ll cases equally violent in going from element
to element, nor are the results the same. Sometimes alpha particles
alone are expelled, sometimes beta, or two of them together, as alpha
and beta.

The new product may remain with the unchanged part of the original
matter. Thus there would be an accumulation of it until its own decay
balances its production, resulting eventually in a state of
equilibrium.

Constitution of the Atom

In order to explain the electrical and optical properties of matter,
the hypothesis was made that the atom consisted of positively and
negatively electrified particles. Later it was shown that negative
electrons exist in all kinds of matter. Various attempts were made to
work out a model of such an atom in which these particles were held in
equilibrium by electrical forces. The atom of Lord Kelvin consisted of
a uniform sphere of positive electrification throughout which a number
of negative electrons were distributed, and J. J. Thomson has
determined the properties of this type as to the number of particles,
their arrangement and stability.

Rutherford's Atom

According to Rutherford, the atom of uranium may be looked upon as
consisting of a central charge of positive electricity surrounded by a
number of concentric rings of negative electrons in rapid motion. The
positively charged centre is made up of a complicated system in
movement, consisting in part of charged helium and hydrogen atoms, and
practically the whole charge and mass of the atom is concentrated at
the centre. The central system of the atom is from some unknown cause
unstable, and one of the helium atoms escapes from the central mass as
an alpha particle.

There are, confessedly, difficulties connected with this conception of
the atom which need not, however, be discussed here. Much remains to
be learned as to the mechanics of the atom, and the hypothesis
outlined above will probably have to be materially altered as
knowledge grows. Perhaps it may have to be entirely abandoned in favor
of some more satisfactory solution. Until such time it at least
suffices as a mental picture around which the known facts group
themselves. In this picture energy and matter lose their old-time
distinctness of definition. Discrete subdivisions of energy are
recognized which may be called charged particles without losing their
significance. Some of these subdivisions charged in a certain way or
with neutralized charge exhibit the prope