1. The colorala martialis, coloured martial glimmer, of which there are many varieties, distinguished by their brown, black, semi-transparent blue, and green shades: they are found chiefly in Lapland, Sweden, and other northern parts of Europe.
2. The alba, pure or colourless mica, which contains several varieties, that are found in Siberia and Sweden, and are perfectly transparent ; whence it has obtained the name of Muscovy-glass.
Both species consist of thin glittering pieces, that are divisible into leaves, or plates; which, when exposed to a moderate heat, become brittle: but, if placed in a strong furnace, they curl or crumple, and are with difficulty reducible into glass. Mica, however, melts easily with borax, by the aid of which it may be converted into glass, with a blow-pipe.
The broad and colourless mica is used in Russia, as a substitute for glass, to which it is preterable ; because it resists the shock on the explosion of a cannon.