shall proceed to point out such necessary cautions for your conduct,
as are easily deduced from them; and which experience confirms; 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