compound salt which
it forms with potash, commonly called _nitre_.

CAROLINE.

Why is it so called? Pray, Mrs. B., let these old unmeaning names be
entirely given up, by us at least; and let us call this salt _nitrat of
potash_.

MRS. B.

With all my heart; but it is necessary that I should, at least, mention
the old names, and more especially those which are yet in common use;
otherwise, when you meet with them, you would not be able to understand
their meaning.

EMILY.

And how is the acid obtained from this salt?

MRS. B.

By the intervention of sulphuric acid, which combines with the potash,
and sets the nitric acid at liberty. This I can easily show you, by
mixing some nitrat of potash and sulphuric acid in this retort, and
heating it over a lamp; the nitric acid will come over in the form of
vapour, which we shall collect in a glass bell. This acid, diluted in
water, is commonly called _aqua fortis_, if Caroline will allow me to
mention that name.

CAROLINE.

I have often heard that aqua fortis will dissolve almost all metals; it
is no doubt because it yields its oxygen so easily.

MRS. B.

Yes; and from this powerful solvent property, it derived the name of
aqua fortis, or strong water. Do you not recollect that we oxydated, and
afterwards dissolved, some copper in this acid?

EMILY.

If I remember right, the nitrat of copper was the first instance you
gave us of a compound salt.

CAROLINE.

Can the nitric acid be completely decomposed and converted into nitrogen
and oxygen?

EMILY.

That cannot be the case, Caroline; since the acid can be decomposed only
by the combination of its constituents with other bodies.

MRS. B.

True; but caloric is sufficient for this purpose. By making the acid
pass through a red hot porcelain tube, it is decomposed; the nitrogen
and oxygen regain the caloric which they had lost in combining, and are
thus both restored to their gaseous state.

The nitric acid may also be partly decomposed, and is by this means
converted into NITROUS ACID.

CAROLINE.

This conversion must be easily effected, as the oxygen is so slightly
combined with the nitrogen.

MRS. B.

The partial decomposition of nitric acid is readily effected by most
metals; but it is sufficient to expose the nitric acid to a very strong
light to make it give out oxygen gas, and thus be converted into nitrous
acid. Of this acid there are various degrees, according to the
proportions of o