CHAPTER XXIV IRRITANT POISONS— (Contd.) II. METALLIC POISONS ARSENIC Metallic arsenic is not poisonous, as it is insoluble in water and therefore incapable of absorption from the alimentary canal, but it oxidizes by exposure to the air, and then becomes poisonous. It is believed that some portion of elementary arsenic may undergo oxidation in the alimentary canal under some conditions and may produce poisonous symptoms. When rubbed on the skin in a finely powdered state it acts as a poison, as it is capable of being absorbed in the form of an oxide. When volatilized by heat, metallic arsenic readily unites with oxygen of the air, forming the poisonous vapour of arsenic trioxide. The vapours emanating during smelting of arsenic ores are destructive to vegetation and animal life, and cause chronic injurious effects to smelters. COMPOUNDS OF ARSENIC Arsenious Oxide or Arsenic Trioxide, As2Oa.—This is the most impor- tant compound of arsenic and is commonly known as white arsenic or merely as arsenic. It is called in the vernacular Sankhya or Somalkhar. It is sold as a white, gritty, crystalline powder, or in the form of a solid mass or cake. The mass first appears transparent and crystalline, but after some time becomes white and opaque, having a porcelain-like appearance. Arsenious oxide is odourless and tasteless, but it is sometimes described a*s having a roughish taste due to mechanical irritation of the tongue caused by the gritty character of the powder. If heated on charcoal it is reduced to metallic arsenic, which, in a vaporous form, has a garlic-like odour, and a very faint sweet taste. Arsenious oxide is almost insoluble in water, one-half to one grain dissolving in one ounce of cold water, and twelve to sixteen grains in one ounce of water kept boiling for an hour. The solution thus formed has a feebly acid reaction and is called arsenious acid, H3AsO3. Arsenious oxide is a heavy substance, its specific gravity being 3.669. A teaspoonful containing finely-powdered arsenious acid weighs 150 grains, a tablespoonful weighs 350 grains and a pinch or the quantity taken up between the finger and the thumb of an adult weigjis 17 graii^, M spite of this heavy weight powdered arsenic has the curioi^", propert^^p noating on water as a white film. If stirred up^ a good deal <lhe &m disappears, but reappears on standing. It is soluble fin spirits and wines in the same pro- portion as in water, but is much more soluble in acids and alkalies. It is also soluble in about 8 parts of glycerin. Arsenious oxide is often found as an impurity in iron pyrites and other sulphide ores, in mineral acids, such as sulphuric acid and hydrochloric acid and in some metals, such as zinc, tin, iron, lead and Antimony. Traces of arsenious oxide also occur in some soils, mineral waters and coal smoke. Arsenious oxide is largely used in the arts, in calico-printing, in taxi- dermy, in the preparation of wall papers and artificial flowers, and as a mordant in dyeing. It constitutes the principal ingredient of fly, papers, and many powders and pastes used for killing rats and vermin, and is an adul- terant of " complexion or violet powders ". In India, it is used for preserv- ing ^timber and skins against white ants. It is not unfrequently used by hakims and voids in the treatmeiitJof certain diseases, such as fevers, rheu- matism, skin diseases, syphilis, and"; impotence.