332 RADIO-ACTIVE PROCESSES 

Ramsay and Soddy state that the amount of helium, present 
in the gases from radium, was very minute. From the above 
estimates, it can readily be shown that the amount of helium 
liberated in the experiments’ described in section 201 was about 
0°5 cubic millimetres. If the a particles are helium, it is to be 
expected that the greater portion of the helium, which is produced 
in a tube contaiming the radium emanation, would be buried in 
the walls of the glass tube; for the a particles are projected with 
sufficient velocity to penetrate some distance into the glass. 
208. Rate of change of the radio-elements. Since the 
atoms of the radio-elements themselves are continuously breaking 
up, they must also be considered to be metabolons, the only dif- 
ference between them and the metabolons such as the emanations, 
Th X and others, being their comparatively great stability and con- 
sequent very slow rate of change. There is no evidence that the 
process of disintegration, traced above, is reversible, and, in the 
course of time, a quantity of radium, uranium or thorium, left 
to itself must gradually be transformed into inactive matter of 
different kinds. 
An approximate estimate of the rate of change of radio-elements 
can be deduced from the number of atoms breaking up per second. 
It has been calculated from several lines of evidence (section 104) 
that from 1 gram of radium about 10" a particles are expelled 
per second. The number for uranium and thorium is about 
MeL Oe 
Now it has been shown that there are at least four rapid changes’ 
in radium, each of which gives rise to a rays. In the absence of 
evidence of the number of a particles expelled at each change, the 
assumption, which seems most probable, will be made, viz., that 
each metabolon expels only one a particle. Since there are four 
changes in radium, the number of atoms in one gram of radium 
breaking up per second is 2°5 x 10". Now it has been shown, from 
data based on experimental evidence, that one cubic centimetre of 
hydrogen, at standard pressure and temperature, contains about 
3°6 x 10" molecules. Taking the atomic weight of radium as 225, 
there will be 1°8 x 10” atoms in 1 gram of radium. The fraction 
» of one gram of radium which changes is thus 1-4 x 10 per 
second and 44x 10 per year. It thus follows that, in each gram 
