: he Bie ray 
= 
194 CONTINUOUS PRODUCTION OF RADIO-ACTIVE MATTER  [CH. 
to liquid air, and is independent of all physical and chemical 
actions. If, however, the production of active matter is supposed 
to be the result of changes, not in the molecule, but im the atom 
uself, it is not to, be expected that the temperature would exert 
much influence. The general experience of chemistry in failing 
to transform the elements by the action of temperature is itself 
strong evidence that wide ranges of temperature have not much 
effect in altering the stability of the chemical atom. 
The view that the atoms of the radio-elements are undergomg 
spontaneous disintegration was put forward by Mr Soddy and the 
writer as a result of evidence of this character. The discovery of 
the material nature of the a rays added strong confirmation to 
the hypothesis; for it has been pointed out (section 87) that the 
expulsion of « particles must be the result of a disintegration 
of the atoms of the radio-element. Taking the case of thorium 
as an example, the processes occurring in the atom may be 
pictured in the following way. It must be supposed that the 
thorium atoms are not permanently stable systems, but, on an 
average, a constant small proportion of them—about one atom in 
every 10" will suffice—breaks up per second. The disintegration 
consists in the expulsion from the atom of one or more @ particles 
with great velocity. For simplicity, 1t will be supposed that each 
atom expels one a particle. It has been shown that the a particle 
of radium has a mass about twice that of the hydrogen atom. 
From the similarity of the a rays from thorium and radium, it is 
probable that the a particle of thorium does not differ much in 
mass from that of radium, and may be equal to it. After the 
escape of an a particle, the part of the atom left behind, which 
has a mass slightly less than that of the thorium atom, tends to 
rearrange its components to form a temporarily stable system. It 
is to be expected that it will differ m chemical properties from 
the thorium atom from which it was derived. The atom of the 
substance Th X is, on this view, the thorium atom minus one @ 
particle. The atoms of Th X are far more unstable than the atoms 
of thorium, and one after the other they break up, each atom ex- 
pelling one a particle as before. These projected a particles give rise 
to the radiation from the Th X. Since the activity of Th X falls to 
half its original value in about four days, on an average half of the 
