ifferent properties which substances composed by
them possess. This is exemplified in common _sulphur_, which is as mild
as air, and may be taken into the stomach with the utmost safety, though
nothing can be more destructive than one of its constituent parts,
separately taken, viz. oil of vitriol. Common air, therefore,
notwithstanding its mildness, may be composed of similar principles, and
be a real _sulphur_.

That the fixed air which makes part of the atmosphere is not presently
imbibed by the waters of the sea, on which it rests, may be owing to the
union which this kind of air also appears to be capable of forming with
phlogiston. For fixed air is evidently of the nature of an acid; and it
appears, in fact, to be capable of being combined with phlogiston, and
thereby of constituting a species of air not liable to be imbibed by
water. Phlogiston, however, having a stronger affinity with acid air,
which I suppose to be the basis of common air, it is not surprising
that, uniting with this, in preference to the fixed air, the latter
should be precipitated, whenever a quantity of common air is made
noxious by an over-charge of phlogiston.

The fixed air with which our atmosphere abounds may also be supplied by
volcanos, from the vast masses of calcareous matter lodged in the earth,
together with inflammable air. Also a part of it may be supplied from
the fermentation of vegetables upon the surface of it. At present, as
fast as it is precipitated and imbibed by one process, it may be set
loose by others.

Whether there be, upon, the whole, an increase or a decrease of the
general mass of the atmosphere is not easy to conjecture, but I should
imagine that it rather increases. It is true that many processes
contribute to a great visible diminution of common air, and that when by
other processes it is restored to its former wholesomeness, it is not
increased in its dimensions; but volcanos and fires still supply vast
quantities of air, though in a state not yet fit for respiration; and it
will have been seen in my experiments, that vegetable and animal
substances, dissolved by putrefaction, not only emit phlogiston, but
likewise yield a considerable quantity of permanent elastic air,
overloaded indeed with phlogiston, as might be expected, but capable of
being purified by those processes in nature by which other noxious air
is purified.

That particles are continually detaching themselves from the surfaces of
all sol