The kinetic energy of each projected particle is enormous, compared with its mass. The kinetic energy of each [Greek: alpha] particle is
(1/2)mV^2 = (1/2)(m/e)V^2e = 5·9 × 10^{-6} ergs.
Taking the velocity of a rifle bullet as 10^5 cms. per second, it is seen that, mass for mass, the energy of motion of the [Greek: alpha] rays is 6 × 10^8 times as great as that of the rifle bullet. In this projection of bodies atomic in size with great velocity probably lies the principal cause of the heating effects produced by radium (chapter XII).
95. Atomic disintegration. The radio-activity of the radio-*elements
is an atomic and not a molecular property. The rate of
emission of the radiations depends only on the amount of the
element present and is independent of its combination with inactive
substances. In addition, it will be shown later that the rate of
emission is not affected by wide variations of temperature, or by
the application of any known chemical or physical forces. Since
the power of radiating is a property of the radio-atoms, and the
radiations consist for the most part of positively and negatively
charged masses projected with great velocity, it is necessary to
suppose that the atoms of the radio-elements are undergoing disintegration,
in the course of which parts of the atom escape from
the atomic system. It seems very improbable that the [Greek: alpha] and β
particles can suddenly acquire their enormous velocity of projection
by the action of forces existing inside or outside the atom. For
example, the [Greek: alpha] particle would have to travel from rest between two
points differing in potential by 5·2 million volts in order to acquire
the kinetic energy with which it escapes. Thus it seems probable
that these particles are not set suddenly in motion, but that they
escape from an atomic system in which they were already in
rapid oscillatory or orbital motion. On this view, the energy is
not communicated to the projected particles, but exists beforehand
in the atoms from which they escape. The idea that the atom is
a complicated structure consisting of charged parts in rapid oscillatory
or orbital motion has been developed by J. J. Thomson,
Larmor and Lorentz. Since the [Greek: alpha] particle is atomic in size, it is
