olished.

EMILY.

Pray what is the true colour of silex, which forms such a variety of
different coloured substances? Sand is brown, flint is nearly black, and
precious stones are of all colours.

MRS. B.

Pure silex, such as is found only in the chemist's laboratory, is
perfectly white, and the various colours which it assumes, in the
different substances you have just mentioned, proceed from the different
ingredients with which it is mixed in them.

CAROLINE.

I wonder that silex is not more valuable, since it forms the basis of so
many precious stones.

MRS. B.

You must not forget that the value we set upon precious stones depends
in a great measure upon the scarcity with which nature affords them;
for, were those productions either common or perfectly imitable by art,
they would no longer, notwithstanding their beauty, be so highly
esteemed. But the real value of siliceous earth, in many of the most
useful arts, is very extensive. Mixed with clay, it forms the basis of
all the various kinds of earthen ware, from the most common utensils to
the most refined ornaments.

EMILY.

And we must recollect its importance in the formation of glass with
potash.

MRS. B.

Nor should we omit to mention, likewise, many other important uses of
silex, such as being the chief ingredient of some of the most durable
cements, of mortar, &c.

I said before, that siliceous earth combined with no acid but the
fluoric; it is for this reason that glass is liable to be attacked by
that acid only, which, from its strong affinity for silex, forces that
substance from its combination with the potash, and thus destroys the
glass.

We will now hasten to proceed to the other earths, for I am rather
apprehensive of your growing weary of this part of our subject.

CAROLINE.

The history of the earths is not quite so entertaining as that of the
simple substances.

MRS. B.

Perhaps not; but it is absolutely indispensable that you should know
something of them; for they form the basis of so many interesting and
important compounds, that their total omission would throw great
obscurity on our general outline of chemical science. We shall, however,
review them in as cursory a manner as the subject can admit of.

ALUMINE derives its name from a compound salt called _alum_, of which it
forms the basis.

CAROLINE.

But it ought to be just the contrary, Mrs. B.; the simple body should
give, instead of taking, its name from