attraction generates the motion of the mass, and the stoppage of
that motion produces heat. In this sense, and in this sense only,
is there a transformation of magnetic work into heat. And if by the
mechanical action of heat, brought to bear by means of a suitable
machine, the sphere be torn from the magnet and again placed at a
distance, a power of exerting a pull through that distance, and
producing a new motion of the sphere, is thereby conferred upon the
magnet; in this sense, and in this sense only, is the heat converted
into magnetic potential energy.

When, therefore, writers on the conservation of energy speak of
tensions being 'consumed' and 'generated,' they do not mean thereby
that old attractions have been annihilated and new ones brought into
existence, but that, in the one case, the power of the attraction to
produce motion has been diminished by the shortening of the distance
between the attracting bodies, and that in the other case the power
of producing motion has been augmented by the increase of the
distance. These remarks apply to all bodies, whether they be
sensible masses or molecules.

Of the inner quality that enables matter to attract matter we know
nothing; and the law of conservation makes no statement regarding
that quality. It takes the facts of attraction as they stand, and
affirms only the constancy of working-power. That power may exist
in the form of MOTION; or it may exist in the form of FORCE, with
distance to act through. The former is dynamic energy, the latter
is potential energy, the constancy of the sum of both being affirmed
by the law of conservation. The convertibility of natural forces
consists solely in transformations of dynamic into potential, and of
potential into dynamic, energy, which are incessantly going on.