the energy thus liberated, and the amount of food needed is proportional
to the amount of work done. When the individual simply walks across the
floor, or even rises from his chair, this is accompanied by an increase
in the amount of food material broken up and a consequent increase in
the amount of refuse matter eliminated and the heat given off. The
income and outgo of the body in both matter and energy is balanced. If,
during the experimental period, it is found that less energy is
liberated than that contained in the food assimilated, it is also found
that the body has gained in weight, which simply means that the extra
energy has been stored in the body for future use. No more energy can be
obtained from the body than is furnished, and for all furnished in the
food an equivalent amount is regained. There is no trace of any creation
or destruction of energy. While, on account of the complexity of the
experimenting, an absolutely strict balance sheet cannot be made, all
the results are of the same nature. So far as concerns measurable
energy, all the facts collected bear out the theoretical conception that
the living body is to be regarded as a machine which converts the
potential energy of chemical composition, stored passively in its food,
into active energy of motion and heat.

It is found, however, that the body is a machine of a somewhat superior
grade, since it is able to convert this potential energy into motion
with less loss than the ordinary machine. As noticed above, in all
machines a portion of the energy is converted into heat and rendered
unavailable by radiating into space. In an ordinary engine only about
one-fifteenth of the energy furnished in the coal can be regained in the
form of motive power, the rest being radiated from the machine as heat.
Some of our better engines to-day utilize a somewhat larger part, but
most of them utilize less than one-tenth. The experiments with the
living body in the respiration apparatus above described, give a means