_Of Dephlogisticated Air._
Dephlogisticated air, which is one of the component parts of
atmospherical air, is a principal element in the composition of acids,
and may be extracted by means of heat from many substances which contain
them, especially the nitrous and vitriolic; as from nitre, red
precipitate, the vitriols, and turbith mineral, and also from these two
acids themselves, exposed to a red heat in an earthen tube. This kind of
air is also contained in several substances which had attracted it from
the atmosphere, as from precipitate _per se_, _minium_, & _manganese_.
Dephlogisticated air is likewise produced by the action of light upon
green vegetables; and this seems to be the chief means employed by
nature to preserve the purity of the atmosphere.
It is this ingredient in atmospheric air that enables it to support
combustion and animal life. By means of it the most intense heat may be
produced, and in the purest of it animals will live nearly five times as
long as in an equal quantity of atmospherical air.
In respiration, part of this air, passing the membrane of the lungs,
unites with the blood, and imparts to it its florid colour, while the
remainder, uniting with phlogiston exhaled from the venous blood, forms
fixed air. It is dephlogisticated air combined with water that enables
fishes to live in it.
Dephlogisticated air is something heavier than atmospherical air, and
the purity of it measured by mixing with it two equal quantities of
nitrous or inflammable air, deducing the residuum after the diminution
from the three measures employed, and dividing the remainder by 3, as in
the process for common air.
_Of Phlogisticated Air._
The other ingredient in the composition of atmospherical air is
phlogisticated air. It is procured by extracting the dephlogisticated
part of the common air, as by the calcination of metals, &c. &c. by
dissolving animal substances in nitrous acid, and also by the union of
phlogiston with nitrous air, as by heating iron in it, and by a mixture
of iron-filings and sulphur.
Phlogisticated air extinguishes a candle, is entirely unfit for
respiration, and is something lighter than common air. It is not capable
of decomposition, except by exploding it together with a superabundance
of dephlogisticated air, and a quantity of inflammable air, or by taking
the electric spark repeatedly in a mixture of it and dephlogisticated
air. In these cases nitrous acid is formed.