fied by the
structure and mode of operation of certain muscles (Fig. 9, _a_) in the
torpedo and other electrical animals. The mechanical contraction of them
results in an electrical excitation, and, if a proper circuit be
provided, in an electric current. The energy of a muscle is derived from
food, which is itself but a molecular compound loaded with energy of a
kind available for muscular transformation. Bread-and-butter has more
available energy, pound for pound, than has coal, and can be substituted
for coal for running an engine. It is not used, because it costs so much
more. There is nothing different, so far as the factors of energy go,
between the food of an animal and the food of an engine. What becomes of
the energy depends upon the kind of structure it acts on. It may be
changed into translatory, and the whole body moves in one direction; or
into molecular, and then appears as heat or electrical energy.

If one confines his attention to the only variable factor in the energy
in all these cases, and traces out in each just what happens, he will
have only motions of one sort or another, at one rate or another, and
there is nothing mysterious which enters into the processes.

We will turn now to the mode in which electricity manifests itself, and
what it can do. It may be well to point out at the outset what has
occasionally been stated, but which has not received the philosophical
attention it deserves--namely, that electrical phenomena are reversible;
that is, any kind of a physical process which is capable of producing
electricity, electricity is itself able to produce. Thus to name a few:
If mechanical motion develops electricity, electricity will produce
mechanical motion; the movement of a pith ball and an electric motor are
examples. If chemical action can produce it, it will produce chemical
action, as in the decomposition of water and electro-plating. As heat
may be its antecedent, so will it produce heat. If magnetism be an
antecedent factor, magnetism may be its product. What is called
induction may give rise to it in an adjacent conductor, and, likewise,
induction may be its effect.

[Illustration: FIG. 9.--Torpedo.]

[Illustration: FIG. 10.--Dynamo and Motor.]

Let us suppose ourselves to be in a building in which a steam-engine is
at work. There is fuel, the furnace, the boiler, the pipes, the engine
with its fly-wheel turning. The fuel burns in the furnace, the water is
superheated in the boi