s the most
detailed and suggestive is that of J.J. Thomson. ("Phil. Mag." March,
1904.) Thomson regards the atom as composed of a number of mutually
repelling negative corpuscles or electrons held together by some central
attractive force which he represents by supposing them immersed in a
uniform sphere of positive electricity. Under the action of the two
forces, the electrons space themselves in symmetrical patterns, which
depend on the number of electrons. Three place themselves at the corner
of an equilateral triangle, four at those of a square, and five form
a pentagon. With six, however, the single ring becomes unstable, one
corpuscle moves to the middle and five lie round it. But if we imagine
the system rapidly to rotate, the centrifugal force would enable the
six corpuscles to remain in a single ring. Thus internal kinetic energy
would maintain a configuration which would become unstable as the energy
drained away. Now in a system of electrons, electromagnetic radiation
would result in a loss of energy, and at one point of instability we
might well have a sudden spontaneous redistribution of the constituents,
taking place with an explosive violence, and accompanied by the ejection
of a corpuscle as a beta-ray, or of a large fragment of the atom as an
alpha-ray.

The discovery of the new property of radio-activity in a small number of
chemical elements led physicists to ask whether the property might not
be found in other elements, though in a much less striking form. Are
ordinary materials slightly radio-active? Does the feeble electric
conductivity always observed in the air contained within the walls of
an electroscope depend on ionizing radiations from the material of the
walls themselves? The question is very difficult, owing to the wide
distribution of slight traces of radium. Contact with radium emanation
results in a deposit of the fatal radium-D, which in 40 years is but
half removed. Is the "natural" leak of a brass electroscope due to
an intrinsic radio-activity of brass, or to traces of a radio-active
impurity on its surface? Long and laborious researches have succeeded in
establishing the existence of slight intrinsic radio-activity in a
few metals such as potassium, and have left the wider problem still
unsolved.

It should be noted, however, that, even if ordinary elements are not
radio-active, they may still be undergoing spontaneous disintegration.
The detection of ray-less changes by Rutherfo