324 RADIO-ACTIVE PROCESSES [CH. 
In order to explain the phenomena of radio-activity, Rutherford 
and Soddy have advanced the theory that the atoms of the radio- 
elements suffer spontaneous disintegration, and that each disinte- 
grated atom passes through a succession of well-marked changes, 
accompanied in most cases by the emission of a rays. 
A preliminary account of this hypothesis to explain special 
phenomena has already been given in sections 87 and 127. It 
will now be applied generally to the radio-elements, and the con- 
sequences, which follow from it, will be considered. It is supposed 
that, on an average, a definite small proportion of the atoms of each 
radio-element becomes unstable at a given time. As a result of 
this instability, an a particle is expelled with great velocity. The 
continuous expulsion of these a particles gives rise to the non- 
separable activity of the radio-elements, which has been shown to 
consist entirely of a rays (section 193). The expulsion of an « par- 
ticle, of mass about twice that of the hydrogen atom, leaves behind it 
a new system lighter than the original one, and possessing chemical 
and physical properties quite different from those of the original 
element. This new system again becomes unstable, and expels 
another a particle. The process of disintegration, once started, 
proceeds from stage to stage at a definite measurable rate in each 
case. At each stage, with the exception of one change in thorium 
and one in radium which are not accompanied by rays at all, one 
or more a particles are projected, until the last stages are reached, 
when the 8 and y rays appear. The term metabolon has been 
suggested as a convenient expression for each of these changing 
atoms, derived from the successive disintegration of the atoms of 
the radio-elements. Each metabolon, on an average, exists only 
for a hmited time. In a collection of metabolons of the same kind 
the number NV, which are unchanged at a time ¢ after production, 
is given by V =N,e-* where JN, is the original number. Now 
S =—XJ, or the fraction of the metabolons present, which change 
in unit time, is equal to A. The value 1/X may thus be taken as 
the average life of each metabolon. 
The various metabolons from the radio-elements are distin- 
guished from ordinary matter by their great instability and conse- 
quently rapid rate of change. Since a body which is radio-active 
