re very serious difficulties, and we leave this to the future.

[4] The words "positive" and "negative" electricity belong to the
days when it was regarded as a fluid. A body overcharged with the
fluid was called positive; an undercharged body was called negative.
A positively-electrified body is now one whose atoms have lost some
of their outlying electrons, so that the positive charge of
electricity predominates. The negatively-electrified body is one
with more than the normal number of electrons.

Sec. 10

What an Electric Current is

The discovery of these two kinds of electricity has, however, enabled us
to understand very fairly what goes on in electrical phenomena. The
outlying electrons, as we saw, may pass from atom to atom, and this, on
a large scale, is the meaning of the electric current. In other words,
we believe an electric current to be a flow of electrons. Let us take,
to begin with, a simple electrical "cell," in which a feeble current is
generated: such a cell as there is in every house to serve its electric
bells.

In the original form this simple sort of "battery" consisted of a plate
of zinc and a plate of copper immersed in a chemical. Long before
anything was known about electrons it was known that, if you put zinc
and copper together, you produce a mild current of electricity. We know
now what this means. Zinc is a metal the atoms of which are particularly
disposed to part with some of their outlying electrons. Why, we do not
know; but the fact is the basis of these small batteries. Electrons from
the atoms of zinc pass to the atoms of copper, and their passage is a
"current." Each atom gives up an electron to its neighbour. It was
further found long ago that if the zinc and copper were immersed in
certain chemicals, which slowly dissolve the zinc, and the two metals
were connected by a copper wire, the current was stronger. In modern
language, there is a brisker flow of electrons. The reason is that
the atoms of zinc which are stolen by the chemical leave their
detachable electrons behind them, and the zinc has therefore more
electrons to pass on to the copper.

[Illustration: DISINTEGRATION OF ATOMS

An atom of Uranium, by ejecting an Alpha particle, becomes Uranium X.
This substance, by ejecting Beta and Gamma rays, becomes Radium. Radium
passes through a number of further changes, as shown in the diagram, and
finally becomes lead. Some radio-active subst