ces.

This excellent philosopher discovered that it is the presence of the
fixed air in these substances that renders them _mild_, and that when
they are deprived of it, by the force of fire, or any other process,
they are in that state which had been called _caustic_, from their
corroding or burning animal and vegetable substances.

Fixed air had been discovered by Dr. Macbride of Dublin, after an
observation of Sir John Pringle's, which led to it, to be in a
considerable degree antiseptic; and since it is extracted in great
plenty from fermenting vegetables, he had recommended the use of _wort_
(that is an infusion of malt in water) as what would probably give
relief in the sea-scurvy, which is said to be a putrid disease.

Dr. Brownrigg had also discovered that the same species of air is
contained in great quantities in the water of the Pyrmont spring at Spa
in Germany, and in other mineral waters, which have what is called an
_acidulous_ taste, and that their peculiar flavour, briskness, and
medicinal virtues, are derived from this ingredient.

Dr. Hales, without seeming to imagine that there was any material
difference between these kinds of air and common air, observed that
certain substances and operations _generate_ air, and others _absorb_
it; imagining that the diminution of air was simply a taking away from
the common mass, without any alteration in the properties of what
remained. His experiments, however, are so numerous, and various, that
they are justly esteemed to be the solid foundation of all our knowledge
of this subject.

Mr. Cavendish had exactly ascertained the specific gravities of fixed
and inflammable air, shewing the former of them to be 1-1/2 heavier
than common air, and the latter ten times lighter. He also shewed that
water would imbibe more than its own bulk of fixed air.

Lastly, Mr. Lane discovered that water thus impregnated with fixed air
will dissolve a considerable quantity of iron, and thereby become a
strong chalybeate.

These, I would observe, are by no means all the discoveries concerning
air that have been made by the gentlemen whose names I have mentioned,
and still less are they all that have been made by others; but they
comprise all the previous knowledge of this subject that is necessary to
the understanding of this treatise; except a few particulars, which will
be mentioned in the course of the work, and which it is, therefore,
unnecessary to recite in this place.