ry, if we suppose a greater quantity of air to rush
through the tubes, then the fuel will be consumed or exhausted faster
than it is supplied; and in order therefore to reduce the combustion
to the proper degree, the quantity of air supplied must be
diminished, and the quantity of fuel increased.

If we suppose one of the tubes, instead of common air, to supply
oxygen gas, it will represent the action of wine, spirits, ether,
opium, and other powerful stimulants upon the body: a bright and
vivid flame will be produced, which however will only be of short
duration, for the fuel will be consumed faster than it is supplied,
and a state of exhaustion will take place.

We may carry this illustration still further, and suppose that the
air tubes exhaust the fuel every day faster than it can be supplied,
then it will be necessary at night to stop up some of the tubes, so
that the expense of fuel may be less than its supply, in order to
make up for the deficiency. When this is made up, the tubes may in
the morning be opened, and the combustion carried on during the day
as usual. This will illustrate the nature of sleep. In speaking of
this subject, it was observed, that the more violently the exciting
powers have acted, the sooner is sleep brought on; because the
excitability is sooner exhausted. In the same way the more the air
rushes through the tubes, the sooner will the fuel be consumed, and
want replenishing. When the exciting powers have acted feebly, a
person feels no inclination to sleep, because the excitability is not
exhausted to the proper degree, and therefore does not want
accumulating. But any diffusible stimulus, as spirits, or opium, will
soon exhaust it to the proper degree.

In the same way, if the air have not passed rapidly through the
tubes, the fuel will not be exhausted: but it may be brought to a
proper degree of exhaustion by the application of oxygen gas.

When the air which nourishes the flame is so regulated, that it
consumes the fuel as it is supplied, but no faster, a clear and
steady flame will be kept up, which will go on as long as the fuel
lasts, or the grate resists the action of the fire: but at last when
the fuel, which we do not suppose inexhaustible, is burnt out, the
fire must cease.

In the same manner, if the different exciting powers which support
life were properly regulated, all the functions of the body would be
properly performed, and we should pass our life in a state of he