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 dissolve 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.
CHARCOAL. The fixed residuum of vegetables exposed to ignition out of contact of air. In the article [Carbon], I have described the general properties of charcoal and the simplest mode of making it. I shall here detail the best systems of manufacturing this product upon the continent of Europe.
To carbonize wood under a movable covering, the plan of meiler, or heaps, is employed very much in Germany. The wood is arranged either in horizontal layers, or in nearly vertical ones, with a slight slope, so as to form conical rounded heaps of different sizes. The former are called lying meiler, [fig. 272.]; the latter standing meiler, [figs. 273.] and [274.] Both are distributed in much the same way.
