SUCCESSION OF CIIAXI;KS IN RADIOACTIVE KODIKS. 



Kadium Thorium 



{ I 



Radium emanation ThX 



I 



ll.iilmin A ) 



I 



Uranium 



UrX 



H 



, 



H.-iiliilln C 



J 



lladium D 



I 



&c. 



5T 



<* 

 



Thorium emanation Final product 



. I. " 



Thorium A 



\ 



Thorium B 



' 











Thorium C [ 4j 

 (final product) J 



Actiiiiiini 



Actinium X ? 



Actinium emanation 



Actinium A 



I 



Actinium B 



' . I 



Actinium C 



(final product) j 



Each product on this scheme is the parent of the product below it. Since only two 

 products have l>een observed in the active deposit of thorium and actinium, thorium C 

 and actinium C respectively refer to their final inactive products. 



5. Decay of the. Excited Activity of Thorium. If a Ixxiy is exposed for several 

 days in the presence of a constant supply of thorium emanation, the activity imparted 

 to it reaches a constant value. On removal, the activity is found to decay very 

 approximately according to an exponential law with the time, falling to half value in 

 1 1 hours. If, however, the body is only exposed for a few minutes in the presence of 

 the emanation, the activity after removal increases, during the course of about 3 hours, 

 to 5 to 6 times the initial value, passes through a maximum, and then decays 

 approximately according to the normal rate, i.e., the activity falls to half value in 

 about 11 hours.* 



The curve of increase of activity with time (measured by the a. rays) for a rod 

 exposed 10 minutes in the presence of the thorium emanation is shown graphically in 

 fig. 1, curve C. The rod was made active by charging it negatively to 100 volte in a 

 closed vessel, t With increasing times of exposure, the rise of the excited activity 

 after removal becomes less and less marked,, and is almost inappreciable after 6 hours. 



I have discussed elsewhere in some detail (' Treatise on Radioactivity,' p. 258) the 

 explanation of this remarkable effect, and have shown that it can be accounted for 



* KiTHEKFOitn, ' Phil. Mag.,' January, 1903. 



t It is important that the air in the vessel should he tlust-free, for Miss BROOKS has found that the 

 carriers of ' excited activity ' in dusty air adhere to the dust particles, and remain anchored in the gas for 

 several days. On the application of an electric field, some of the dust particles are conveyed to the 

 negative electnxle. The presence of old radioactive matter in the rod masks, to a large extent, the rise 

 nli-iT\ri| fur the rmlioai-tive matter which has just lieen formed 



