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8 PRECIOUS STONES
continued pressure, combined with heat or cold, or perhaps both in turn, doing the rest, as already mentioned.
The moisture falling in dew and rain becomes acidulated with carbonic acid, CO2
(carbon dioxide), from the combustion and decay of organic matter,
vegetation, and other sources, and this moisture is capable of
dissolving certain calcareous substances, which it takes deep into the
earth, till the time comes when it enters perhaps a division-plane in
some rock, or some such cavity, and is unable to get away. The hollow
becomes filled with water, which is slowly more and more charged with
the salts brought down, till saturated ; then super-saturated, so that
the salts become precipitated, or perhaps crystallised out, maybe by
the presence of more or other salts, or by a change in temperature.
These crystals then become packed hard by further supplies and
pressure, till eventually, after the lapse of ages, a natural gem is
found, exactly filling the cavity, and is a precious find in many cases.
If
now we try to find its analogy in chemistry, and for a moment consider
the curious behaviour of some well-known salts, under different
conditions of temperature, what is taking place underground ceases to
be mysterious and becomes readily intelligible.
Perhaps
the best salt for the purpose, and one easy to obtain for experiment,
is the sulphate of sodium—known also as Glauber's Salt.
It
is in large, colourless prisms, which may soon be dissolved in about
three parts of water, so long as the water does not exceed 60° F., and
at this temperature a super-saturated solution may easily be made. But
if the
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