is constantly giving out a large quantity of waste materials by
what is called _insensible perspiration_; a process which is of great
importance to the preservation of health, and which is called
_insensible_, because the exhalation, being in the form of vapor, and
carried off by the surrounding air, is invisible to the eye. But its
presence may often be made manifest, even to the sight, by the near
approach of a dry cool mirror, on the surface of which it will soon be
condensed so as to become visible. It is this which causes so copious
deposits upon the windows of a crowded school-room in cold weather. A
portion of these exhalations, however, proceed from the lungs.

There is an experiment that may be easily tried, which affords
conclusive evidence that the amount of insensible perspiration is much
greater than it is ordinarily supposed to be. Take a dry glass jar, with
a neck three or four inches in diameter, and thrust the hand and a part
of the fore-arm into it, closing the space in the neck about the arm
with a handkerchief. After the lapse of a few minutes, it will be seen,
by drawing the fingers across the inside of the jar, that the insensible
perspiration even from the hand is very considerable. Many attempts have
been made to estimate accurately the amount of exhaled matter carried
off through the skin; but many difficulties stand in the way of
obtaining precise results. There is a great difference in different
constitutions, and even in the same person at different times, in
consequence of which we must be satisfied with an approximation to the
truth.

Although the precise amount of perspiration can not be ascertained, it
is generally agreed that the cutaneous exhalation is greater than the
united excretions of both bowels and kidneys. Great attention has been
given to this subject. Sanctorius, a celebrated medical writer, weighed
himself, his food, and his excretions, daily, for thirty days. He
inferred from his experiments that _five pounds_ of every eight, of both
food and drink, taken into the system, pass out through the skin. All
physiologists agree that from twenty to forty ounces pass off through
the skin of an adult in usual health every twenty-four hours. Take the
lowest estimate, and we find the skin charged with the removal of
_twenty ounces_ of waste matter from the system _every day_. We can thus
see ample reason why checked perspiration proves so detrimental to
health; for every twenty-four h