or of the sulphuret of potash, or such other
substance as is judged proper for examining their action upon the
residuum of air. I shall, in the sequel, give an account of these
methods of analysing air, when I have explained the nature of these
different substances, which are only here in a manner accidentally
mentioned. After this examination, so much water must be let into the
glass as will displace the quicksilver, and then, by means of a shallow
dish placed below the bell-glass, it is to be removed into the common
water pneumato-chemical apparatus, where the air remaining may be
examined at large, and with great facility.

When very soft and very pure iron has been employed in this experiment,
and, if the combustion has been performed in the purest respirable or
vital air, free from all admixture of the noxious or mephitic part, the
air which remains after the combustion will be found as pure as it was
before; but it is difficult to find iron entirely free from a small
portion of charry matter, which is chiefly abundant in steel. It is
likewise exceedingly difficult to procure the pure air perfectly free
from some admixture of mephitis, with which it is almost always
contaminated; but this species of noxious air does not, in the smallest
degree, disturb the result of the experiment, as it is always found at
the end exactly in the same proportion as at the beginning.

I mentioned before, that we have two ways of determining the constituent
parts of atmospheric air, the method of analysis, and that by synthesis.
The calcination of mercury has furnished us with an example of each of
these methods, since, after having robbed the respirable part of its
base, by means of the mercury, we have restored it, so as to recompose
an air precisely similar to that of the atmosphere. But we can equally
accomplish this synthetic composition of atmospheric air, by borrowing
the materials of which it is composed from different kingdoms of nature.
We shall see hereafter that, when animal substances are dissolved in the
nitric acid, a great quantity of gas is disengaged, which extinguishes
light, and is unfit for animal respiration, being exactly similar to the
noxious or mephitic part of atmospheric air. And, if we take 73 parts,
by weight, of this elastic fluid, and mix it with 27 parts of highly
respirable air, procured from calcined mercury, we will form an elastic
fluid precisely similar to atmospheric air in all its properties.