Vitreous lustre.—This
lustre recalls exactly the fracture of glass. It belongs generally to
bodies in which the refracting power is inconsiderable.
ACTION OF LIGHT AND HEAT UPON PRECIOUS STONES.
Light.—When
the most valuable precious stones, and the diamond particularly, are
exposed for a certain time to the rays of the sun, and are then taken
into darkness, they remain luminous, and exhibit the phenomenon of
phosphorescence. This curious effect lasts for some time, but gradually
becomes fainter and fainter, and finally disappears.
Heat.—The
effects produced upon precious stones by heat are even more remarkable
than those due to the action of light. Heat acts upon them in two very
dissimilar ways. It modifies the elementary constitution of the stone
by separating its molecules, but this in a manner altogether
mechanical; or it produces in the stone a veritable chemical reaction.
In the first case the modifications are temporary, and at length the
objects return to their primitive condition; in the second case the
effects produced are permanent.
As an example of the latter case, we may cite a practice whose origin is lost in antiquity, and which
