economical practice depends. These investigations,
while they have thrown much light on the matter, have by no means
exhausted it, and it will be readily understood that the complete
elucidation of a subject of such complexity, touching on so many of the
most abstruse and difficult problems of chemistry and physiology, and in
which the experiments are liable to be affected by disturbing causes,
dependent on peculiarities of constitution of different animals, cannot
be otherwise than a slow process.

In considering the principles of feeding, it is necessary to point out,
in the first instance, that the plant and animal are composed of the
same chemical elements, hence the food supplied to the latter invariably
contains all the substances it requires for the maintenance of its
functions. And not only is this the case, but these elements are to a
great extent combined together in a similar manner,--the fibrine,
caseine, albumen, and fatty matters contained in animals corresponding
in all respects with the compounds extracted from plants under the same
name; and though the starchy and saccharine substances do not form any
part of the animal body, they are represented in the milk, the food
which nature has provided for the young animal. It has been frequently
assumed that the nitrogenous and fatty matters are simply absorbed into
the animal system, and deposited unchanged in its tissues; but it is
probable that the course of events is not quite so simple, although,
doubtless, the decomposition which occurs is comparatively trifling. The
starchy matters, on the other hand, are completely changed, and devoted
to purposes which will be immediately explained.

It is a matter of familiar experience, that if the food be properly
proportioned to the requirements of the animal, its weight remains
unchanged; and the inference to be drawn from this fact obviously is,
that the food does not remain permanently in the system, but must be
again got rid of. It escapes partly through the lungs, and partly by the
excretions, which do not consist merely of the part which has not been
digested, but also of that portion which has been absorbed, and after
performing its allotted functions within the system, has become effete
and useless. When the weights of the excretions, the carbon contained in
the carbonic acid expired by the lungs and the small quantity of matter
which escapes in the form of perspiration, are added together, they are
found