acid that entered into its composition can readily unite.

In order to determine whether the fixed part of common air was deposited
in the diminution of it by nitrous air, I inclosed a vessel full of
lime-water in the jar in which the process was made, but it occasioned
no precipitation of the lime; and when the vessel was taken out, after
it had been in that situation a whole day, the lime was easily
precipitated by breathing into it as usual.

But though the precipitation of the lime was not sensible in this method
of making the experiment, it is sufficiently so when the whole process
is made in lime-water, as will be seen in the second part of this work;
so that we have here another evidence of the deposition of fixed air
from common air. I have made no alteration, however, in the preceding
paragraph, because it may not be unuseful, as a caution to future
experimenters.

It is exceedingly remarkable that this effervescence and diminution,
occasioned by the mixture of nitrous air, is peculiar to common air, or
_air fit for respiration_; and, as far as I can judge, from a great
number of observations, is at least very nearly, if not exactly, in
proportion to its fitness for this purpose; so that by this means the
goodness of air may be distinguished much more accurately than it can be
done by putting mice, or any other animals, to breathe in it.

This was a most agreeable discovery to me, as I hope it may be an useful
one to the public; especially as, from this time, I had no occasion for
so large a stock of mice as I had been used to keep for the purpose of
these experiments, using them only in those which required to be very
decisive; and in these cases I have seldom failed to know beforehand in
what manner they would be affected.

It is also remarkable that, on whatever account air is unfit for
respiration, this same test is equally applicable. Thus there is not the
least effervescence between nitrous and fixed air, or inflammable air,
or any species of diminished air. Also the degree of diminution being
from nothing at all to more than one third of the whole of any quantity
of air, we are, by this means, in possession of a prodigiously large
_scale_, by which we may distinguish very small degrees of difference in
the goodness of air.

I have not attended much to this circumstance, having used this test
chiefly for greater differences; but, if I did not deceive myself, I
have perceived a real difference in the