heric oxygen on the other, constitute a vast store of energy of
this kind--vast, but far from infinite. We have, besides our
coal-fields, metallic bodies more or less sparsely distributed through
the earth's crust. These bodies can be oxydised; and hence they are,
so far as they go, stores of energy. But the attractions of the great
mass of the earth's crust are already satisfied, and from them no
further energy can possibly be obtained. Ages ago the elementary
constituents of our rocks clashed together and produced the motion of
heat, which was taken up by the aether and carried away through
stellar space. It is lost for ever as far as we are concerned. In
those ages the hot conflict of carbon, oxygen, and calcium produced
the chalk and limestone bills which are now cold; and from this
carbon, oxygen, and calcium no further energy can be derived. So it
is with almost all the other constituents of the earth's crust. They
took their present form in obedience to molecular force; they turned
their potential energy into dynamic, and yielded it as radiant heat to
the universe, ages before man appeared upon this planet. For him a
residue of potential energy remains, vast, truly, in relation to the
life and wants of an individual, but exceedingly minute in comparison
with the earth's primitive store.

To sum up. The whole stock of energy or working-power in the world
consists of attractions, repulsions, and motions. If the attractions
and repulsions be so circumstanced as to be able to produce motion,
they are sources of working-power, but not otherwise. As stated a
moment ago, the attraction exerted between the earth and a body at a
distance from the earth's surface, is a source of working-power;
because the body can be moved by the attraction, and in falling can
perform work. When it rests at its lowest level it is not a source of
power or energy, because it can fall no farther. But though it has
ceased to be a source of _energy_, the attraction of gravity still acts
as a _force_, which holds the earth and weight together.

The same remarks apply to attracting atoms and molecules. As long as
distance separates them, they can move across it in obedience to the
attraction; and the motion thus produced may, by proper appliances, be
caused to perform mechanical work. When, for example, two atoms of
hydrogen unite with one of oxygen, to form water, the atoms are first
drawn towards each other--they move, they clash