bodies,
it seems to be not impossible that in course of time the carbon may
undergo a slow combustion, and the colour of the stone be more or less
destroyed.
It
is impossible, too, that a substance so corrosive as sulphuric acid
should not make some impression on the stone. Silica, it is true, is
not attacked by this liquid, but it is altered in a remarkable manner;
and then precious stones of the agate class are not formed exclusively
of silica, but contain small quantities of different substances on
which sulphuric acid has a very decided effect.
The
stones artificially coloured, then, may be used with great success for
works of secondary value; but they should never be employed by
veritable artists, who work " not for a day, but for all time."
