
126 NATURE OF THE RADIATIONS [CH. 
87. 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 1s necessary to 
suppose that the atoms of the radio-elements are undergoing dis- 
integration, in the course of which parts of the atom escape from 
the atomic system. It seems very improbable that the a and 8 
particles can suddenly acquire their enormous velocity of projection 
by the action of forces existing inside or outside the atom. For. 
example, the a particle would have to travel from rest between two 
points differig 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 1s 
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 oscil- 
latory or orbital motion has been developed by J. J. Thomson, 
Larmor and Lorentz. Since the a particle is atomic in size, it is 
natural to suppose that the atoms of the radio-active elements 
consist not only of the electrons in motion, but also of positively 
charged particles whose mass is about the same as that of the 
hydrogen or helium atom. 
It will be shown later that only a minute fraction of the atoms 
of the radio-element need break up per second in order to account 
for the radiations even of an enormously active element like 
radium. The question of the possible causes which lead to this 
atomic disintegration and the consequences which follow from it 
will be discussed later in chapter X. 
88. Experiments with a zinc sulphide screen. A screen 
