ARSENIOUS ACID.
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Characteristics. —These may be conveniently and usefully discussedunder three heads:—(a.) The characteristics of solid arsenious acid;(b.) the characteristics of a pure solution of arsenious acid ; (c.) the cha-racteristics of arsenious acid in organic mixtures.
{a.) Of Solid Arsenious Acid. —The characteristics of solid arse-nious acid are (besides its physical properties before mentioned) principallythree—its volatility, the garlic odour evolved by throwing it on ignited char-coal, and the qualities of the metallic crust obtained by reducing the acid.
1. Its volatility. —Heated on the point of a penknife in the flame of aspirit lamp arsenious acid produces a white smoke, and speedily disap-pears. If the acid be heated in a test tube, a crystalline sublimate isobtained: the crystals when examined by a magnifying glass are foundto be regular octahedra. The impediments to the operation of this testare alkaline or earthy bases which retain a portion of the arsenious acid,and prevent its rising in vapour : boracic acid may be used to counteracttheir influence. The fallacy of this test is, that other white solids (ashydrochlorate of ammonia, oxalic acid, &c.) are volatile, and produce awhite smoke when heated.
2. Garlic odour. —If arsenious acid or an arsenite be put on a red-hotcinder (placed for convenience in a saucer), it evolves a scarcely visiblevapour (of metallic arsenicum) having a garlic odour, and which, at thedistance of an inch or two from the cinder, is converted into a dense,white, odourless smoke (arsenious acid.) The deoxidation of the acid isessential to the production of the garlic odour : hence no odour is per-ceived when arsenious acid is placed on a heated metallic or glass plate.The impediment to the action of this test is the presence of organic matter(as flour): this, by burning, developes a strong odour, which masks thesmell of the vapour of arsenicum. The fallacy attending it is, that someother bodies (as phosphorus, with certain of its compounds and someorganic matters) evolve when heated a garlic odour. Vauquelin, Barruel ,and Orfila, have shewn that a compound of albumen and fat, whichexhaled this odour when heated, did not contain a particle of arseniousacid. “ It is true,” say these experimenters, “ that arsenicum does evolvea garlic odour when volatilized; but even when this is well characterized,it is insufficient to establish the existence of the oxide of arsenic, since itbelongs to some other substances ; and it is not impossible that there maybe developed in the stomach, during digestion, substances which exhalean analogous odour, when heated.”
3. Formation of a metallic crust. Reduction test. —If arsenious acid beintimately mixed with freshly-ignited but coldcharcoal, and heated in a glass tube, the acid isdeoxidized, and yields arsenicum, which is su-blimed into a cooler portion of the tube, where itcondenses, and forms a metallic crust. A com-mon cylindrical test tube answers very well, butthe reduction tube of Berzelius (fig. 59) is to bepreferred. The characters of the arsenical crustare—the brilliancy of its outer surface, which isfrequently equal to polished steel or looking-glass ; the crystalline appearance and greyishwhite colour of its inner surface ; its volatility ;its conversion, by sublimation, up and down the
Fig. 59.