no energy or power created,
for the machine must be always supplied with an amount of energy equal
to that which it gives back in another form. Indeed, a larger amount of
energy must be furnished the machine than is expected back, for there is
always an actual loss of available energy. In the process of the
conversion of one form of energy into another some of the energy, from
friction or other cause, takes the form of heat, and is then radiated
into space beyond our reach. It is, of course, not destroyed, for energy
cannot be destroyed; but it has assumed a form called radiant heat,
which is not available for our uses. A machine thus neither creates nor
destroys energy. It receives it in one form and gives it back in another
form, with an inevitable loss of a portion of the energy as radiant
heat. With this understanding, we may now ask if the living body can be
properly compared with a machine.

==A General Comparison of a Body and a Machine==.--That the living body
exhibits the ordinary types of energy is of course clear enough when we
remember that it is always in motion and is always radiating heat--two
of the most common types of physical energy. That this energy is
supplied to the body as it is to other machines, in the form of the
energy of chemical composition, will also need no further proof when it
is remembered that it is necessary to supply the body with appropriate
food in order that it may do work. The food we eat, like coal,
represents so much solar energy which is stored up by the agency of
plant life, and the close comparison between feeding the body to enable
it to work and feeding the engine to enable it to develop energy is so
evident that it demands no further demonstration. The details of the
problem may, however, present some difficulties.

The first question which presents itself is whether the only power the
body possesses is, as in the case with other machines, to _transform_
energy without being able to create or destroy it? Can every bit of
energy shown by the living organism be accounted for by energy furnished
in the food, and conversely can all the energy furnished in the food be
found manifested in the living organism?

The theoretical answer to this question in terms of the law of the
conservation of energy is clear enough, but it is by no means so easy to
answer it by experimental data. To obtain experimental demonstration it
would be necessary to make an accurate determination of th