are
entirely due to the refraction and polarization of light. They show
that the substances in which they are produced have not identically the
same constitution in all their parts.
ELECTRICAL PROPERTIES.
In
a general manner, all bodies acquire electricity by friction; only, one
kind keep for a longer or shorter time the electricity confined as it
were in their pores, while the other kind lose it instantly. The first
are isolating bodies, the others are conducting bodies.
Precious
stones belong to the first of these categories ; but they exhibit
great difference in the time during which they remain electrified; and
this characteristic affords, in experienced hands, a very useful test
for distinguishing one from another.
There
are certain precious stones which possess the curious quality of
becoming electric when they are subjected to heat. The tourmaline is
especially susceptible to this thermotic electricity.
When
precious stones are rubbed with the same material, usually a bit of
cloth, some of them acquire positive electricity and the others
negative electricity. Tourmaline, and other substances electrified by
heat, usually exhibit positive electricity at one extremity and
negative at the other.
