Elementary photographic chemistry Author:Eastman Kodak Company Purchase of this book includes free trial access to www.million-books.com where you can read more than a million books for free. This is an OCR edition with typos. Excerpt from book: CHAPTER III. The Chemistry of Development When a light sensitive material is exposed for a short time to light, although the change which takes place may b... more »e 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 bromide 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 bromide 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, such as the charcoal which, as explained in Chapter I, 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 s...« less