ygen
gas is essential both to respiration and combustion, while neither of
these processes can be performed in nitrogen gas.

CAROLINE.

But if nitrogen gas is unfit for respiration, how does it happen that
the large proportion of it which enters into the composition of the
atmosphere is not a great impediment to breathing?

MRS. B.

We should breathe more freely than our lungs could bear, if we respired
oxygen gas alone. The nitrogen is no impediment to respiration, and
probably, on the contrary, answers some useful purpose, though we do not
know in what manner it acts in that process.

EMILY.

And by what means can the two gasses, which compose the atmospheric air,
be separated?

MRS. B.

There are many ways of analysing the atmosphere: the two gasses may be
separated first by combustion.

EMILY.

You surprise me! how is it possible that combustion should separate
them?

MRS. B.

I should previously remind you that oxygen is supposed to be the only
simple body naturally combined with negative electricity. In all the
other elements the positive electricity prevails, and they have
consequently, all of them, an attraction for oxygen.*

[Footnote *: If chlorine or oxymuriatic gas be a simple body,
according to Sir H. Davy's view of the subject, it must be
considered as an exception to this statement; but this subject
cannot be discussed till the properties and nature of chlorine
come under examination.]

CAROLINE.

Oxygen the only negatively electrified body! that surprises me
extremely; how then are the combinations of the other bodies performed,
if, according to your explanation of chemical attraction, bodies are
supposed only to combine in virtue of their opposite states of
electricity?

MRS. B.

Observe that I said, that oxygen was the only _simple_ body, naturally
negative. Compound bodies, in which oxygen prevails over the other
component parts, are also negative, but their negative energy is greater
or less in proportion as the oxygen predominates. Those compounds into
which oxygen enters in less proportion than the other constituents, are
positive, but their positive energy is diminished in proportion to the
quantity of oxygen which enters into their composition.

All bodies, therefore, that are not already combined with oxygen, will
attract it, and, under certain circumstances, will absorb it from the
atmosphere, in which case the nitrogen gas will remain alone, and