3. _When in a state of compound mixture_, nitric acid, like sulphuric acid in similar circumstances, may be after a time partly decomposed and partly neutralized; and when the matter with which it is mixed belongs to either of the organic kingdoms, more particularly to the animal world, its decomposition is more rapid than that of sulphuric acid. Still it is an important fact, that some of the acid may be discovered after a considerable interval. M. Ollivier detected it in various stains on the skin at least a day after it had been applied;[308] Dr. O’Shaughnessey detected it in a stain on cloth sent to him from Ireland to Edinburgh;[309] and I have found it in stains made on broad-cloth with detached drops seven weeks before. _Process for Stains._ Nitric acid produces on the skin a yellow stain, which gradually becomes dirty orange, and finally of a dirty yellowish-brown; but in all of these states it is at once rendered for a time lively yellow by the action of ammonia. I am not aware that any other yellow stain is similarly affected. Stains on cloth are generally yellow, reddish-yellow, or brownish-yellow, and are attended with more or less disintegration of the texture of the cloth. The method of analyzing all these stains is as follows:—The stained parts is to be boiled in a few drachms of pure water several times in succession; and the liquid is then filtered, and may be subjected to litmus-paper for the purpose of ascertaining its acidity. It is then to be rendered neutral, or for the sake of greater facility, feebly alkaline, by adding a few drops of a diluted solution of caustic potass, after which the whole is evaporated to dryness, and in a vapour-bath, if practicable. The residuum is then to be decomposed by sulphuric acid in the same way as recommended above for the simple diluted acid.—Orfila thinks it advantageous to let the stains macerate for some hours in a solution of bicarbonate of soda rather than to boil them in water. In that case, however, it is necessary to ascertain the acidity of the stains with litmus-paper before proceeding to macerate them. _Process for Mixtures._ The detection of nitric acid in compound mixtures, such as the contents of the stomach, is not so easy a matter as its detection in stains; and indeed a sure and delicate process is still a desideratum in medico-legal chemistry. The process varies, as in the case of sulphuric acid, according as the subject of analysis is acid or neutral.