CHALK

( Craie , Fr.; Kreide , Germ.) A friable carbonate of lime, white, opaque, soft, dull, or without any appearance of polish in its fracture. Its specific gravity varies from 2·4 to 2·6. It usually contains a little silica, alumina, and oxide of iron. It may be purified by trituration, and elutriation. The siliceous and ferruginous matters subside first, and the finer chalky particles floating in the supernatant liquid, may be decanted with it, and obtained by subsidence. When thus purified, it is called whitening and Spanish white, in England; schlemmkreide , in Germany; blanc de Troyes , and blanc de Meudon , in France. Pure chalk should di olve readily in dilute muriatic acid, and the solution should afford no precipitate with water of ammonia. CHALK— Black . A mineral, called also drawing-slate . CHALK— French . Steatite , or soap stone ; a soft magnesian mineral. CHALK— Red . A clay coloured with the peroxide of iron, of which it contains about 17 per cent.