To examine a piece of cloth stained with this poison it is only necessary to boil it in distilled water and then apply to the liquid the barium test as before. NITRIC ACID (_Aqua fortis_, _Red Spirit of Nitre_).—This substance has been employed as a poison for upwards of four centuries. Like the oil of vitriol it is found in commerce in a concentrated and in a diluted state. Cases of poisoning by it are rare. It produces a yellow stain on the skin. Two drachms is the smallest quantity which has destroyed life; but less than this would probably prove fatal, if it produced much corrosion about the wind-pipe. Death has occurred from it, in one hour and three quarters; the average would be within twenty-four hours. Several cases ending fatally have followed the inhalation of the fumes of this acid, probably by inducing very extensive inflammation of the lung. _Tests._—The concentrated acid may be known by its orange-colored irritating fumes, and by its action on copper, tin, or mercury. 1. When poured on copper-filings, effervescence takes place, a red acid vapor is given off, and a green liquid remains (solution of nitrate of copper). 2. In a diluted state it is detected by its acid reaction; by no precipitate being obtained by nitrate of barium or by nitrate of silver, proving the absence of sulphuric and hydrochloric acids; further, by neutralizing the liquid with potass, evaporating it, and then procuring crystals of nitrate of potassium, in the form of lengthened fluted prisms, which are permanent in the air. These crystals may be powdered and moistened with strong sulphuric acid, when a colorless acid vapor (nitric acid) will be evolved. Or the powdered crystals may be mixed with an equal bulk of fine copper filings, moistened with water, and treated with a few drops of sulphuric acid; when ruddy acid fumes will be given off. 3. Other tests for nitric acid are, (_a_,) its action on morphia, which it turns red: (_b_,) its action on green iron sulphate, which it blackens; (_c_,) a trace of it along with sulphuric acid gives with narcotine a blood red color; and finally, (_d_,) along with hydrochloric acid it dissolves gold. HYDROCHLORIC ACID (_Muriatic Acid_, _Spirit of Salt_).—Not more than half a dozen cases of poisoning by this acid have occurred in the last fifteen or twenty years in this country. In May, 1859, a woman sixty-three years old was admitted into King’s College Hospital within three-quarters of an hour of swallowing half an ounce of the strong acid. She had burning pain in the throat and stomach, vomiting of brown shreddy matters, and great prostration. Death occurred in eighteen hours, from the corrosive action of the poison. This is the smallest dose which has been known to prove fatal. _Tests._—The concentrated hydrochloric acid of commerce is of a yellowish color, it fumes in the air when strong, and produces dense white fumes with the vapor of ammonia.