ARSENIOUS ACID. (Fa., Eleide arsenieux ; GER., ArSenii:Siiire.) Formula A:4,0,.
Arsenious acid, commonly called arsenic, or white arsenic, is usually met with in the form of a white powder resembling flour, or of thick, white, brittle masses which are transparent when first made, but on exposure to the air for a short time become opaque; this change extends from the surface of the lump to tho centre, giving it an appearance and fracture closely resembling that of porcelain, It may also be obtained iu octohedral crystals by cooling in hot, saturated, aqueous solution. Arsenious acid is sparingly soluble iu cold, but dissolves readily and in boiling water ; iu hot, dilute acids it also dissolves, though not as a rule forming salts. It is easily sublimed, volatilizing at 218° (121 F.); the vapour of arsenic is colourless and inoderous, provided that carbon and ether reducing agents are absent ; if heated in the presence of charcoal the vapour emits the characteristic velour of garlic, by means of which very small quantities of arsenic may readily be detected. Arsenious acid is one of the deadly poisons; two or three grains are sufficient to cause death. Iu case of po'soning by arsenic, an effective antidote may be prepared by precipating the hydrated sesqui-oxide of iron from ferric chloride or sulphate by means of a solution of magnesia or ammonia in excess ; the arsenic on being brought into contact with this oxide of iron iu the stomach is converted into an insoluble arsenite of iron, and thus prevented from entering the system.
Arsenious acid occurs native in the mineral arsenite or arsenolite; this mineral, however, does not occur in sufficient abundance to be valuable as a source of arsenic. By far the largest quantity of English-made commercial arsenic is obtained from mispichel or arsenical pyrites. This mineral contains from 43 to 46 per cent. of arsenic ; it is found in large quantities in Devon, Cornwall, and Smith Wales, in association with tinstone, iron and copper pyrites, galena, and various other minerals. These latter are separated from the arsenical pyrites by mechanical means, arsenic being obtained from them, if present in sufficient quantity, during the subsequent smelting processes, as a bye-product. Arsenical pyrites, being practically worthless as an ore of iron, is treated solely for the arsenic which it contains. In Saxony, Silesia, and the Harz, arsenic is also obtained from ores of cobalt and nickel, some of which contain upwards of 70 per cent.
The manufacture of arsenious acid on a commercial scale has within the last twenty years risen to a position of considerable importance among the industries of Great Britain, and there is reason to believe that it has yet to see its best days, both in this country and on the Continent ; owing to the discovery of new applications, and the further development of old ones, the demand for it is steadily increasing.
The details of the manufacture of arsenious acid will be given under the heading of "Arsenic."