When a light sensitive material is exposed for a short time to light, although the change which takes place may be so minute that it cannot be detected by any ordinary means, if the exposed material is placed in a chemical solution, which is termed the "developer," the chlorine or bromine is taken away from the silver, and the black metallic silver which remains behind forms the image. This image is, of course, made up of grains, because the original emulsion contains the silver bromide in the form of microscopic crystals, and when the bromine is taken away from each of these, the crystal breaks up and a tiny coke-like mass of metallic silver remains behind in exactly the same position as the bromide crystal from which it was formed, so that, whereas the original emulsion consisted of microscopic crystalline grains of the sensitive silver salt, the final image consists of equally microscopic grains of black metallic silver. This removal of the bromine from the metallic silver is known chemically as reduction. (It must be remembered that chemical reduction has nothing to do with the photographic operation known as the reducing of a negative, that is, the weakening of an over-dense negative, where the word simply refers to the removal of the silver and is not used in the chemical sense.)

Chemical reducers are substances which have an affinity for oxygen and which can liberate the metals from their salts, in much the same way as charcoal is used to reduce iron from its ore. A developing solution is therefore one which contains a chemical reducer. All substances which are easily oxidized, are, however, not developers, since in order that a reducer may be used as the photographic developer it is necessary that it should be able to reduce exposed silver bromide but should not affect unexposed silver bromide, so that its affinity for oxygen must be within certain narrow bounds. It must be a sufficiently strong reducer to reduce the exposed silver salt, and at the same time must not affect that which has not been exposed. For practical purposes the developing agents are limited to a very few substances, almost all of which are chemically derived from benzol, the light oil which is distilled from coal tar.

The commonest developing agents are pyrogallol, hydroquin-one, paramidophenol or Kodelon, Elon and diamidophenol.

Pyrogallol (or pyrogallic acid) is made from gallic acid, which is obtained from gall nuts imported from China, the gall nuts being fermented to obtain the gallic acid, and the gallic acid being then heated in a still from which the pyrogallol is distilled over. Before the war most of the pyrogallol used in this country was made in Europe, but the shortage was met by the erection of a plant by the Eastman Kodak Company, which to day makes all the pyrogallol needed for their cus -tomers. Pyrogallol is made in two forms: a flaky powder form and a crystal form. When the powdered pyrogallol is opened in the darkroom or studio, the fine particles fly about and are likely to settle on paper or plates, producing spots on the photographs. For this reason the Eastman Kodak Company supply pyrogallol in the crystal form, which can be handled without any danger of particles flying about and giving trouble.

Hydroquinone is made from benzol, which is first converted into aniline and then oxidized in order to get the hydroquinone. It is made in several places in the United States. It is a less powerful reducing agent than pyrogallol but gives no stain and is very useful in conjunction with Elon or Kodelon for developing papers. When used with these substances it also gives developers which keep very well in tanks and are convenient where a developer must be kept for a long time, as in motion picture work.

Some time after pyrogallic acid and hydroquinone were in general use by photographers, there were introduced a number of new developing agents made from coal tar, which are very useful as supplements to the older developers. Several of these are based on a substance called paramido-phenol, which is made in the manufacture of dyes. When para-midophenol is treated with methyl alcohol the methyl attaches itself to it and forms a compound called methyl-paramidophenol, which is a more active developing agent than the paramidophenol itself. Another developing agent of the same type is diamidophenol, which is prepared in a way similar to paramidophenol.

Paramidophenol, methyl-paramidophenol and diamidophenol are all bases and the developing agents are their salts, the chlorides (or hydrochlorides) of paramidophenol and diamidophenol being used, and the sulphate of methyl-paramidophenol.

Paramidophenol Hydrochloride is manufactured by the Eastman Kodak Company under the name of Kodelon. Many of the so-called "new" developing agents on the market consist entirely or mainly of paramidophenol hydrochloride. A good sample should be light in color and should burn entirely when heated to redness, leaving no ash behind.

Monomelhyl Paramidophenol Sulphate is sold by the Eastman Kodak Company under the name of Elon. It is a more powerful developer than paramidophenol and is used with hydroquinone as one of the standard developing agents, Elon-hydroqui-none being used almost exclusively for the development of papers and very largely for the development of other sensitive materials. Elon is distinguished sharply from paramidophenol hydrochloride by the fact that it is soluble in the cold in its own weight of strong hydrochloric acid, whereas the paramidophenol hydrochloride is insoluble.

Diamidophenol Hydrochloride is sold by the Eastman Kodak Co. under the trade name of Acrol. It is a steel gray powder darkening easily in the air, and is oxidized so rapidly in solution that it is usual to dissolve it only when required for use.