Volltext Seite (XML)
244 GYPSUM FAMILY. The crystals occur of all degrees of magnitude, and are sometimes acicular. The lateral planes of the prism are sometimes smooth, sometimes longitudinally streaked, and shining; the con vex terminal faces, and the lens, are rough and dull. Internally the lustre is splendent and pearly. The fracture is generally straight, seldom curved fo liated, with a threefold cleavage, of which one of the cleavages is perfect, and very distinct, and the two others imperfect, and at right angles to the former. The fragments are rhomboidal, in which two of the sides are smooth and splendent, and four are streaked. It sometimes occurs in granular concretions, which are large, coarse, and small, and their surfaces are gene rally uneven. It alternates from semi transparent to transparent, and . in the latter case is observed to refract double. It is very soft; yields readily to the nail. It is sectile. It is very easily frangible. In thin pieces it is flexible, but not elastic. Specific gravity, 2.322, Muschenbrock. 2.3065,2.3117, 2.3846, Kopp. Chemical Characters. It exfoliates before the blowpipe, and, if the flame is directed towards the edge of the folia, it melts into a white enamel, which, after a time, falls into a white powder. Constituent
