by burning some of its
basis, which you see here, in a retort full of oxygen gas. The heat of a
candle is all that is required for this combustion.--

EMILY.

The light is astonishingly brilliant, and what beautiful sparks it
throws out!

MRS. B.

The result of this combustion is the boracic acid, the nature of which,
you see, is proved both by analytic and synthetic means. Its basis has
not, it is true, a metallic appearance; but it makes very hard alloys
with other metals.

EMILY.

But pray, Mrs. B., for what purpose is the boracic acid used in
manufactures?

MRS. B.

Its principal use is in conjunction with soda, that is, in the state of
_borat of soda_, which in the arts is commonly called borax. This salt
has a peculiar power of dissolving metallic oxyds, and of promoting the
fusion of substances capable of being melted; it is accordingly employed
in various metallic arts; it is used, for example, to remove the oxyd
from the surface of metals, and is often employed in the assaying of
metallic ores.

Let us now proceed to the FLUORIC ACID. This acid is obtained from a
substance which is found frequently in mines, and particularly in those
of Derbyshire, called _fluor_, a name which it acquired from the
circumstance of its being used to render the ores of metals more fluid
when heated.

CAROLINE.

Pray is not this the Derbyshire spar, of which so many ornaments are
made?

MRS. B.

The same; but though it has long been employed for a variety of
purposes, its nature was unknown until Scheele, the great Swedish
chemist, discovered that it consisted of lime united with a peculiar
acid, which obtained the name of _fluoric acid_. It is easily separated
from the lime by the sulphuric acid, and unless condensed in water,
ascends in the form of gas. A very peculiar property of this acid is its
union with siliceous earths, which I have already mentioned. If the
distillation of this acid is performed in glass vessels, they are
corroded, and the siliceous part of the glass comes over, united with
the gas; if water is then admitted, part of the silex is deposited, as
you may observe in this jar.

CAROLINE.

I see white flakes forming on the surface of the water; is that silex?

MRS. B.

Yes it is. This power of corroding glass has been used for engraving, or
rather etching, upon it. The glass is first covered with a coat of wax,
through which the figures to be engraved are to be scratched with a pin;