ive or
negative electricity, such bodies would mutually repel each other; but
if one was charged with positive, while the other was charged with
negative electricity, the two bodies would mutually attract each other.

Either of these two theories may be used to illustrate the phenomena,
and so have done good service in systematizing the facts. It is evident
that both of them cannot be true, and it is in the highest degree
probable that neither of them is true.

Some have supposed that there was a kind of electric atmosphere about
every atom of matter; and still another theory, now advocated by Edlund
of Stockholm, assumes that electricity is identical with the ether by
which radiant energy, light and heat, is transmitted.

Before a correct judgment can be formed of the nature of any force, it
is necessary to know what it can do, what kind of phenomena it can
produce. Let us, then, take a brief survey of what electricity can do.

1st, It can directly produce _motion_, through the attractions and
repulsions of electrified bodies,--as indicated by electrometers, the
rotation of the fly-wheel, the deflection of the galvanometer needle. It
has been proved by the mathematical labors of Clausius, and confirmed by
experiment, that, when electricity performs any mechanical work, so much
electricity is lost, annihilated as electricity.

2d, It can directly produce _heat_, as shown by passing a sufficient
quantity of electricity through a fine platinum wire: the wire becomes
heated, and glows, and it may even be fused by the intensity of the
heat. The heat developed in the so-called electric arc is so great as to
fuse the most refractory substances. If a current of electricity from a
battery be sent through a thermo-pile, one of the faces of the pile will
be heated. The heat of the spark from a Leyden jar may be made to ignite
gunpowder, and dissipate gold into vapor. The heat produced by lightning
is seen when a live tree is struck by a powerful flash: the sap of the
tree is instantly converted into steam of so high a tension as to
explode the tree, scattering it in small fragments over a wide area. The
tips of lightning-rods often exhibit this heating effect, being fused by
the passage of too great a quantity of electricity.

In the early part of the present century it was demonstrated by Count
Rumford, and also by Sir Humphry Davy, that heat was but a form of
molecular motion. Since then the exact relations between the mot