more electrifications can be combined
experimentally with a result of the same kind as when two quantities are
added algebraically. We, therefore, are entitled to use language fitted
to deal with electrification as a quantity as well as a quality, and to
speak of any electrified body as "charged with a certain quantity of
positive or negative electricity."

While admitting electricity to the rank of a physical quantity, we must
not too hastily assume that it is, or is not, a substance, or that it
is, or is not, a form of energy, or that it belongs to any known
category of physical quantities. All that we have proved is that it
cannot be created or annihilated, so that if the total quantity of
electricity within a closed surface is increased or diminished, the
increase or diminution must have passed in or out through the closed
surface.

This is true of matter, but it is not true of heat, for heat may be
increased or diminished within a closed surface, without passing in or
out through the surface, by the transformation of some form of energy
into heat, or of heat into some other form of energy. It is not true
even of energy in general if we admit the immediate action of bodies at
a distance.

There is, however, another reason which warrants us in asserting that
electricity, as a physical quantity, synonymous with the total
electrification of a body, is not, like heat, a form of energy. An
electrified system has a certain amount of energy, and this energy can
be calculated. The physical qualities, "electricity" and "potential,"
when multiplied together, produce the quantity, "energy." It is
impossible, therefore, that electricity and energy should be quantities
of the same category, for electricity is only one of the factors of
energy, the other factor being "potential."

Electricity is treated as a substance in most theories of the subject,
but as there are two kinds of electrification, which, being combined,
annul each other, a distinction has to be drawn between free electricity
and combined electricity, for we cannot conceive of two substances
annulling each other. In the two-fluid theory, all bodies, in their
unelectrified state, are supposed to be charged with equal quantities of
positive and negative electricity. These quantities are supposed to be
so great than no process of electrification has ever yet deprived a body
of all the electricity of either kind. The two electricities are called
"fluids" because