Excited Activity of TIioi 



643 



Time of 





Percentage removed. 



1 



heating 



in minutes. 



Temp. 







1 







1 







Thorium A. 



Thorium B. 



1 



3 



630° 













3 



640° 



24 









4 



680° 



40 







1 



3 



730° 



74 



10 





U 



830° 



57 



13 





•"'» 



955° 



95 



74 





(5 



1050° 



98 



77 





2 



1090° 



99 



91 





3 



1280° 



100 



99 





This method is thus a very convenient one for obtaining 

 pure thorium B. The separation of the two substances also 

 gives the answer to another question which has been hitherto 

 left undetermined. It shows that the loss of activity of the 

 wire is not due to its own disintegration, but that the process 

 is a true distillation of the active substances, each of which 

 volatilizes at a definite temperature. 



Results of Chemical Separations. 



This theory of the simultaneous presence, on any surface 

 made active by exposure to thorium, of two substances having 

 different properties and different rates of decay, gives a simple 

 explanation of most of the results of investigations by chemical 

 and electrolytic methods, which would otherwise seem to show 

 the existence of a very large number of different substances, 

 varying in their rates of decay. An examination of the 

 numbers given by v. Lei-ch* shows that in the majority of 

 cases the decay of activity (when it departs from the normal 

 rate) does not follow an exponential law at all. For instance, 

 the active BaS0 4 precipitate obtained from a solution of the 

 active deposit in hydrochloric acid rose 8 per cent, in activity 

 during the first 3 hours, then fell off, very slowly at first, 

 and ' s hours after its separation was decaying at the normal 

 rate of half value in 11 hours. Here there is, therefore, at 

 first an excess of thorium A. Again, the activity obtained 

 on a platinum cathode during the electrolysis of an active 

 hydrochloric acid solution was reduced to one-fourth of its 

 initial value in the first 3 hours, while the mean rate of 

 decay over the next 2<> hours was less than one-third of this. 

 In this case there was, therefore, a large excess of thorium B. 

 There i- only one case in all v. Lerch's results to which this 



* Drnde'e Annalen. Nov. 1903. 



