nt of
electricity, but one of these definite parcels.

It was not, however, until the later years of the nineteenth century
that the natural unit of electricity was manifested by itself and
without a carrier. At a famous address to the British Association at
York in 1881 Sir William Crookes described the first marvellous
experiments in which this feat had been accomplished, though there was
still to come a long controversy before the interpretation was clearly
accepted. It is now definitely established that there is a fundamental
atom of electricity which we now call the electron. As we all know
electrification is of two kinds--a positive and a negative. The electron
is of the negative kind. There does not appear to be a corresponding
positive atom of electricity, or at least not one that is so singular in
its properties as the electron. Electrons go to the making of all atoms,
just as atoms go to the making of molecules. The atom which is neutral,
that is, shows neither positive nor negative electrification, must
contain positive electricity in some form to balance the electrons which
we know it contains. When we strip an atom, as we know how to do, of one
or more of these electrons, the remainder is positively charged. The
positive ion is any sort of an atom or molecule which has become
positively electrified in this way. An atom which has become positive by
the loss of one or more of its electrons exercises a force on any spare
electrons in its neighbourhood or on any atom carrying a spare electron.
When there are large numbers of atoms seeking in this way to become
neutral once more, as occurs often in Nature, the forces generated may
be tremendous. They are shown, for example, in the lightning-stroke. But
indeed it would seem that all the chemical forces of which we have
already spoken depend ultimately upon the electric state of the atom
concerned.

It is because the force which a positively-charged atom exerts on an
electron is so great and because the electron is so light and easily
moved compared to an atom that the electron has not been isolated at
will until recent years. The isolation in fact depends upon the electron
being endowed with a sufficient speed to carry it through or past the
action of an atom which is seeking to absorb it into its system. A lump
of matter flying in space might enter our solar system with such speed
as to be able to pass through and go on its way almost undeflected. Or
again, it