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.

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 ordinary price, which may induce the unwary to purchase it. Thi3 reduction of price is effected by fusing with the salt of silver a proportion of some other metallic salt. The fraud is readily detected by observing if the salt becomes moist on exposure to the air,—the adulterated nitrate of silver being 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 character and changes be clearly understood.

Experiment 1

Dry nitrate of silver, free of organic matter, will not blacken by sunshine; and, when dissolved in perfectly 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 the presence of organic matter in water.

Experiment 2

Place a stick of charcoal in pure water containing nitrate of silver, most beautiful crystals of silver will form around the charcoal. We here see that carbonaceous matter has the power of effecting the decomposition of the silver salt. In the first example, we have the metal precipitated as a black powder—oxide of silver and metallic silver; in the last, it is revived as a pure white metal, the crystals being of exceeding brilliancy. Thus we learn that the organic matter of the paper or of the size, is necessary to determine the change on which the photographic phenomena depend.