_Of Fixed Air._
Having considered the properties of those kinds of air which are not
readily absorbed by water, and therefore may be confined by it, I
proceed to those which _are_ absorbed by it, and which require to be
confined by mercury. There are two kinds, however, in a middle state
between these, being absorbed by water, but not very readily; a
considerable time, or agitation, being necessary for that purpose. The
first of these is _fixed air_.
This kind of air is obtained in the purest state by dissolving marble,
lime-stone, and other kinds of mild calcareous earth in any acid. It is
also obtained by the burning, or the putrefaction, of both animal and
vegetable substances, but with a mixture of both phlogisticated and
inflammable air. Fixed air is also produced by heating together iron
filings and red precipitate; the former of which would alone yield
inflammable air, and the latter dephlogisticated. Fixed air is therefore
a combination of these two kinds of air.
Another fact which proves the same thing is, that if charcoal of copper
be heated in dephlogisticated air, almost the whole of it will be
converted into fixed air. On the same principle fixed air is produced
when iron, and other inflammable substances, are burned in
dephlogisticated air, and also when minium, and other substances
containing dephlogisticated air, are heated in inflammable air.
That water is an essential part of fixed air is proved by an experiment
upon _terra ponderosa aerata_, which yields fixed air when it is
dissolved in an acid, but not by mere heat. If steam, however, be
admitted to it in that state, it will yield as much fixed air as when it
is dissolved in an acid.
Water absorbs something more than its own bulk of fixed air, and then
becomes a proper acid. Iron dissolved in this water makes it a proper
chalybeate; as without iron it is of the same nature with Pyrmont or
Seltzer water, which by this means may be made artificially.
Ice will not imbibe this air, and therefore freezing expels it from
water.
Fixed air extinguishes flame, and is fatal to animals breathing in it.
Also water impregnated with this air is fatal to fishes, and highly
injurious to plants. But water thus impregnated will prevent, in a great
measure, the putrefaction of animal substances.
Fixed air thrown into the intestines, by way of glyster, has been found
to give relief in some cases of putrid disease.
_Of Hepatic Air._
Another species of air absorbed by water, but not instantly, is termed
_hepatic air_, being produced by the solution of liver of sulphur, or of
sulphurated iron, in vitriolic or marine acid.
Water imbibes about twice its bulk of this kind of air, and it is then
the same thing with the sulphureous waters of Harrowgate.
_Of Phosphoric Air._
Phosphoric air is produced by the solution of phosphorus in caustic
fixed alkali. If this air be confined by mercury, it will take fire on
being admitted to atmospheric, and much more to dephlogisticated air.
After agitation in water it loses this property, and the residuum is
merely inflammable air, with no great diminution of its bulk. This kind
of air, therefore, probably consists of phosphorus dissolved in
inflammable air; though it cannot be made by melting it in inflammable
air.