a
solution at a great temperature and under enormous pressure. Suppose we
dissolve in cold water as much Glauber's salts (sodium sulphate) as
possible and then heat the solution to nearly the boiling point, on
adding more of the salt we shall find it dissolve readily, and
conversely on cooling it will be again deposited in a visible form as
crystals. In such a case the temperature to which we can raise the
solution is very nearly fixed by the pressure of the atmosphere ; if,
however, we heated the solution in a strong metal vessel capable of
withstanding a pressure of several hundred pounds to the square inch,
we should find that crystals of the Glauber's salts which remained
undissolved at the ordinary boiling point were dissolved. Thus we can
reason that given sufficient heat and sufficient pressure and
sufficient time, many substances, which in the chemical
laboratory seem insoluble, may be readily dissolved. Conversely on the
lowering of the temperature and the relief of the pressure, it is
possible to have such mineral substances deposited from solution ; in
other words, they may appear as crystals. It is inore than probable
that some gems are actually deposited as crystals from a molten magma,
but it is even more certain that some are formed on the cooling of
solutions at high temperatures.
While
most precious stones are crystalline, some few, as the Opal, are
probably not, but the distinction is now-regarded as a somewhat
arbitrary one; whether crystalline or colloid, Opal is probably
deposited by water charged with silica slowly percolating through
fissures in rocks.
In dealing with the origin of the various gems, the classification proposed by J. G. Goodchild' will be followed in the
1 " Proc. Royal Physical Soc," vol. xiv., p. 183.
