376

ELEMENTS OF MATERIA MEDICA.

(Fig. 58.) Properties. —When recently prepared, arsenious

explanation. Moreover, 1 find the opacity takes place in an air-tight vessel.Does not the opacity (as in opaque gum arabic) depend on innumerablecracks, imperceptible from their minuteness ? Professor Guibourt, Mr.Phillips, and Mr. Taylor, have each found the density of the opaquevariety to be less than that of the transparent. Transparent arseniousacid has a sp. gr. of 3'7391, according to Guibourt (3'715, Phillips;3'208 to 3'333, Mitchell and Durand ; 3'798, Taylor.) It dissolves, ac-cording to the same authority, in 103 parts of water at 59°, or in 9'33parts of boiling water, and the solution feebly reddens litmus. Opaquearsenious acid, on the other hand, according to Guibourt, has a sp. gr. of3'095 (3'529, Taylor; 3'620, Phillips), is soluble in 80 parts of waterat 59°, or in 7'72 parts of boiling water, and the solution restores the bluecolour of reddened litmus. But I find both kinds redden litmus, andDr. Christison has observed the same. Mr. Taylor ( Guy's Hospital Re-ports, vol. ii. p. 83), did not find any difference in the solubility of thetwo varieties. He found that water boiled for an hour on this substancedissolved of its weight; that this water on perfect cooling did notretain more than of its weight; and that water at ordinary tempera-tures will dissolve from about to of its weight. It appears, then,that water perfectly cooled from a boiling saturated solution will retainfrom ten to twenty or more times the quantity of acid in solution than itwill take up at common temperatures without heat,—a fact which is ascurious as it is inexplicable {op. cit. p. 96.) Arsenious acid is solublein alcohol and oils. It is of importance to know that the presence oforganic matters very much impairs the solvent power of water for thisacid—a circumstance which readily explains why arsenious acid has not,in some cases, been found in the liquid contents of the stomach ofpersons poisoned by it. Arsenious acid has little or no taste, as Plenck{Toxicologia , ed. 2 nda - 26), Addison, and Christison, have remarked : andneither in the solid nor vaporous form has it odour. The acid may bereadily obtained in a crystalline condition by sublimation or by coolinga boiling saturated solution : the crystals are transparent, usually regularoctahedra, sometimes tetrahedra or acicular. At a temperature of 380° P.it volatilizes: when heated under pressure it liquefies, and is convertedinto a transparent glass.