engine, again, is designed to convert potential or passive energy into
active energy. Potential energy in the form of chemical composition
(coal) is supplied to the engine, and this energy is first liberated in
the active form of heat and then is converted into the motion of the
great fly-wheel. In all these cases there is 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