A mineral, which Professor Jameson divides into ten species; hut the term is generally understood to imply talc. or Muscovy glass, which is one of the species. Most of the mica or talc of commerce is brought from Siberia, where it is used as a substitute for window-glass. In this country it is employed for similar purposes where violent agitation or great heat would he destructive of common glass. It is also used for enclosing objects for microscopes, for which it is admirably adapted; consisting, as it does, of an unlimited series of transparent laminae adhering to each other, which easily separate into extremely thin flexible plates, by the application of the fine edge of a pen-knife.