nfirms; and
I shall follow an arrangement in the consideration of the subject,
which naturally presents itself to us. The chief exciting powers
which act upon us are, air and food; these I shall respectively
consider, and afterwards make a few remarks on exercise.

The air is the main-spring in the animal machine; the source of heat
and activity, without which our blood would soon become a black and
stagnant mass, and life would soon stop.

It is now known, that only a part of atmospheric air, is necessary
for respiration: the atmosphere near the surface of the earth,
consists of two kinds of air; one, which is highly proper for
respiration, and combustion, and in which, an animal immersed, will
live much longer than in the same quantity of common air; and one,
which is perfectly improper for supporting respiration, or
combustion, for an instant.

The first of these airs, has been called vital air, from its
property of supporting life, and constitutes about one fourth of the
atmosphere. [3] The other, from its property of destroying life, is
called azote, and forms of course the remaining three fourths of the
atmosphere.

These two airs may be separated from each other by various methods.
If a candle be inclosed in a given quantity of atmospheric air, it
will burn only for a certain time, and then be extinguished; and
from the rising of the water in the vessel in which it is inclosed,
it is evident that a quantity of air has been absorbed. What has
been absorbed is the vital air, and what remains, the azote, which
is incapable of supporting flame. If an animal be immersed in a
given quantity of common air, it will live only a certain time; at
the end of this time, the air will be found diminished, about one
fourth being extracted from it, and the remainder will neither
support flame nor animal life; this experiment might easily be made,
but it seems a piece of unnecessary cruelty.

By similar experiments to those I have mentioned, we get the azote
pure; here is some, in which a candle has burnt out, and in which
nothing but azote, or the impure part of the atmosphere is left. [4] I
shall plunge a lighted match into it, and you see it is instantly
extinguished.

Some metals, and particularly manganese, when exposed to the
atmosphere, attract the vital air from it, without touching the
azote; and it may be procured from these metals by the application
of heat, in very great purity. Here is a bottle of that kind o