B.

You have guessed it; as soon as it comes in contact with the atmosphere,
it absorbs from it the additional quantity of oxygen necessary to
convert it into nitrous acid gas. And, if I now remove the bottle
entirely from the water, so as to bring at once the whole of the gas
into contact with the atmosphere, this conversion will appear still more
striking--

EMILY.

Look, Caroline, the whole capacity of the bottle is instantly tinged of
an orange colour!

MRS. B.

Thus, you see, it is the most easy process imaginable to convert
_nitrous oxyd gas_ into _nitrous acid gas_. The property of attracting
oxygen from the atmosphere, without any elevation of temperature, has
occasioned this gaseous oxyd being used as a test for ascertaining the
degree of purity of the atmosphere. I am going to show you how it is
applied to this purpose. --You see this graduated glass tube, which is
closed at one end, (PLATE X. Fig. 2.) --I first fill it with water, and
then introduce a certain measure of nitrous gas, which, not being
absorbable by water, passes through it, and occupies the upper part of
the tube. I must now add rather above two-thirds of oxygen gas, which
will just be sufficient to convert the nitrous oxyd gas into nitrous
acid gas.

CAROLINE.

So it has! --I saw it turn of an orange colour; but it immediately
afterwards disappeared entirely, and the water, you see, has risen, and
almost filled the tube.

MRS. B.

That is because the acid gas is absorbable by water, and in proportion
as the gas impregnates the water, the latter rises in the tube. When the
oxygen gas is very pure, and the required proportion of nitrous oxyd gas
very exact, the whole is absorbed by the water; but if any other gas be
mixed with the oxygen, instead of combining with the nitrous oxygen, it
will remain and occupy the upper part of the tube; or, if the gases be
not in the due proportion, there will be a residue of that which
predominates. --Before we leave this subject, I must not forget to
remark that nitrous acid may be formed by dissolving nitrous oxyd gas in
nitric acid. This solution may be effected simply by making bubbles of
nitrous oxyd gas pass through nitric acid.

EMILY.

That is to say, that nitrogen at its highest degree of oxygenation,
being mixed with nitrogen at its lowest degree of oxygenation, will
produce a kind of intermediate substance, which is nitrous acid.

MRS. B.

You have stated the fact with great prec