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 of
seeing with an ordinary light.

We find, that we are not always equally capable of performing the
functions of life. When we have been engaged in any exertion, either
mental or corporeal, for some hours only, we find ourselves
fatigued, and unfit to pursue our labours much longer; if in this
state, several of the exciting powers, particularly light and noise,
be withdrawn; and if we are laid in a posture which does not require
much muscular exertion, we soon fall into that state which nature
intended for the accumulation of the excitability, and which we call
Sleep. In this state, many of the exciting powers cannot act upon us,
unless applied with some violence, for we are insensible to their
moderate action. A moderate light, or a moderate noise, does not
affect us, and the power of thinking, which exhausts the
excitability very much, is in a great measure suspended. When the
action of these powers has been suspended for six or eight hours,
the excitability is again capable of being acted on, and we rise
fresh, and vigorous, and fit to engage in our occupations.