ergy, 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. In no other sense has
the convertibility of force, at present, any scientific meaning.

By the contraction of a muscle a man lifts a weight from the earth. But
the muscle can contract only through the oxidation of its own tissue or
of the blood passing through it. Molecular motion is thus converted into
mechanical motion. Supposing the muscle to contract without raising the
weight, oxidation would also occur, but the whole of the heat produced
by this oxidation would be liberated in the muscle itself. Not so when
it performs external work; to do that work a certain definite portion of
the heat of oxidation must be expended. It is so expended in pulling the
weight away from the earth. If the weight be permitted to fall, the heat
generated by its collision with the earth would exactly make up for that
lacking in the muscle during the lifting of the weight. In the case
here supposed, we have a conversion of molecular muscular action into
potential energy of gravity; and a conversion of that potential energy
into heat; the heat, however, appearing at a distance from its real
origin in the muscle. The whole process consists of a transference of
molecular motion from the muscle to the weight, and gravitating force is
the mere go-between, by means of which the transference is effected.

These considerations will help to clear our way to the conception of
the transformations which occur when a wire is moved across the lines
of force in a magnetic field. In this case it is commonly said we have
a conversion of magnetism into electricity. But let us endeavour to
understand what really occurs. For the sake of simplicity, and with a
view to its translation into a different one subsequently, let us adopt
for a moment the provisional conception of a mixed fluid in the wire,
composed of positive and negative electricities in equal quantities, and
therefore perfectly neutralizing each other when the wire is still. By
the motion of the wire, say with the hand, towards the magnet, what the
Germans call a Scheidungs-Kraft--a separating force--is brought into
play. This force tears the mixed fluids asunder, and drives them in
two currents, the one positive and