t be
another constituent part of nitrous air, entering the common air, is the
cause of the diminution it suffers in this process; as it is the cause
of a similar diminution, in a variety of other processes.

I would observe, that it is not peculiar to nitrous air to be a test of
the fitness of air for respiration. Any other process by which air is
diminished and made noxious answers the same purpose. Liver of sulphur
for instance, the calcination of metals, or a mixture of iron filings
and brimstone will do just the same thing; but the application of them
is not so easy, or elegant, and the effect is not so soon perceived. In
fact, it is _phlogiston_ that is the test. If the air be so loaded with
this principle that it can take no more, which is seen by its not being
diminished in any of the processes above mentioned, it is noxious; and
it is wholesome in proportion to the quantity of phlogiston that it is
able to take.

This, I have no doubt, is the true theory of the diminution of common
air by nitrous air, the redness of the appearance being nothing more
than the usual colour of the fumes, of spirit of nitre, which is now
disengaged from the superabundant phlogiston with which it was combined
in the nitrous air, and ready to form another union with any thing that
is at hand, and capable of it.

With the volatile alkali it forms nitrous ammoniac, water imbibes it
like any other acid, even quicksilver is corroded by it; but this action
being slow, the redness in this mixture of nitrous and common air
continues much longer when the process is made in quicksilver, than when
it is made in water, and the diminution, as I have also observed; is by
no means so great.

I was confirmed in this opinion when I put a bit of volatile alkaline
salt into the jar of quicksilver in which I made the mixture of nitrous
and common air. In these circumstances, the vessel being previously
filled with the alkaline fumes, the acid immediately joined them, formed
the white clouds above mentioned, and the diminution proceeded almost
as far as when the process was made in water. That it did not proceed
quite so far, I attribute chiefly to the small quantity of calx formed
by the slight solution of mercury with the acid fumes not being able to
absorb all the fixed air that is precipitated from the common air by the
phlogiston.

In part, also, it may be owing to the small quantify of surface in the
quicksilver in the vessels that I made use