work. This amount of energy can neither be increased nor decreased;
energy can no more be created or destroyed than matter. It exists,
however, in a variety of forms, which may be either active or passive.
In the active state it takes some form of motion. The various forces
which we recognize in nature--heat, light, electricity, chemism,
etc.--are simply forms of motion, and thus forms of this energy. These
various types of energy, being only expressions of the universal energy,
are convertible into each other in such a way that when one disappears
another appears. A cannon ball flying through the air exhibits energy of
motion; but it strikes an obstacle and stops. The motion has apparently
stopped, but an examination shows that this is not the case. The cannon
ball and the object it strikes have been heated, and thus the motion of
the ball has simply been transformed into a different form of motion,
which we call heat. Or, again, the heat set free under the locomotive
boiler is converted by machinery into the motion of the locomotive. By
still different mechanism it may be converted into electric force. All
forms of motion are readily convertible into each other, and each form
in which energy appears is only a phase of the total energy of nature.

A second condition of energy is energy at rest, or potential energy. A
stone on the roof of a house is at rest, but by virtue of its position
it has a certain amount of potential energy, since, if dislodged, it
will fall to the ground, and thus develop energy of motion. Moreover, it
required to raise the stone to the roof the expenditure of an amount of
energy exactly equal to that which will reappear if the stone is allowed
to fall to the ground. So in a chemical molecule, like fat, there is a
store of potential energy which may be made active by simply breaking
the molecule to pieces and setting it free. This occurs when the fat
burns and the energy is liberated as heat. But it required at some time
the expenditure of an equal amount of energy to make the molecule. When