temperature, and thus leaving behind the one producing a reddish tint.
210. Quartz.
This
mineral is more abundantly distributed in Nature than any other, yet
though so plentiful, in its different varieties, it is largely drawn
upon to provide material for use as precious stones. It was the
Crystallus of Pliny and other early writers. Theophrastus mentions it
as one of the stones set in rings. Amongst the Romans it was very
highly prized for the purpose of making vases and cups, and fashion
decreed that such "crystal cups" should only be used for iced drinks,
hot liquids being served in the Murrhine vases (King).
The
known varieties of this mineral are so many that we must confine the
general account of the physical properties to the purer forms, and
point out, in the consideration of the different kinds that more
particularly concern us, any exceptional features in which they differ
from the normal.
Pure
Quartz is colourless, but small amounts of impurities or enclosures may
give it various shades of yellow, brown, green, red, or blue. The
lustre is characteristically vitreous, and often in a splendent degree;
some kinds have a greasy lustre. The diaphaneity may range from
perfectly transparent to opaque. It shows weak double refraction, the
indices for the yellow ray being 1"544 and l-553; and the
dispersion is feeble also, the refractive index of the ordinary ray for
red light being 1*541, and for violet light 1*558. It follows from this
that the separation of the two images seen on looking through a cut
piece of the mineral is never great, and that the " fire" is not
marked. Also if the
