Symbol, AgS. Atomic weight, 124.
This compound is formed by the action of sulphur upon metallic silver, or of sulphuretted hydrogen, or hydrosulphate of ammonia, upon the silver salts; the decomposition of hyposulphite of silver also furnishes the black sulphuret.
Sulphuret of silver is insoluble in water, and nearly so in those substances which dissolve the chloride, bromide, and iodide, such as ammonia, hyposulphites, cyanides, etc.; but it dissolves in nitric acid, being converted into soluble sulphate and nitrate of silver.
Symbol, AgO NO{5}. Atomic weight, 170.
Nitrate of silver is prepared by dissolving metallic silver in nitric acid. Nitric acid is a powerfully acid and corrosive substance, containing two elementary bodies united in definite proportions. These are nitrogen and oxygen; the latter being present in greatest quantity.
Nitric acid is a powerful solvent for the metallic bodies generally. To illustrate its action in that particular, as contrasted with other acids, place pieces of silver foil in two test-tubes, the one containing dilute sulphuric, the other dilute nitric acid; on the application of heat a violent action soon commences in the latter, but the former is unaffected. In order to understand the cause of the difference, it must be borne in mind that when a metallic substance dissolves in an acid, the nature of the solution is unlike that of an aqueous solution of salt or sugar. If you take salt water, and boil it down until the whole of the water has evaporated, you obtain the salt again, with properties the same as at first; but if a similar experiment be made with a solution of silver in nitric acid, the result is different: in that case you do not get metallic silver on evaporation, but silver combined with oxygen and nitric acid, both of which are tightly retained, being, in fact, in a state of chemical combination with the metal.
If we closely examine the effects produced by treating silver with nitric acid, we find them to be of the following nature:—first, a certain amount of oxygen is imparted to the metal, so as to form an oxide, and afterwards this oxide dissolves in another portion of the nitric acid, producing nitrate of the oxide, or, as it is shortly termed, nitrate of silver.
It is therefore the instability of nitric acid, its proneness to part with oxygen, which renders it superior to sulphuric acid in the experiment of dissolving silver. Nitric acid stands high in the list of "oxidizing agents," and it is important that the photographer should bear this fact in mind.
Properties of Nitrate of Silver.—In the preparation of nitrate of silver, when the metal has dissolved, the solution is boiled down in order to drive off the excess of nitric acid, and set aside to crystallize. The salt, however, as so obtained is still acid to test-paper, and requires either recrystallization, or a careful heating to about 300° Fahrenheit, to render it perfectly neutral.