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. Pure nitrate of silver occurs in the form of white crystalline plates, which are very heavy and dissolve readily in an equal weight of cold water. The solubility is much lessened by the presence of free nitric acid, and in the _concentrated_ nitric acid the crystals are almost insoluble. Boiling alcohol takes up about one-fourth part of its weight of the crystallized nitrate, but deposits nearly the whole on cooling. Nitrate of silver has an intensely bitter and nauseous taste; acting as a caustic, and corroding the skin by a prolonged application. Its aqueous solution is perfectly neutral to test-paper.