All airs are compounds. Medium of space is the most voluminous
constituent of every aëriform body, which accounts for an air or gas
and steam being of so much greater volume than that from which it had
been obtained; steam has fifteen hundred times the volume of the water
it was produced from.
Oxygen air is decomposed in converting it with hydrogen to water:
there is no oxygen or hydrogen air in water; their _elements_ are
the constituents of water. Oxygen is decomposed by respiration; when
inspired, it is not expired, but nitrogen, which must have been one
of its constituents, and from there being nothing to constitute the
expiration but the previous inspiration the proposition is proved.
The constituents of oxygen are--nitrogen, _a highly rare imponderable
element_ and medium of space. The first is the most ponderable element
of nitrogen air; its atoms are the largest of all others of the
elements of matter, and, it may be said, they constitute the substance
of the framework of all ponderable or gross formations. Davy says,
"the properties of nitrogen are altogether negative;" the same applies
to every other kind of air, all being constituted of _inert_ atomic
substance, consequently of inactive essence; and all being alike in
every respect but in the size of their atoms. The imponderable element
being highly evanescent, is never found alone, and is always connected
with nitrogen; hence simple nitrogen is obtainable only from bodies, or
by deoxygenating atmospheric air. Atmospheric air is nitrogen, plus the
imponderable element; and when the nitrogen is saturated with the same
element, the air is oxygen: hence, whichever is inspired, nitrogen is
expired.
From nitrogen being evolved copiously from water in vacuo, and from
ice being convertible to nitrogen, according to Priestley, so is
nitrogen a constituent of water, also of the gases into which water is
decomposable; but as it cannot belong to the hydrogen, owing to its
superior levity, it must to the oxygen; which is confirmation of the
above, that nitrogen is a constituent of oxygen air or gas.