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
apparatus to stir water vigorously, that apparatus being driven by
falling weights or a rotating flywheel or by any other mechanical means,
the water became heated. A certain amount of mechanical energy had been
used up and a certain amount of heat had appeared. The relation between
these two things was found to be invariable. Every physical change in
nature involves a transformation of energy, but the total quantity of
energy in the universe remains unaltered. This is the great doctrine of
the Conservation of Energy.
§ 13