ts, 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 deg. F., and
at this temperature a super-saturated solution may easily be made. But
if the water is heated the salt then becomes more and more insoluble as
the temperature increases, till it is completely insoluble.

If a super-saturated solution of this Glauber's Salt is made in a glass,
at ordinary atmospheric temperature, and into this cold solution,
without heating, is dropped a small crystal of the same salt, there will
be caused a rise in temperature, and the whole will then crystallise out
quite suddenly; the water will be absorbed, and the whole will solidify
into a mass which exactly fits the inner contour of the vessel.

We have now formed what _might_ be a precious stone, and no doubt would
be, if continuous pressure could be applied to it for perhaps a few
thousand years; at any rate, the formation of a natural jewel is not
greatly different, and after being subjected for a period, extending to
ages, to the washings of moisture, the contact of its containing bed
(its later matrix), the action of the changes in the temperature of the
earth in its vicinity, it emerges by volcanic eruption, earthquake,
landslip and the like, or is discovered as a rare and valuable specimen
of some simple compound of earth-crust and water, as simple as Glauber's
Salt, or as the pure crystallized carbon.

It is also curious to note that in some cases the stones have not been
caused by aqueous deposit in an already existing hollow, but the aqueous
infusion has acted on a portion of the rock on which it rested,
absorbing the rock, and, as it were, replacing it by its own substance.
This is evidenced in cases where gems have been found encrusted on their
matrix, which latter was being slowly transformed to the character of
the jewel encrusted, or "scabbed" on it.

The character of the matrix is also in a great measure the cause of the
variety of the stone, for it is obvious that the same salt-charged
aqueous solution which undergoes change in and on ironstone would result
in an entirely different product from that resting on o