is
still resorted to daily by lapidaries. It consists in submitting a
coloured stone (diamond, topaz, &c.) to a temperature more or less
elevated. Nearly always in these conditions the stone changes colour
permanently.
A remarkable communication made to the Academy of Sciences will serve as an example of the first case.
"
MM. Halphen have the honour to present to the Academy a diamond of the
weight of 4 grammes (about 20 carats), presenting a phenomenon which
has never been before observed, at least to their knowledge.
"This
stone is, in its normal state, of a white colour, faintly tinged with
brown. When it is subÂjected to the action of fire, it acquires a very
clear rose-tint, which it retains for eight or ten days, and which it
loses gradually, to return to its primitive normal colour.
"This
change and return to the primitive state may be repeated indefinitely,
for the diamond subÂmitted to the Academy has been subjected five times
to this test.
"The
phenomenon in question arrested at first the attention of an observer,
who was trying at random upon this diamond the prolonged action of
fire. Experiments made since upon other diamonds have not produced the
same result.
