production of a blue colour when supersaturated with ammonia. The yellow tinge of the acid usually met with in commerce, may depend either upon the presence of iron, vegetable extractive, or a small portion of chlorine. This latter body may be recognised by the odour, or by its power of dissolving gold leaf. ACIDUM NITRICUM. L.E.D. _Nitric Acid._ Aqua Fortis. QUALITIES. A limpid liquid of the specific gravity 1·500, a fluid-ounce of which is equal to about 11 drachms, 1 scruple by weight, and ought to decompose of pure limestone an ounce; it emits white fumes of a suffocating odour. _Taste_, extremely acid; it is highly corrosive, and tinges the skin indelibly yellow; an effect which is considerably heightened by the subsequent application of an alkali, so that these agents afford the means of detecting minute portions of animal matter, and were ingeniously employed for such an object by Mr. Hatchett. CHEMICAL COMPOSITION. When of the specific gravity 1·500, it contains 74·895 per cent. of dry acid; (whose ultimate elements are one portion of nitrogen and five of oxygen) the compliment 25·105 parts is water. It is decomposed with violent action by all combustibles, and when mixed with volatile oils, it causes their inflammation. It boils at 210°, and when its specific gravity is below 1·4, it is strengthened, when stronger than 1·45 it is weakened by ebullition. USES. It is principally employed as a pharmaceutical agent; _viz._ for the preparation of _Argenti Nitras_; _Liquor Ferri Alkalini_; _Hydrargyri Nitri-co-oxydum_; _Spiritus Etheris Nitrici_; and _Unguentum Hydrargyri Nitratis_. As an escharotic it has been frequently employed for the destruction of tumours, and is certainly of value where an immediate destruction of diseased parts is required. The method of using the strong nitric acid in such cases is to smear all the sound parts in the immediate vicinity of the ulcer with _Ung: Resin: Nig:_ and then to apply pledgets of lint firmly upon the ulcer for a few seconds, by which the whole surface will be deadened, and a deep slough remain, underneath which healthy suppuration and granulations will ensue.[352] ADULTERATIONS. _Sulphuric acid_ may be detected by a precipitate being produced on the addition of nitrate of baryta; in the application however of this test, Mr. Hume has shewn that unless this as well as the nitric acid be diluted, a precipitate will occur, although sulphuric acid should not be present; a circumstance which depends upon the barytic salt yielding its water of solution to the acid under examination, and becoming insoluble. _Muriatic acid_ is discovered by nitrate of silver, affording a precipitate at first white, but becoming coloured by exposure to light; the nitric acid ought to be perfectly colourless, but to preserve it in such a state it must be closely stopped, and kept in a dark place, or it will soon be converted into nitrous acid.