bdivided in it that they
are scarcely visible even in the sun's rays. Water vapours always have
the preponderance amongst these foreign particles. The air, however, is
also mixed with another elastic substance resembling air, which differs
from it in numerous properties, and is, with good reason, called aerial
acid by Professor Bergman. It owes its presence to organised bodies,
destroyed by putrefaction or combustion.

+5.+ Nothing has given philosophers more trouble for some years than
just this delicate acid or so called fixed air. Indeed it is not
surprising that the conclusions which one draws from the properties of
this elastic acid are not favourable to all who are prejudiced by
previously conceived opinions. These defenders of the Paracelsian
doctrine believe that the air is in itself unalterable; and, with Hales,
that it really unites with substances thereby losing its elasticity; but
that it regains its original nature as soon as it is driven out of these
by fire or fermentation. But since they see that the air so produced is
endowed with properties quite different from common air, they conclude,
without experimental proofs, that this air has united with foreign
materials, and that it must be purified from these admixed foreign
particles by agitation and filtration with various liquids. I believe
that there would be no hesitation in accepting this opinion, if one
could only demonstrate clearly by experiments that a given quantity of
air is capable of being completely converted into fixed or other kind of
air by the admixture of foreign materials; but since this has not been
done, I hope I do not err if I assume as many kinds of air as experiment
reveals to me. For when I have collected an elastic fluid, and observe
concerning it that its expansive power is increased by heat and
diminished by cold, while it still uniformly retains its elastic
fluidity, but also discover in it properties and behaviour different
from those of common air, then I consider myself justified in believing
that this is a peculiar kind of air. I say that air thus collected must
retain its elasticity even in the greatest cold, because otherwise an
innumerable multitude of varieties of air would have to be assumed,
since it is very probable that all substances can be converted by
excessive heat into a vapour resembling air.

+6.+ Substances which are subjected to putrefaction or to destruction by
means of fire diminish, and at the same