Nitric Acid
The two principal constituent parts of our atmosphere--oxygen and nitrogen gases--when in certain proportions are capable under particular circumstances of combining chemically into one of the most powerful acids--the nitric. For all practical purposes, nitric acid is obtained from nitrate of potash, from which it is expelled by sulphuric acid. The nitric acid is of considerable use in the arts. It is employed for etching on copper; as a solvent of tin to form with that metal a mordant for some of the finest dyes; in metallurgy and assaying, in various chemical processes, on account of the facility with which it parts with oxygen and dissolves metals. For the purposes of the arts it is commonly used in a diluted state and adulterated with sulphuric and muriatic acids, by the name of aquafortis. Two kinds are made: one called double aquafortis, which is one half the strength of nitric acid; the other simply aquafortis, which is half the strength of the double. A compound made by mixing two parts of the nitric acid with one of muriatic, known formerly by the name of aqua regia, and now by that of nitro-muriatic acid, has the property of dissolving gold and platina. On mixing the two acids heat is given out, an effervescence takes place, and the mixture acquires an orange color. See AQUA REGIA; ATMOSPHERE.