advantage of in the various bichromate printing processes. 148 CHAPTER XIV. BENZENE AND THE ORGANIC DEVELOPERS. Cyclic or Aromatic Compounds . — The most im- portant cyclic substances used in photographic work are the developers, pyrogallol, hydroquinone, metol, amidol, etc. Before these compounds are considered it is necessary to have some idea of the properties of a few simple cyclic substances, the nomenclature used, etc. * Benzene. C — This is the parent hydro- carbon from which an immense number of cyclic compounds can be derived. Benzene is of very stable behaviour towards chemical reagents. The halogens act in two ways towards the hydro- carbon : (1) they replace hydrogen atoms (substi- tuted compounds) ; (2) they simply add themselves on to the benzene (additive compounds). (1) + CL = CeH.Cl + HCl C,H,C1 + CL = CeH.Cl^ -h HCI, etc. (2) C,H, + Br, - C.HeBr, + HBr CyH^Br -1- Bi\ = CgHeBr^ + HBr, etc. Another point to be noticed is that benzene does not polymerise. If the properties of benzene are compared with those of the fatty or open chain compounds they are found to differ in a very marked manner. Consequently the constitution of benzene cannot be represented as related in any manner to the open chain hydrocarbons. Distinction between Bem.^ne and Benzine . — Benzene is used directly in photography as a sol- vent. Attention may here be drawn to the dis- tinction between benzene and benzme, since both these substances are used as solvents, and appar- ' BENZENE AND THE OKGANIC DEVELOPERS. 149 ently a great amount of confusion exists as to the identity of the two. Benzene, the compound at present under consideration, is obtained during the distillation of coal tar. Benzine is a mixture of open chain paraffin hydrocarbons, a totally different substance. Indirectly, benzene is used in the preparation of most of the organic de- velopers and coloured substances used in colour photography. Benzene Derivatives. — By replacing one or more hydrogen atoms in the benzene molecule, by other atoms, or group of atoms, various deriva- tives are obtained. The introduction of hydroxyl groups produces the various phenolic bodies : — C,He C.H^OH C,H,(OH), C,H3(OH)3 Benzene. Phenol. ' ^ Hydroquinone. Pyrogallol. These compounds are acid in behaviour, conse- quently they dissolve in alkalis. They are designated mono, di, tri hydric phenols accord- ing to the number of hydroxyl groups present. Phenol or Carbolic Acid. C — This com- pound is obtained during the distillation of coal tar. It is the simplest phenol and readily dis- solves in alkalis. It is employed as a preserva- tive in solutions of albumen or gelatine, and for mountants. Nearly all phenols give definite colour reactions with a solution of ferric chloride. Ordinary phenol gives a violet coloration. Ben- zene is formed when phenol vapour is passed over heated zinc dust. Pyrocatecliol^ Resorcinol., Three com- pounds are known, having different chemical and physical properties, yet all having the molecular formula CcH 4 (OH) 2 . These three substances are pyrocatechol, resorcinol, and hydroquinone. In order to distinguish between three isomeric di- hydroxy-benzenes they have received special names. Formula (1) is termed the ortho ^ (2) the metaj and (3) the para derivative. 150 PHOTOGRAPHIC CHEMISTRY. OH 1 OH 1 OH 1 A A A HC C-OH 1 II HC CH HC CH 1 II HC CH 1 II HC .C-OH HC iu / •c C c E 1 I H OH Ortlio-di-hydroxy- Meta-di-liydroxy- Para-di-hydroxy- benzene. benzene. benzene. Pyrocatechol. Resorcinol. Hydroquinone. All these compounds can act as photographic developers, but the most important is the para derivative, hydroquinone. Hydroquinone, Para-di-hydroxy-benzene. — This compound was first suggested as a photographic developer by Captain (now Sir W.) Abney in 1880. It acts more slowly than pyrogallol, and produces rather hard negatives. It is most con- veniently prepared by reducing a body known as quinone (obtained by oxidising aniline, CeHgNHg) with sulphurous acid. + Ha Quinone. O + HaSOa = CeH.(OH), + Hydroquinone. HaSO, Hydroquinone is di-morphous and dissolves readily in water. Its aqueous solution is coloured brown with ammonia. Oxidising agents convert it into quinone. Tri-hydroxy Derivatives of Benzene. — These compounds exist in three isomeric forms, pro- duced, like the di-hydric phenols, by the position of the hydroxyl groups in the benzene molecule. BENZENE AND THE ORGANIC DEVELOPERS. 151 OH ci OH // \ ^ N HC C-OH H-C C-H OH I C //\ H-C C-H Hi C-OH H u C OH OH HO-C C-OH V H Pyrogallol Hydroxy Phlorogluciuol. or • hydroquinone. Pyrogallic acid. V ^ > Isomeric tri-hydric phenols. Pyrogallol. G is the only compound of the three isomeric substances used in photographic work. It melts at 132° C., and is produced by heating gallic acid. The reaction expressing the change is this : — OH OH A HO-C CH HO-i ^-COOH H A HO-C CH 1 II + COa HO-C. CH H Gallic acid. Pyrogallol. Or it may be written : c„h,(oh)3COoh z= c,n,{on), + co^ Gallic acid. Pyrogallic acid. Pyrogallol is extremely soluble in water, and with more difficulty in alcohol and ether. Its alkaline solutions absorb oxygen with great readiness, carbon di-oxide, oxalic and acetic acid, and vari- ous brown colouring matters are produced during the oxidation. A blue colour is imparted to pyro- gallol solutions by ferrous sulphate, and a red by ferric chloride. An iodine solution is turned a purple red on the addition of an aqueous or 162 PHOTOGRAPHIC CHEMISTRY. alcoholic solution of pyrogallol. The latter is a powerful reducing agent, readily precipitating gold, silver, and mercury from their solutions. Amido-Phenolic Substances . — These compounds are, like the di- and tri-hydric phenols, used in photography as developers. If ordinary phenol is taken and treated with nitric acid, under the proper conditions, a mixture of ortho and para* nitro-phenols is obtained. That is, a hydrogen O O 93 Cl 0—0 •• ^ 93 / \ C 0 2: C O -2: k — \ / % * 0-0 X ' ,53 ^ o X / 0-0^ o • X 0=0 \ o-= 0--0 93 93 s o o \ / X fO o O a > o n c X CM o 2: ii: CM X X 0=0 X / \ 0-0 ox ^0-0^ X X 93 o o CM BENZENE AND THE OllGANIC DEVELOPERS. 153 atom in the phenol is replaced by a nitro group, NO 2 (see diagram, p. 152). It must be carefully noted that only hydrogen connected with a carbon atom of the benzene ring is replaced by the nitro group ; the hydroxylic hydrogen remains intact; this is proved by the in- creased acid properties of the compound formed. If the para-nitro-phenol is then separated and treated with any reducing mixture, the nitro group (NO 2 ) is converted into an amido group (NH 2 ), and para-amido-phenol results. Thus : — NO2 + 3H2 = NH2 + 2H2O Nitro group. Amido group. OH 1 OH 1 I c HC HC CH 1 II + 3H2 = 1 11 HC CH HC CH 1 NOe 1 NHj Para-nitro- Para-amido- phenol. phenol. + 2H2O These amido-phenols are both basic and acid in character. The hydroxyl group confers acidic, and the amido group basic properties upon the compounds. RodinaL — This developer is a strong solution of the hydrochloride of para amido-phenol. The hydrochloric acid unites with the amido group. .OH C.H.^ + HCl ''NH, Para-ainido-plienol. .OH - C.H / ^NK.HCl Rodinal, or hydrochloride of i>ara-aniido-phenoI. Di-ainido-phenols . — By introducing two nitro group into ordinary phenol, or another nitro group into para-nitro-phenol, di-nitro-phenol is obtained. 154 PHOTOGEAPHIO CHEMISTEY. This compound, on treatment with reducing agents, is converted into di-amido-phenol. The change taking place may be represented as fol- lows : — + 2H«0 QH H HC CH + 2HNO, = 1 II HC\ CH HC, CH ^C^ H k'o. or T C.H.OH .2HNO, Phenol. Di-nitro-f)henol. On reduction: — OH OH + 2H,0 NO, A + 6H., = HC C-NH + 4H^O HC C-H HC CH ^C^ 1 1 NO, NH, I . T + 6H, » +4HjO \(NO,), Nnhj. Di-nitro-phenol. Di-amido-phenol. Amidol. — The salts of di-amidol-phenol are employed in photography under the name of “ amidol,’’ as developers. It is a powerful reduc- ing agent, but is very unstable, and does not keep for any length of time. It might be noted here that the greater the number of amido groups a developer contains, the more unstable it becomes. BENZENE AND THE OIIGANIC DEVELOPERS. 165 Metol . — Another important developer deserves consideration in connection with these amido phenolic bodies, namely, metol. This compound is a derivative of methyl phenol, or, to give it its common name, cresol. OH 1 OH OH 1 HC CH 1 11 - HC C-CHa H T h HC C-H 1 li- ne C-CHa 1 A H-C CH 1 II HC C-CHj V 1 NHCH3 or, 1 T T /OH -4 C,H 3 <-CH 3 - /OH CeH 3