826 PHOTOGRAPHY was prepared in the jisual way and a solution of zinc nitrate and silver nitrate in water was made to flow over it. The hygroscopic nature of the zinc salt kept sufficient moisture on the plate to attain the desired end. Various modifications in procedure have been made since, but it is scarcely necessary to record them here ; for details the reader may consult the volumes of the Photographic Journal, 1854-55. Collo- Dry Plates. It would appear that the first experiments dion dry w Jth collodion dry plates were due to M. Gaudin. In plates. La L um ^ re O f 22d April and 27th May 1854 he describes his researches on the question ; whilst in England Mr G. R. Muirhead, on the 4th August 1854, stated that light acts almost as energetically on a dry surface as on a wet after all the silver has been washed away from the former previous to desiccation. Dr Taupenot, however, seems to have been the first to use a dry -plate process that was really workable. His original plan was to coat a plate with collodion, sensitize it in the ordinary manner, wash it, cause a solution of albumen to flow over the surface, dry it, dip it in a bath of silver nitrate, acidified with acetic acid, and wash and dry it again. The plate was then in a condition to be exposed, and was to be developed with pyro- gallic acid and silver. In this method we have a double manipulation, which is long in execution, though perfectly effective, as we know from experience. Alkaline A great advance was made in all dry -plate processes vel by the introduction of what is known as the "alkaline developer," which is, however, inapplicable to all plates on which silver nitrate is present in the free state. It will be remembered that the developers previously described, either for collodion or paper processes, were dependent on the reduction of metallic silver by some such agent as ferrous sulphate, the reduction taking place gradually and the reduced particles aggregating on those portions of the film which had been acted upon by light. The action of light being to reduce the silver iodide, bromide, or chloride to the state of sub-salts (e.g., sub-iodide of silver), these re duced particles really acted as nuclei for the crystallized metal. It will be evident that in such a method of develop ment the molecular attraction acts at distances relatively great compared with the diameters of the molecules them selves. If it were possible to reduce the altered particles it was plain that development would be more rapid, and also that the number of molecules reduced by light would be smaller if the metallic silver could be derived from silver compounds within shorter distances of the centres of mole cular attraction. Alkaline development accomplished this to a very remarkable extent ; but the method is only really practicable when applied to films containing bromide and chloride of silver, as iodide is only slightly amenable to the alkaline body. We have not been able to trace the exact date of the introduction of this developer. It is be lieved to be of American origin ; and it is known that in the year 1862 Major Russell used it with the dry plates he introduced. An alkaline developer consists of an alkali, a reducing agent, and a restraining agent. These bodies, when combined and applied to the solid bromide or chloride of silver, after being acted upon by light, as when a plate was exposed to the- camera image, were able to reduce the sub-bromide or sub-chloride, and to build up an image upon it, leaving the unaltered bromide intact, except so far as it was used in the building up. In 1877 Abney investigated this action and was able to demonstrate what actually occurred during the development. One of the experiments will show on what grounds this conclusion was arrived at. A dry plate was prepared by the bath process in the usual manner (to be described below), and exposed in the camera. The exposed film was covered with another film of collodio- bromide emulsion, which of course had not seen the light. An image was obtained from the double film by means of the developer, which penetrated through the upper unexposed film, and the development was prolonged until an image appeared through the same film, when the plate was fixed, washed, and dried. A piece of gelatinous paper was cemented on the upper film, and a similar piece on the lower after both had been stripped off the glass. When quite dry the two papers were forcibly separated, a film adhering to each. The upper film, altlvmgh never exposed to light, showed an image in some cases more intense than the under film. The action of the alkaline developer was here manifest : the bromide of silver in close contiguity to the exposed particles was reduced to the metallic state. Hence, from this and similar experi ments Abney was able to announce that silver bromide could not exist in the presence of freshly precipitated or reduced metallic silver, and that a sub-bromide was immediately formed. Thus Ag 2 Br 2 + Ag 2 = 2Ag 2 Br. From this it will be seen that the deposited silver is well within the sphere of molecular attraction, and that consequently a less ex posure (i.e., the reduction of fewer molecules of the sensi tive salt) would give a developable image. The alkalis used embraced the alkalis themselves and the mono -carbonates. The sole reducing agent up till recent times was pyrogallic acid. In the year 1880 Abney found that hydrokinone was even more effective than pyrogallic acid, its reducing power being stronger. Various other experimentalists tried other kindred substances, but without adding to the list of really useful agents. In 1884, however, Herr Egli and Arnold S} tiller brought out hydroxylamin as a reducing agent, which promises to be of great use if it can be prepared cheaply enough. Another set of developers for dry plates dependent on Other the reduction of the silver bromide and the metallic state is founded on the fact that certain organic salts of iron can be utilized. In 1877 Mr Carey Lea of Philadelphia and Mr William illis announced almost simultaneously that a solution of ferrous oxalate in neutral potassium oxalate was effective as a developer, and from that time it has been universally acknowledged as a useful agent in that capacity; and it is a rare favourite, more especially amongst Continental photographers. In 1881 Abney showed that the addition of a small quantity of sodium hyposulphite very greatly increased its rapidity of action by reducing the time of exposure necessary to get a developable image. In 1882 Dr Eder demonstrated that gelatin chloride of silver plates could be developed with ferrous citrate, which could not be so readily accomplished with ferrous oxalate The exposure for chloride plates when developed by the latter was extremely prolonged. In the same year Abney showed that if ferrous oxalate were dissolved in potassium citrate a much more powerful agent was formed, which allowed not only gelatine -chloride plates to be readily developed but also collodio-chloride plates. These, it may be said, were undevelopable except by the precipitation method until the advent of the agents last -mentioned ; the chloride being as readily reduced as the sub-chloride rendered the development of an image impracticable. Amongst the components of an alkaline developer we Re- mentioned a restrainer. This factor, generally a bromide Cramer or chloride of an alkali, serves probably to form a com- j! " d ^~ pound with the silver salt which has not been acted upon veloper. by light, and which is less easily reduced than is the silver salt alone, the altered particles being left intact. The action of the restrainer is regarded by some as due to its combination with the alkali. But whichever theory is correct the fact remains that the restrainer does make the primitive salt less amenable to reduction. Such restrainers as the bromides of the alkalis act through chemical means ; but there are others which act through physical means, an devel opers.
