f air,
which I have expelled by heat from manganese; I shall plunge a taper
into it, and you will perceive that it burns with great brilliancy.
An animal shut up in it, would live about four times as long as if
shut up in an equal quantity of atmospheric air.

If I take three parts of azote, and one of vital air, I shall form a
compound which is similar to the atmosphere, and which is the
mixture best suited to support the health of the body; for if there
were a much greater proportion of vital air, it would act too
powerfully upon the system, and bring on inflammatory diseases; it
would likewise by its stimulus exhaust the excitability, and bring
us sooner to death; and in the same manner that a candle burns
brighter in vital air, and would therefore be sooner exhausted, so
would the flame of life be sooner burnt out.

On the contrary, if the atmosphere contained a much less proportion
of vital air, it would not stimulate the body sufficiently; the
excitability would morbidly accumulate, and diseases of debility
would occur.

Combustion, putrefaction, and the breathing of animals, are
processes which are continually diminishing the quantity of vital
air contained in the atmosphere; and if the all-wise author of
nature had not provided for its continual re-production, the
atmosphere would in all probability have long since become too
impure to support life; but this is guarded against in a most
beautiful manner.

Water is not a simple element, as has been supposed, but is composed
of vital air, and a particular kind of air which is called
_inflammable_; the same that is used to fill balloons. It has been
found by experiment, that one hundred pounds of water, are composed
of eighty-five pounds of vital air, and fifteen of inflammable
air. [5]

Water may be decompounded by a variety of means, and its component
parts separated from each other.

Vegetables effect this decomposition; they absorb water, and
decompose it in their glands; and taking the inflammable air for
their nourishment, breathe out the vital air in a state of very
great purity; this may be ascertained by a very easy experiment.

This vital air is received by animals into their lungs, gives them
their heat, and communicates a red colour to their blood; when
animals die for want of vital air, their blood is always found
black.

From what I have said, it is evident, that in large and populous
towns, where combustion and respiration are continuall