584 On the Cause and Nature of Radioactivity. 



IX. Further Theoretical Considerations. 



Enough has been brought forward to make it clear that in 

 the radioactivity of thorium, and. by analogy, o£ radium, we 

 are witnessing the effect of a most complex series of changes, 

 each of which is accompanied by the continuous production 

 of a special kind of active matter. The complexity of the 

 phenomenon gives rise to an important question concerning 

 the fundamental relation between the changes which occur 

 and radioactivity. So far it has been assumed, as the simplest 

 explanation, that the radioactivity is preceded by chemical 

 change, the products of the latter possessing a certain amount 

 of available energy dissipated in the course of time. A 

 slightly different view is at least open to consideration, and 

 is in some ways preferable. Radioactivity may be an accom- 

 paniment of the change, the amount of the former at any 

 instant being proportional to the amount of the latter. On 

 this view the non-separable radioactivities of thorium and 

 uranium would be caused by the primary change in which 

 ThX and UrX are produced. The activity of ThX would be 

 caused by the secondary change producing the emanation, 

 the activity of the emanation by a tertiary change in which 

 the matter causing the excited activity is produced, the 

 activity of the latter being derived from still further changes. 

 The law of the decay of the activity with time (equation 1 

 first part) in all cases but the primary then appears as the 

 expression of the simple law of chemical change, in which 

 one substance only alters at a rate proportional to the amount 

 remaining. In the primary change the amount remaining is 

 infinitely great compared with the amount that alters in short 

 time, and therefore the velocity of reaction is constant. This 

 view certainly affords an explanation of why the emanating 

 power of ThX is proportional to the radioactivity. So long- 

 as the latter is considered a consequence of what has occurred 

 there is no reason why this should be so. But if it is considered 

 the accompaniment of the change in which the emanation is 

 formed the result follows naturally. Further and more exact 

 determinations of the rate of rise and decay of emanating 

 power are therefore called for. 



In the case of uranium the changes so far as they can be 

 followed by the radioactivity appear to be at an end with 

 that which causes the activity of UrX. It is of interest that 

 this substance gives only cathode-rays, and that it continues 

 to do so for many weeks after its separation from uranium. 

 This gives rise to the question whether any connexion can be 

 established between the nature of the radiation and the kind 

 of change producing it. 



