xg] RADIO-ACTIVE PROCESSES 327. 
presence by the property of radio-activity. Consequently, there 
will always be a residual inactive product or products of the 
changes occurring in each of the radio-elements. In addition, 
since the a particles, projected from the radio-elements, are 
material in nature and are not radio-active, they must also be 
considered as a residual product. 
The value of +, which may be taken as a comparative measure 
of the stability of the different metabolons, varies over a very wide 
range. The thorium emanation is the most unstable of the meta- 
bolons, and (leaving out of account the fourth change in emanation 
X of radium) uranium X the least unstable. The metabolons 
constituting uranium X are about 30,000 times as stable as the 
metabolons constituting the thorium emanation. 
The only two metabolons of about the same stability are the 
metabolons constituting thorium X and the radium emanation. 
In each case the activity falls to half value in about four days. I 
consider that the approximate agreement of the numbers is a mere 
coincidence, and that the two types of matter are quite distinct 
from one another; for, if the metabolons were identical, it would 
be expected that the changes which follow would take place in the 
same way and at the same rate, but such is not the case. More- 
over Th X and the radium emanation have chemical and physical 
properties quite distinct from one another. 
201. Production of helium from radium and the radium 
emanation. Since the final products, resulting from a disinte- 
gration of the radio-elements, are not radio-active, they should in 
the course of geologic ages collect in some quantity, and should 
always be found associated with the radio-elements. Now the 
inactive products resulting from the radio-active changes are the a 
particles expelled at each stage, and the final inactive product or 
products which remain, when the process of disintegration can no 
longer be traced by the property of radio-activity. 
Pitchblende, in which the radio-elements are mostly found, 
contains in small quantity a large proportion of all the known 
elements. In searching for a possible disintegration product 
common to all the radio-elements, the presence of helium in the 
radio-active minerals is noteworthy; for helium is only found in 
