s,
I believe, necessary for the composition of porcelain, as well as a
finer kind of clay; and these materials are, no doubt, more carefully
prepared, and curiously wrought, in the one case than in the other.
Porcelain owes its beautiful semitransparency to a commencement of
vitrification.

EMILY.

But the commonest earthen-ware, though not transparent, is covered with
a kind of glazing.

MRS. B.

That precaution is equally necessary for use as for beauty, as the ware
would be liable to be spoiled and corroded by a variety of substances,
if not covered with a coating of this kind. In porcelain it consists of
enamel, which is a fine white opake glass, formed of metallic oxyds,
sand, salts, and such other materials as are susceptible of
vitrification. The glazing of common earthen-ware is made chiefly of
oxyd of lead, or sometimes merely of salt, which, when thinly spread
over earthen vessels, will, at a certain heat, run into opake glass.

CAROLINE.

And of what nature are the colours which are used for painting
porcelain?

MRS. B.

They are all composed of metallic oxyds, so that these colours, instead
of receiving injury from the application of fire, are strengthened and
developed by its action, which causes them to undergo different degrees
of oxydation.

Alumine and silex are not only often combined by art, but they have in
nature a very strong tendency to unite, and are found combined, in
different proportions, in various gems and other minerals. Indeed, many
of the precious stones, such as ruby, oriental sapphire, amethyst, &c.
consist chiefly of alumine.

We may now proceed to the alkaline earths, I shall say but a few words
on BARYTES, as it is hardly ever used, except in chemical laboratories.
It is remarkable for its great weight, and its strong alkaline
properties, such as destroying animal substances, turning green some
blue vegetable colours, and showing a powerful attraction for acids;
this last property it possesses to such a degree, particularly with
regard to the sulphuric acid, that it will always detect its presence in
any substance or combination whatever, by immediately uniting with it,
and forming a sulphat of barytes. This renders it a very valuable
chemical test. It is found pretty abundantly in nature in the state of
carbonat, from which the pure earth can be easily separated.

The next earth we have to consider is LIME. This is a substance of too
great and general importance