ACTION OF LIGHT ON POTASSIUM BICHILOMATE.
If a solution of potassium or ammonium bichromate be poured over a sheet of paper, and then exposed to the light, it turns brown on the exposed portion, the protected portion remaining reddish yellow. Under the influence of the light the organic matter of the paper brings about reduction of the bichro mate, and is itself superficially oxidised ; hence the brown colour. At the present moment it may be said that compara tively little is certain as to the exact effect of light on chromic salts, ex cept that the presence of organic matter of some kind is necessary. There are two chromates of potassium, the normal yellow chromate, and the acid orange chromate, or bichromate, 7. When the latter is incorporated with a colloid organic substance, such as gelatine, albumen, fish glue, etc., and exposed to light under a negative, two changes take place in the uncovered portions. First, the organic matter is rendered more or less insoluble ; and, secondly, it loses its power of absorbing water and swelling or becoming tacky. On the differentiation
thus effected between the exposed and un exposed portions of the bichromated film a large number of important photographic processes depend, among which may be mentioned carbon and gum bichromate printing, and the various branches of half tone, photo-mechanical, and collotype work. According to R. .Namias (" Chem. Centr." 868) chromium sulphate is the best compound for ren dering the-gelatine insoluble, owing to the ease with which the substance undergoes dissociation into and Chromium salts also render albumen and casein insoluble. The tungstates, molyb dates, and ceranates can replace potas sium bichromate for sensitising gelatine, but are not so active as that compound (T. Baker, " Brit. Jour. Pilot.," 1900 [18]).