effect are found in the feeling of discomfort and lassitude which result
when stimulants to which the body has become accustomed are withdrawn. Not
until one gets back his bodily reserve is he able to work normally and
effectively. Increase in bodily energy comes through health and not
through the use of stimulants.

*Summary.*—The body requires a continuous supply of energy. To obtain this
supply, materials possessing potential, or stored-up, energy are
introduced into it. The free oxygen of the air and the substances known as
foods, on account of the chemical relations which they sustain to each
other, contain potential energy and are utilized for supplying the body.
So long as the foods are not oxidized, the energy remains in the potential
form, but in the process of oxidation the potential energy is changed to
kinetic energy and made to do the work of the body.

*Exercises.*—1. In what different ways does the body use energy?

2. Show that a stone lying against the earth has no energy, while the same
stone above the earth has energy.

3. How does potential energy differ from kinetic energy?

4. What kind of energy is possessed by a bent bow? By a revolving wheel?
By a coiled spring? By the wind? By gunpowder?

5. How does decomposing water with electricity store energy?

6. Account for the energy possessed by the oxygen of the air and food
substances.