sometimes
very large. It is unequally opaque and semi-transparent. It has
vitreous light, and is of all colours of the prism in all their
gradations. It cuts quartz, but is cut by the topaz. It gives a white
powder. Its breakage is concave and imperfect.
Its specific gravity is from 2-99 to 3-33.
It has double refraction and great power in polarizing light, which
power increases when it is heated. Under the action of the blow-pipe it
swells without melting, while its thin extremities vitrify. United with
borax, it easily reduces into a clear and vitreous substance.
Tourmaline
cut into plates is used in the polaroscope to ascertain the optical
properties of other substances. The analyses made of this gem are many
and complicated, and amongst these the following is the most simple ;
it is of the red tourmaline :
