e constant exercise of the intellectual faculties--lawyers,
writers, statesmen, students, scientific men, and other
brain-workers--excrete more urea than do men engaged in the most
physically laborious occupations. An activity of thoughts and ideas
involves a corresponding destruction of the tissues, and these require,
for their reparation, the consumption of food. Here, then, we have a
physical meaning for the common expression--"food for thought."

That the amount of heat developed in the animal organism, is
proportionate to the quantity of fatty matters (or of substances capable
of forming them) supplied to it in the shape of food, is a proposition
which admits of easy demonstration. The natives of warm regions do not
require the generation of much heat within their bodies, because the
temperature of the medium in which they exist is generally as high as,
or higher than, that of their blood. But as they must consume food for
the purpose of repairing the waste of their nitrogenous tissues, and as
every kind of food contains heat-producing elements, an excess of heat
is developed within their bodies, which, if allowed to accumulate, would
speedily produce fatal results. The means by which nature removes this
superabundant heat are admirably simple, as indeed all its contrivances
are. The skin is permeated with millions of pores, and through these
openings a large quantity of vapor is given off, and carries with it the
surplus heat. The pores are the orifices of minute convoluted tubes
which lie beneath the skin, and when straightened measure each about the
tenth of an inch, or, according to a writer in the _British and Foreign
Medico-Chirurgical Review_ (1859, page 349), the one-fifteenth of an
inch in length. According to Erasmus Wilson, the number of these tubes
which open into every square inch of the surface of the body is 2,800.
The total number of square inches on the surface of an average sized man
is 2,500, consequently the surface of his body is drained by not less
than twenty-eight miles of tubing, furnished with 7,000,000 openings.
The cooling of the body, by the evaporation of water from it, admits of
explanation by well-known natural laws. Water, in the state of vapor,
occupies a space 1,700 fold greater than it does in its liquid
condition. It is heat which causes its vaporous form, but it ceases to
be heat when it has accomplished this change in the condition of the
liquid; for, suffering itself an altera