this subject here. It is
sufficient if we briefly outline its salient aspects. Energy is
recognised in two forms, kinetic and potential. The form of energy which
is most apparent to us is the _energy of motion_; for example, a rolling
stone, running water, a falling body, and so on. We call the energy of
motion _kinetic energy_. Potential energy is the energy a body has in
virtue of its position--it is its capacity, in other words, to acquire
kinetic energy, as in the case of a stone resting on the edge of a
cliff.

Energy may assume different forms; one kind of energy may be converted
directly or indirectly into some other form. The energy of burning coal,
for example, is converted into heat, and from heat energy we have
mechanical energy, such as that manifested by the steam-engine. In this
way we can transfer energy from one body to another. There is the energy
of the great waterfalls of Niagara, for instance, which are used to
supply the energy of huge electric power stations.

What Heat is

An important fact about energy is, that all energy _tends to take the
form of heat energy_. The impact of a falling stone generates heat; a
waterfall is hotter at the bottom than at the top--the falling particles
of water, on striking the ground, generate heat; and most chemical
changes are attended by heat changes. Energy may remain latent
indefinitely in a lump of wood, but in combustion it is liberated, and
we have heat as a result. The atom of radium or of any other
radio-active substance, as it disintegrates, generates heat. "Every hour
radium generates sufficient heat to raise the temperature of its own
weight of water, from the freezing point to the boiling point." And what
is heat? _Heat is molecular motion._ The molecules of every substance,
as we have seen on a previous page, are in a state of continual motion,
and the more vigorous the motion the hotter the body. As wood or coal
burns, the invisible molecules of these substances are violently
agitated, and give rise to ether waves which our senses interpret as
light and heat. In this constant movement of the molecules, then, we
have a manifestation of the energy of motion and of heat.

That energy which disappears in one form reappears in another has been
found to be universally true. It was Joule who, by churning water, first
showed that a measurable quantity of mechanical energy could be
transformed into a measurable quantity of heat energy. By causing an
ap