In Silesia the crude arsenious anhydride obtained from arsenical pyrites is refined by
sublimation as follows:—For this purpose the cast-iron vessels (a) are employed. Upon these are placed iron rings or collars (b, c, d) and a hood (e), communicating by means of tubes with a series of chambers, of which the first only is shown in i. The flanges of the cast-iron collars and all other joints having been thoroughly luted, the fire is lighted and the heat so increased as to cause the semi-fusion of the arsenious anhydride, which, after cooling, exhibits a peculiarly porcelain-like appearance, at first being as transparent as glass.
Prop. Crystals (obtained by careful sublimation, or by cooling a boiling aqueous solution), usually transparent, regular octahedrons (fig. 1), but sometimes, though rarely, assume the form of tetrahedrons (see 2). When prepared on the large scale it forms large, glassy, colourless or yellowish-white, transparent or semi-transparent cakes or porcelain-like masses (vitreous arsenious anhydride, glacial a. a.), which soon becomes opaque on their exterior, and often friable and pulverulent; odourless; volatilises at 380° Fahr.; fumes odourless, unless carbonaceous organic matter be present, when they smell strongly like garlic; heated under pressure it liquefies and forms a transparent glass; taste faintly sweetish, with a slight acidity and astringency, not perceived until some minutes after being swallowed. The opaque variety is soluble in 80 parts of water at 59° Fahr., and 7·72 parts of boiling water; but on cooling to 60°, only about one third of this quantity continues in solution. The transparent variety is soluble in 103 parts of water at 59°, and 9·3 parts of boiling water. Both soluble in alcohol, syrups, oils, and spirits, and freely so in alkaline lyes and hydrochloric acid; organic matter generally impedes its solution; solutions redden litmus; heated with organic matter it is reduced to the metallic state. Sp. gr. 3·5 (lowest opaque var.) to 3·8 (highest transp. var.).
Arsenites. True arsenious acid (HAsO2) has never been obtained in a satisfactory condition, but its salts are readily obtained by dissolving arsenious anhydride in a solution of the base, or by double decomposition. They are generally white, nearly all insoluble, except those of the alkalies, and all soluble in acids.
Tests, Detec., &c. Owing to the importance of the subject, and for convenience and facility of reference, the leading tests for the arsenites and arsenious anhydride are noticed alphabetically below; to which a few general remarks on their application, under the various circumstances that occur to the chemist and toxicologist, are appended. When not otherwise stated, it is to be understood that they are to be applied to pure, or nearly pure and colourless solutions of arsenious acid or the arsenites. Ammonio-nitrate of silver gives a well-marked yellow precipitate of arsenite of silver in an aqueous or arsenious anhydride solution which is soluble in ammonia and in dilute nitric acid.
Crystallisation Test.—A very minute quantity of arsenious acid placed in a small tube (arsenic-tube), and heated in the flame of a spirit lamp, gives a crystalline sublimate, which collects on the cooler portion of the tube, and which, when examined by a pocket lens, is found to consist of sparkling octahedral crystals (see engr.)
(Magnified.)
Ellis’s Test.—This is a modification of the ‘nascent hydrogen test,’ in which the suspected gas is passed through a tube containing slips of copper leaf or riband, or still better pure oxide of copper, gently heated; the end of the tube communicating with the atmosphere being drawn to a capillary size, at which the gas may be inflamed and tested, as in ‘Marsh’s Apparatus.’ (See engr.) If arsenic be abundant in the gas, the copper will be almost instantly covered over with a coating of metallic arsenic; and after continuing the heat for a few minutes it will present a beautiful silvery surface, and may then be submitted to further examination.