THE DEVELOPMENT OF THE NEGATIVE IN this chapter the subject is treated under the following heads: (a) General Principles ; (b) Inorganic Developers ; (c) Organic Devel opers; (d) Substances chiefly used in inaking Developing Solutions; (e) General Practice of Dcvelopnent; (f) Various Methods of Devel opment ; (g) Tests of a Developer ; and (h) Physical Dove1opment, before or after Fixing.
334. Chemical Development. Chemical devel opment by means of a solution of pyrogallol, made alkaline with ammonia or ammonium carbonate, was used for dry-collodion plates as early as 1862, and has been used for gelatino bromide emulsions since their first introduction,. Since 1877, when Carey Lea introduced a solu tion of ferrous oxalate, the number of developers in use has continually increased.
During development, the silver bromide 1 of the grains which have been affected by light is converted (i.e. reduced) into black metallic silver, which constitutes the final image, 2 whilst the bromine goes to form hydrobromic acid, which would soon arrest development unless it were converted into a less active bromide by reaction with one of the constituents of the developer.
A developer is thus always a reducing mixture, using the word in its chemical sense, but all reducers cannot be used as developers. Some would reduce the grains of silver bromide to metallic silver irrespective of whether they had been affected by light or not. Others, on the contrary, would not be active enough to reduce even the exposed silver Within the limits of reducing power (or t o use the correct term, reduction potential) over which a reducer can be used as a developer, there are very marked differences in the action on silver bromide grains which have received only a very small exposure, these grains being reduced by the most active developers, but not by those of low reduction potential.' No visible change occurs at first when a sensit ive emulsion which has been exposed to light is placed in a developing solution. The liquid must have time to penetrate the gelatine, to diffuse within it, and to swell it before reaching the grain of silver bromide on which it has to react. Nor is this latter reaction instantaneous.
Chemical progress has not belied the old adage of the alchemists, " Substances react only when dissolved," and traces of silver bromide must be dissolved in the developer before reduction can occur. And this reduction must even proceed until the solution is saturated with reduced silver before the latter can begin to deposit itself on the germs of the latent image. ' This delay in the appearance of the image (induction period) is still further increased by the fact that the image does not become visible (especially in a dim light) until it has acquired a certain degree of dens;ty.
From the moment that the image has begun to appear, its density increases progressively, but with a constantly decreasing speed. 1 This speed varies according to various conditions, e.g. composition of the developing solution, its concentration and temperature. It also varies, and often very appreciably, from one emulsion to another, and even with successive batches of a given type of emulsion.
33s. Theory of Development. The theory, advanced by J. Waterhouse (1891), that pheno mena of an electrolytic nature occur in develop ment is based on the fact that t is possible to develop images by coating the film of emulsion on a silver plate used as a cathode in the electro lysis of weakly ammoniacal water (P. Chenevier 1894; E. Banks, 1896) and on the facts, ob served by A. von Eltibl (19o1), that the duration of development is almost exactly proportional to the electric resistance of the bath (both with ferrous oxalate and the various organic devel opers), and that with alkaline developers the alkalis roust be substituted for each other, not in the ratio of their chemical equivalence nor in that of the amounts of heat produced in their neutralization, but in quantities affording a mixture of the same resistance.