re is not sufficient left
for properly running the vital processes. Evidences of their weakening
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when stimulants to which the body has become accustomed are withdrawn.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.

7. Trace the energy supply of the body back to the sun.

8. Why must both oxygen and food be introduced into the body in order to
supply it with energy?

9. How may overwork and overexercise diminish the energy supply of the
body?

10. How may one increase the amount of his energy?

PRACTICAL WORK

*Suggested Experiments.*--1. The change of kinetic into potential energy
may be shown by stretching a piece of rubber, by lifting a weight, and by
separating the armature from a magnet.

2. The change of potential into kinetic energy may be shown by letting
weights fall to the ground, by releasing the end of a piece of stretched
rubber, and by burning substances.

3. The change of one form of kinetic energy to another may be illustrated
by rubbing together two pieces of wood until they are heated, by ringing a
bell, and by causing motion in air or in water by heating them. If
suitable apparatus is at hand, the transformation of electrical energy
into h