t least a month
after its preparation. The evolution of heat from radium and the
radio-active substances is, in a sense, a secondary effect, as it
measures the radiant energy transformed into heat energy by the
active matter itself and whatever surrounds it. Let us repeat,
therefore, that the total amount of energy pent up in a single atom of
radium almost passes our powers of conception.

Necessity for a Disintegration Theory

The facts gathered so far justify and necessitate a theory which shall
satisfactorily explain them, and since these phenomena are not caused
by nor subject to the influence of external agencies, they must refer
to changes taking place within the atom--in other words, a theory of
disintegration. In the main, these facts may be summed up as the
emission of certain radiations from known elemental matter: the
material alpha particles with positive charge, the beta particles or
negative electrons, and the gamma rays analogous to _X_ rays. The
emission of these rays results in the production of great heat. Then
there is the law of transformations by which whole series of new
elements are generated from the original element and maintain a
constant equilibrium of growth and decay in the series. Lastly, we
have the production of helium from the alpha particles.

Disintegration Theory

In explanation of these phenomena, Rutherford offered the hypothesis
that the atoms of certain elements were unstable and subject to
disintegration. The only elements definitely known to come under this
description are the two having atoms of the greatest known mass,
thorium (232) and uranium (238).

The atoms of uranium, for instance, are supposed to be not permanent
but unstable systems. According to the hypothesis, about 1 atom in
every 10^{18} becomes unstable each second and breaks up with a
violent explosion for so small a mass of matter. One, or possibly two
alpha particles are expelled with great velocity. This alpha particle
corresponds to an atom of helium with an atomic weight of 4, and its
loss reduces the original atomic weight to 234 with the formation of a
new element, having changed properties corresponding to the new atomic
weight. This new element is uranium X_{1}.

These new atoms are far more unstable than those of uranium, and the
decomposition proceeds at a new rate of 1 in 10^{7} per second. So at
a definite, measurable rate this stepwise disintegration proceeds. The
explosions are not in a