known to us as an atom.

The most natural idea is perhaps the one suggested by comparison with
those astronomical phenomena where our observation most readily allows
us to comprehend the laws of motion. It corresponds likewise to the
tendency ever present in the mind of man, to compare the infinitely
small with the infinitely great. The atom may be regarded as a sort of
solar system in which electrons in considerable numbers gravitate
round the sun formed by the positive ion. It may happen that certain
of these electrons are no longer retained in their orbit by the
electric attraction of the rest of the atom, and may be projected from
it like a small planet or comet which escapes towards the stellar
spaces. The phenomena of the emission of light compels us to think
that the corpuscles revolve round the nucleus with extreme velocities,
or at the rate of thousands of billions of evolutions per second. It
is easy to conceive from this that, notwithstanding its lightness, an
atom thus constituted may possess an enormous energy.[43]

[Footnote 43: This view of the case has been made very clear by M.
Gustave le Bon in _L'Evolution de la Matiere_ (Paris, 1906). See
especially pp. 36-52, where the amount of the supposed intra-atomic
energy is calculated.--ED.]

Other authors imagine that the energy of the corpuscles is principally
due to the extremely rapid rotations of those elements on their own
axes. Lord Kelvin lately drew up on another model the plan of a
radioactive atom capable of ejecting an electron with a considerable
_vis viva_. He supposes a spherical atom formed of concentric layers
of positive and negative electricity disposed in such a way that its
external action is null, and that, nevertheless, the force emanated
from the centre may be repellent for certain values when the electron
is within it.

The most prudent physicists and those most respectful to established
principles may, without any scruples, admit the explanation of the
radioactivity of radium by a dislocation of its molecular edifice. The
matter of which it is constituted evolves from an admittedly unstable
initial state to another stable one. It is, in a way, a slow
allotropic transformation which takes place by means of a mechanism
regarding which, in short, we have no more information than we have
regarding other analogous transformations. The only astonishment we
can legitimately feel is derived from the thought that we are suddenly
and deep