_Of Atmospherical Air._
The first species of air that offers itself to our consideration is that
of the atmosphere, which appears to consist of a mixture of two kinds of
air, of different and opposite qualities, viz., dephlogisticated and
phlogisticated, in the proportion of about one third of the former to
two thirds of the latter. It is by means of the former of these two
ingredients that it is capable of supporting flame and animal life.
This composition of atmospherical air is proved by several substances
absorbing the dephlogisticated air, and leaving the phlogisticated. All
these processes have been termed _phlogistic_, because the effect is not
produced but by substances supposed to contain phlogiston in a volatile
state; and by the affinity between phlogiston and the dephlogisticated
part of the air, the one is separated from the other. Of these processes
are the calcination of metals, a mixture of iron-filings and sulphur,
liver of sulphur, the burning of phosphorus, and the effluvia of
flowers.
In some cases, however, it is not so clear that any thing is emitted
from the substance that produces this effect; for water deprived of all
air will absorb the dephlogisticated part of the atmospherical in
preference to the phlogisticated part.
As the purity of atmospherical air, or its fitness for respiration,
depends upon the proportion of the dephlogisticated air that it
contains, any of the above-mentioned processes will suffice to determine
it. The more any given quantity of air is diminished by any of them, the
purer it was before the diminution. But this effect is produced the most
quickly by a mixture of nitrous air, or firing inflammable air in it,
being almost instantaneous.
In order to measure the purity of air, it is convenient to take more of
the nitrous or inflammable air than is necessary to saturate the
dephlogisticated air it contains. Equal quantities of each best answer
the purpose. Supposing a given quantity of atmospherical air to be mixed
with an equal quantity of nitrous air, and the residuum to be 1.1
measure, the proportion of dephlogisticated and phlogisticated air in it
may be found by the following arithmetical operation, it being here
taken for granted that one measure of pure dephlogisticated air will
saturate two measures of pure nitrous air.
2.0 viz. one of each.
1.1 the residuum.
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3)0.9 the quantity that has disappeared.
0.3 the dephlogisticated air contained
in the measure of the air examined.
And this substracted from 1 leaves .7 for the proportion of
phlogisticated air in it.