38 SODDY, Evolution of Matter by Radio-active Elements. 



delineate the complete course of the disintegration process 

 causing the radio-activity of pitchblende, are too numerous 

 for any definite prediction to be made. Certain con- 

 siderations seem to point to actinium being an inter- 

 mediate transition-form between uranium and radium. 



These considerations have been introduced to show 

 that it is at least possible, although not yet definitely 

 proved, that all radio-active matter may be transition- 

 forms in the disintegration of the heaviest known elements, 

 uranium and thorium, and that these two may be con- 

 sidered as the prototypes of the fifteen other types now 

 known. The advantage of this is obvious. We have 

 seen that the average lives of these two elements 

 is of the order of at least 10^ years. This is a period 

 so great that it carries us back to the probable limit of 

 the existence of the earth as a separate planet. We have 

 no difficulty in supposing that the disintegration of these 

 elements has been proceeding continuously at a rate similar 

 to that at present, during the past epochs demanded by 

 other considerations as the necessary age of the earth. It 

 is not absolutely necessary, therefore, to assume that there 

 has been any concomitant process of reproduction. The 

 geological estimate of the necessary age of the earth, 

 which is the maximum that has yet been demanded, is only 

 a hundred million years, or one-tenth of the average 

 life of the uranium and thorium atoms. Hence the 

 cosmical processes of separation which occurred in the 

 early history of the earth, and resulted in its present 

 non-homogeneous composition and the grouping of the 

 elements into the various minerals known to us to-day, 

 may well have occurred within the lives of the uranium 

 and thorium which exist in the earth's crust at the present 

 day. The theory of atomic disintegration, without com- 



