the more wonderful when it it is
considered that the range of the temperature of the medium in which
they exist exceeds 200 degrees Fahrenheit. In India, the mercury in the
thermometer has been observed to stand at 145 degrees in the direct
sunlight, and at 120 degrees in the shade. In high latitudes the
temperature is sometimes so low as 100 degrees below zero. A Russian
army, in an expedition to China, in 1839, was exposed for several
successive days to a temperature of 42 degrees below zero, and suffered
severely in consequence.

The facts which I have cited clearly prove that the animal body
possesses the power of generating, or, to speak more correctly,
liberating heat, either from portions of its own mechanism or from
substances placed within that mechanism.

At one time it was the general belief amongst physiologists that one
portion of the food consumed by an animal was employed in repairing
the waste of its body, and the remaining part was burned as fuel,
evolving heat just in the same way as if it had been consumed in a
furnace. It was this theory that led to the classification of food into
flesh-formers, and heat-givers. It is now doubted if any portion of the
food be really burned in this way; and I, for one, think it far more
probable that, before its conversion into carbonic acid gas and water
(whereby, according to this theory, it develops the heat which keeps the
body warm), it first becomes assimilated, that is, becomes an integral
part of the animal body--blood, fat, muscle. Perhaps we would be
nearer the truth if we were to assume that heat is evolved during the
decomposition of both the nitrogenous and fatty constituents of the
body.

The constantly recurring contractions of the muscles must alone be a
source of much heat. The development of animal motive power is said to
be strictly proportionate to the amount of muscular tissue decomposed.
As the nitrogen of the latter is almost completely excreted under the
form of urea, the quantity of the latter daily eliminated from the
body of an animal is a measure of the decomposed muscular tissue, and
consequently of the amount of muscular power generated in the animal
organism.[7] The correspondence between the amount of the motive power
of an animal, and the quantity of effete nitrogen excreted from the
body, is limited to laboring men and to the lower animals. Strange as
it may appear, it is an incontrovertible fact that men whose pursuits
require th