e
away before the nitrous oxyd appears.

EMILY.

Watery vapour! Whence does that proceed? There is no water in nitrat of
ammonia?

MRS. B.

You must recollect that there is in every salt a quantity of water of
crystallisation, which may be evaporated by heat alone. But, besides
this, water is actually generated in this experiment, as you will see
presently. First tell me, what are the constituent parts of nitrat of
ammonia?

EMILY.

Ammonia, and nitric acid: this salt, therefore, contains three different
elements, nitrogen and hydrogen, which produce the ammonia; and oxygen,
which, with nitrogen, forms the acid.

MRS. B.

Well then, in this process the ammonia is decomposed; the hydrogen quits
the nitrogen to combine with some of the oxygen of the nitric acid, and
forms with it the watery vapour which is now coming over. When that is
effected, what will you expect to find?

EMILY.

Nitrous acid instead of nitric acid, and nitrogen instead of ammonia.

MRS. B.

Exactly so; and the nitrous acid and nitrogen combine, and form the
gaseous oxyd of nitrogen, in which the proportion of oxygen is 37 parts
to 63 of nitrogen.

You may have observed, that for a little while no bubbles of air have
come over, and we have perceived only a stream of vapour condensing as
it issued into the water. --Now bubbles of air again make their
appearance, and I imagine that by this time all the watery vapour is
come away, and that we may begin to collect the gas. We may try whether
it is pure, by filling a phial with it, and plunging a taper into
it--yes, it will do now, for the taper burns brighter than in the common
air, and with a greenish flame.

CAROLINE.

But how is that? I thought no gas would support combustion but oxygen or
chlorine.

MRS. B.

Or any gas that contains oxygen, and is ready to yield it, which is the
case with this in a considerable degree; it is not, therefore,
surprising that it should accelerate the combustion of the taper.

You see that the gas is now produced in great abundance; we shall
collect a large quantity of it, and I dare say that we shall find some
of the family who will be curious to make the experiment of respiring
it. Whilst this process is going on, we may take a general survey of the
most important combinations of the nitric and nitrous acids with the
alkalies.

The first of these is _nitrat of potash_, commonly called _nitre_ or
_saltpetre_.

CAROLINE.

Is not that the