derived
through the plant from the sun. When a person eats the food and breathes
the oxygen, this energy becomes the possession of the body. It is then
converted into kinetic energy as the needs of the body require.

[Fig. 84]

Fig. 84--*Simple apparatus* for illustrating transformation of energy.
Potential energy is converted into heat and heat into motion.

*From the Sun to the Cells.*--It thus appears that the body comes into
possession of energy, and is able to use it, through a series of
transferences and transformations that can be traced back to the sun.(72)
Coming to the earth as kinetic energy, it is transformed into potential
energy and stored in the compounds of plants and in the oxygen of the air.
Through the food and the oxygen the potential energy is transferred to the
cells of the body. Then by the uniting of the food and the oxygen at the
cells (oxidation), the potential becomes kinetic energy and is used by the
body in doing its work. The phrase "Child of the Sun" has sometimes been
applied to man to express his dependence upon the sun for his supply of
energy.

*Why Oxygen and Food are Both Necessary.*--The necessity for introducing
both oxygen and food into the body for the purpose of supplying energy is
now apparent. The energy which is used in the body is not the energy of
food alone. Nor is it the energy of oxygen alone. It belongs to both. It
is due to their attraction for each other and their condition of
separation. It cannot, therefore, become kinetic except through their
union. To introduce one of these substances into the body without the
other, would neither introduce the energy nor set it free. They must both
be introduced into the body and there caused to unite.

*Bodily Control of Energy.*--A fact of importance in the supply of energy
to the body is that the rate of transformation (changing of potential to
kinetic) is just sufficient for its needs. It is easily seen that too
rapid or too slow a rate would prove injurious. The oxidations at the
cells are, therefore, under such control that the quantity of kinetic
energy supplied to the body as a whole, and to the different organs, is
proportional to the work that is done. This is attained, in part at least,
through the ability of the body to store up the food materials and hold
them in reserve until they are to be oxidized (page 180).

*Animal Heat and Motion.*--Most of the body's energy is