slowly evaporated, leaving behind it the particle of earth
which it held in solution? You know that crystallisation is more regular
and perfect, in proportion as the evaporation of the solvent is slow and
uniform; nature, therefore, who knows no limit of time, has, in all
works of this kind, an infinite advantage over any artist who attempts
to imitate such productions.

EMILY.

I can now conceive that the arrangement of the particles of earth,
during crystallisation, may be such as to occasion transparency, by
admitting a free passage to the rays of light; but I cannot understand
why crystallised earths should assume such beautiful colours as most of
them do. Sapphire, for instance, is of a celestial blue; ruby, a deep
red; topaz, a brilliant yellow?

MRS. B.

Nothing is more simple than to suppose that the arrangement of their
particles is such, as to transmit some of the coloured rays of light,
and to reflect others, in which case the stone must appear of the colour
of the rays which it reflects. But besides, it frequently happens that
the colour of a stone is owing to a mixture of some metallic matter.

CAROLINE.

Pray, are the different kinds of precious stones each composed of one
individual earth, or are they formed of a combination of several earths?

MRS. B.

A great variety of materials enters into the composition of most of
them; not only several earths, but sometimes salts and metals. The
earths, however, in their simple state, frequently form very beautiful
crystals; and, indeed, it is in that state only that they can be
obtained perfectly pure.

EMILY.

Is not the Derbyshire spar produced by the crystallisation of earths, in
the way you have just explained? I have been in some of the
subterraneous caverns where it is found, which are similar to those you
have described.

MRS. B.

Yes; but this spar is a very imperfect specimen of crystallisation; it
consists of a variety of ingredients confusedly blended together, as you
may judge by its opacity, and by the various colours and appearances
which it exhibits.

But, in examining the earths in their most perfect and agreeable form,
we must not lose sight of that state in which they are commonly found,
and which, if less pleasing to the eye, is far more interesting by its
utility.

All the earths are more or less endowed with alkaline properties; but
there are four, barytes, magnesia, lime, and strontites, which are
called _alkaline earths