A Manual Of Photography

CHAPTER II. CHEMICAL CHANGES ON SENSITIVE PREPARATIONS. It is of some moment to the photographic artist that he is acquainted with the changes which occur in the several agents which he employs. A few of these are therefore selected. Section I.—Nitrate of Silver. 1. The crystallised salt, in a pure state, should be procured. The commercial salt often contains nitrate of potash. The fused nitrate, which is sold in cylindrical sticks, is yet more liable to contamination. A preparation is sometimes sold for nitrate of silver, at from sixpence to ninepence the ounce less than the or- dinary price, which may induce the unwary to purchase it. This reduction of price is effected by fusing with the salt of silver a proportion of some cupreous salt, generally the nitrate. This fraud is readily detected by observing if the salt becomes moist on exposure to the air,—a very small admixture of copper rendering the nitrate of silver deliquescent. The evils to the photographer are, want of sensibility upon exposure, and the perishability (even in the dark) of the finished drawing. As all the silver salts are prepared from the nitrate, it is of consequence that its chemical character be clearly understood. Section II.—Chloride of Silver. Experiment 1.—Dry nitrate of silver, free of organic matter, will not blacken by sunshine ; and, even wiren dissolved in per- fectly pure distilled water, it may be exposed for a long time to solar influence without undergoing any visible change. Add, however, to the solution the smallest appreciable quantity of any organic matter, and it will almost immediately begin to blacken. This is so certain, that nitrate of silver is the most sensitive test that we have for tire presence of organic matter in solution.