388 Prof. E. Rutherford and Mr. F. Soddy on 



The work of Crookes and Becquerel on the separation of 

 UrX and the recovery of the activity of the uranium with 

 time, makes it appear extremely probable that the same 

 explanation holds true for this element. The work of M. and 

 Mme. Curie, the discoverers of radium, goes to show that this 

 body easily suffers a temporary decrease of its activity by 

 chemical treatment, the normal value being regained after 

 the lapse of time, and this can be well interpreted on the new 

 view. All known types of radioactivity can thus be brought 

 under the same category. 



IX. The Initial Portions of the Curves of Decay and Recovery* 



The curves of the recovery and decay of the activities of 

 thorium and ThX with time suggested the explanation that 

 the radioactivity of thorium was being maintained by tbe 

 production of ThX at a constant rate. Before this can be 

 considered rigidly established, two outstanding points remain 

 to be cleared up. 1. What is the meaning of the early 

 portion of the curves ? The recovery curve drops before it 

 rises, and the decay curve rises before it drops. 2. Why 

 does not the removal of ThX render thorium completely 

 inactive ? A large proportion of the original radioactivity 

 is not affected by the removal of ThX. 



A study of the curves (fig. 2) shows that in each case a 

 double action is probably at work. It may be supposed that 

 the normal decay and recovery are taking place, but are 

 being masked by a simultaneous rise and decay from other 

 causes. From what is known of thorium radioactivity, it was 

 surmised that an action might be taking place similar to that 

 effected by the emanation of exciting radioactivity on sur- 

 rounding inactive matter. It will be shown later that the 

 ThX, and not thorium, is the cause of the emanating power 

 of thorium compounds. On this view, the residual activity of 

 thorium might consist in whole or in part of a secondary or 

 excited radioactivity produced on the whole mass of the 

 thorium compound by its association with the ThX. The 

 drop in the recovery-curve on this view would be due to the 

 decay of this excited radioactivity proceeding simultaneously 

 with, and at first reversing the effect of the regeneration of 

 ThX. The rise of the decay-curve would be the increase 

 due to the ThX exciting activity on the matter with which it 

 is associated, the increase from this cause being greater than 

 the decrease due to the decay of the activity of the ThX. It 

 is easy to put this hypothesis to experimental test. If the 

 ThX is removed from the thoiium as soon as it is formed 

 over a sufficient period, the former will be prevented from 



