Such
experiments with the topaz are highly interesting, and if we take a
little of the powdered stone and mix with it a small portion of the
microcosmic salt, we may apply the usual test for analysing and proving
aluminium, thus : a strongly brilliant mass is seen when hot, and if we
moisten the powder with nitrate of cobalt and heat again, this time in
the inner flame, the mass becomes blue. Other phenomena are seen during
the influence of heat. Some stones, as stated, become pink on heating,
but if the heating is continued too long, or too strongly, the stone is
decoloured. Others, again, suffer no change, and this has led to a
slight difference of opinion amongst chemists as to whether the colour
is due to inorganic or organic matter. Heating also produces
electricity, and the stone, and even splinters of it, will give out a
curious phosphorescent light, which is sometimes yellow, sometimes
blue, or green. Friction or pressure produces strong electrification ;
thus the stones may be electrified by shaking a few together in a bag,
or by the tumbling of the powdered stone-grains over each other as they
roll down a short inclined plane. The stones are usually found in the
primitive rocks, varying somewhat in different localities in their
colour ; many of the Brazilian stones, when cut as diamonds, are not
unlike them.
In
testing, besides those qualities already enumerated, the crystalline
structure is specially perfect and unmistakable. It is doubly
refractive, whereas spinel and the diamond, which two it closely
resembles, are singly refractive. Topaz is readily electrified, and, if
perfect at terminals, becomes polarised ; also the commercial
