The terms unity and convertibility, as applied to natural forces,
are often employed in these investigations, many profound and
beautiful thoughts respecting these subjects being expressed in
Faraday's memoirs. Modern inquiry has, however, much augmented our
knowledge of the relationship of natural forces, and it seems worth
while to say a few words here, tending to clear up certain
misconceptions which appear to exist among philosophic writers
regarding this relationship.
The whole stock of energy or working-power in the world consists of
attractions, repulsions, and motions. If the attractions and
repulsions are so circumstanced as to be able to produce motion,
they are sources of working-power, but not otherwise. Let us for
the sake of simplicity confine our attention to the case of
attraction. 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 to
the earth can perform work. When it rests upon the earth's surface
it is not a source of power or energy, because it can fall no
further. 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.
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