liquors, will be
intoxicated by a quantity that will produce no effect upon one who
has been for some time accustomed to take them; and when a person
has used himself to these stimulants for some time, the ordinary
powers which in common support life, will not have their proper
effects upon him, because his excitability has been in some measure
exhausted by the stimulants.

The same holds good with respect to tobacco and opium; a person
accustomed to take opium will not be affected by a quantity that
would completely intoxicate one not used to it; because the
excitability has been so far exhausted by the use of that drug, that
it cannot be acted on by a small quantity.

These facts, with innumerable others, which will easily suggest
themselves to you, prove the truth of our second proposition,
namely, that when the exciting powers have acted violently, or for a
considerable time, the excitability is exhausted, or less fit to be
acted on.

This exhaustion of the excitability, may, however, be either finite,
or temporary; we see animals, while the exciting powers continue to
act, at first appear in their greatest vigour, then gradually decay,
and at last come into that state, in which, from the long continued
action of the exciting powers, the excitability is entirely
exhausted, and death takes place.

We likewise see plants in the spring, while the exciting powers have
acted on them, moderately, and for a short time, arrayed in their
verdant robes, and adorned with flowers of "many mingling hues;"
but, as the exciting powers which support the life of the plant,
continue to be applied, and some of them, for instance heat, as the
summer advances become increased, they first lose their verdure,
then grow brown, and at the end of summer cease to live; because
their excitability is exhausted by the long continued action of the
exciting powers; and this does not happen merely in consequence of
the heat of summer decreasing, for they grow brown and die, even in
a greater degree of heat than that which in spring made them grow
luxuriantly.

These are examples of the finite, or irreparable exhaustion of the
excitability, but we find also, that it may be exhausted for a time,
and accumulated again. Though the eye has been so dazzled by the
splendour of light, that it cannot see an object moderately
illuminated, yet, if it be shut for some time, the excitability of
the optic nerve accumulates again, and we are again capable