they are capable of being transferred from one body to
another, and are, within conducting bodies, extremely mobile.

In the one-fluid theory everything is the same as in the theory of two
fluids, except that, instead of supposing the two substances equal and
opposite in all respects, one of them, generally the negative one, has
been endowed with the properties and name of ordinary matter, while the
other retains the name of the electric fluid. The particles of the fluid
are supposed to repel each other according to the law of the inverse
square of the distance, and to attract those of matter according to the
same law. Those of matter are supposed to repel each other and attract
those of electricity. This theory requires us, however, to suppose the
mass of the electric fluid so small that no attainable positive or
negative electrification has yet perceptibly increased or diminished the
mass or the weight of a body, and it has not yet been able to assign
sufficient reasons why the positive rather than the negative
electrification should be supposed due to an _excess_ quantity of
electricity.

For my own part, I look for additional light on the nature of
electricity from a study of what takes place in the space intervening
between the electrified bodies. Some of the phenomena are explained
equally by all the theories, while others merely indicate the peculiar
difficulties of each theory. We may conceive the relation into which the
electrified bodies are thrown, either as the result of the state of the
intervening medium, or as the result of a direct action between the
electrified bodies at a distance. If we adopt the latter conception, we
may determine the law of the action, but we can go no further in
speculating on its cause.

If, on the other hand, we adopt the conception of action through a
medium, we are led to inquire into the nature of that action in each
part of the medium. If we calculate on this hypothesis the total energy
residing in the medium, we shall find it equal to the energy due to the
electrification of the conductors on the hypothesis of direct action at
a distance. Hence, the two hypotheses are mathematically equivalent.

On the hypothesis that the mechanical action observed between
electrified bodies is exerted through and by means of the medium, as the
action of one body on another by means of the tension of a rope or the
pressure of a rod, we find that the medium must be in a state of
mechanica