sting the substance of the atom. Even
so, the evidence of so great a capacity to give out energy is startling,
and has given rise to various theories (all as yet tentative) in
explanation. Thus J. Perrin*9* has suggested that atoms may consist of
parts not unlike a miniature planetary system, and in the atoms of the
radio-elements the parts more distant from the centre are continually
escaping from the central attraction, thus giving rise to the
radiations. Monsieur and Madame Curie have suggested that the energy may
be borrowed from the surrounding air in some way, the energy lost by
the atom being instantly regained. Pilipo Re,*10* in 1903, advanced the
theory that the various parts of the atom might at first have been free
particles constituting an extremely tenuous nebula.

These parts gradually becoming collected around condensed centres have
formed what we know as the atoms of elements, the atom thus becoming
like an extinct sun of the solar system. From this point of view the
radio-active atoms represent an intermediate stage between nebulae
and chemical atoms, the process of contraction giving rise to the heat
emissions.

Lord Kelvin has called attention to the fact that when two pieces of
paper, one white and the other black, are placed in exactly similar
glass vessels of water and exposed to light, the temperature of the
vessel containing the black paper is raised slightly higher than the
other. This suggests the idea that in a similar manner radium may keep
its temperature higher than the surrounding air by the absorption of
other radiations as yet unknown.

Professor J. J. Thompson believes that the source of energy is in the
atom itself and not external to it. "The reason," he says, "which
induces me to think that the source of the energy is in the atom of
radium itself and not external to it is that the radio-activity of
substances is in all cases in which we have been able to localize it a
transient property. No substance goes on being radio-active very long.
It may be asked, how can this statement be reconciled with the fact
that thorium and radium keep up their activity without any appreciable
falling off with time. The answer to this is that, as Rutherford and
Soddy have shown in the case of thorium, it is only an exceedingly small
fraction of the mass which is at any one time radio-active, and that
this radio-active portion loses its activity in a few hours, and has to
be replaced by a fresh supply