Since they differ in nature from the silver bromide, it was suspected that the specks were derived from the gelatine, since some gelatines give emulsions which are much more sensitive than others. A material was extracted from gelatine which was identi fied by Dr. S. E. Sheppard with allyl mustard oil, which in am moniacal solution becomes allyl thiocarbamide. Allyl thiocar bamide reacts with silver bromide, and the compound the two in alkaline solution produces silver sulphide, so that the sensitizing specks may consist of minute traces of silver sulphide on the surface of the silver bromide grain. The ripening process in the preparation of gelatine-bromide emulsions may therefore be ascribed partly to the growth of grains of larger sizes from those of small dimensions and partly to the production of the sensitiz ing specks of silver sulphide on the 'surface of the grain. Silver itself may also be associated with the specks.
The exact action of light upon the silver halide and the nature of the product which renders the grains developable is not well understood, although recent work is tending to define the issue and to suggest that we are not far from the complete solution of the problem. The exposed silver halide, which changes to silver in development, is known in photography as the "latent image," and the problem of the nature of the latent image has been a subject of speculation from the earliest days of the science.
The theories put forward as to the nature of the latent image may be roughly divided into physical and chemical: The physical theories include theories of disintegration and of depolymerization as well as theories of the formation of the image without involving any assumption as to its actual nature, such as the theories of molecular strain and of photo-electric emission of electrons.
The latent image has certain definite chemical properties. It is normally stable, since it has been known to persist for many years. Its fundamental property is that of facilitating the depo sition of silver in a reducing agent, such as a developer. It can be destroyed by powerful oxidizing agents, such as chromic acid or acid permanganate. These properties seem to call for the existence of some definite chemical individual, and it would appear, therefore, that the latent image must consist of some re duction product of silver bromide. Those which have been sug gested are silver sub-bromide and metallic silver.
The existence of silver sub-bromide is doubtful; the formula has been proposed but it has never been prepared in a pure state. All attempts to prepare it have resulted in mixtures or solid solutions of silver bromide with metallic silver. While the theory that silver sub-bromide was the material of the latent image was accepted for many years, it is now generally agreed that the image consists of metallic silver in very small amounts formed by the reduction of silver bromide.
When light falls upon a crystal of silver bromide, therefore, it produces a small quantity (perhaps a few hundred or a few thousand atoms) of silver. The size or condition of this speck of silver determines whether or not it will act as a nucleus to make the crystal developable. The production of the silver ap pears to be greatly facilitated by the existence on the crystal of small specks of foreign material, presumably, from Sheppard's work, mainly silver sulphide. The mechanism of this action is still under discussion.
Dr. Sheppard and his colleagues have developed a theory which they term the "concentration speck theory," according to which the action of the sensitizing speck is to concentrate the energy falling upon the silver bromide and enable silver to be liberated at the boundary of the speck. Toy and Trivelli have suggested independently that the chief action of light is to increase the con ductivity of the silver bromide, and Trivelli suggests that an elec trolytic action then ensues as a result of which metallic silver is deposited in contact with the silver sulphide speck. Hickman and others have postulated that a sensitizing speck must be an accep tor for the halogen set free, and many mechanisms have been sug gested for the reaction. Much work remains to be done on this problem, however.