430 H. 31. Dadourian — Radio-activity of Thorium. 



Column V gives, in centimeters per minute,* the average 

 of the ionization produced by the active-deposit measured 

 between 4 and 5 hours after the removal of the potential dif- 

 ference from the negatively charged plate. Some of the sub- 

 stances tested contained more or less radium and actinium. 

 Therefore a comparison of the activities observed during the 

 first few hours, after the removal of the potential difference, 

 could not have been a comparison of the activities due to 

 thorium. But thanks to the rapid rates of decay of the 

 excited activities due to radium and actinium compared with 

 that due to thorium, the activities observed after 4 hours from 

 the removal of the potential difference were entirely due to 

 thorium, the part contributed by radium and actinium having 

 become negligible. Two sets of observations were taken for 

 thorite and thorianite, of which the second set was taken 

 under more favorable experimental conditions. The average 

 observed activities obtained from the first set were 25 "7 for 

 thorite and 21*2 for thorianite, while those obtained from the 

 second set were 26 - 3 and 25*2 respectively. 



In column YI the activity per gram of thorium oxide of 

 each substance is given. It is obtained by dividing the num- 

 bers of column Y by those of column TV. For brevity this 

 will be called the specific thorium activity of the substance. 



The agreement between the specific thorium activities of 

 thorite and thorianite is striking, when the difference in nature 

 and origin of these minerals and the experimental difficulties 

 are taken into consideration. On the other hand, there is just as 

 close an agreement among the specific thorium activities of the 

 first three thorium nitrates in the table. Yet there is a marked 

 difference between the S23ecific activities of these two groups, 

 the thorium in the salts being only about half as active as the 

 thorium in the minerals. This difference could not be explained 

 by possible experimental or accidental errors. It was to be 

 accounted for in some other manner. At first it was assumed 

 that thorium might be composed of two or more simple ele- 

 ments of which only one was undergoing radio-active disinte- 

 gration and giving off the characteristic thorium emanation. 

 According to this, the specific thorium activity will be different 

 for different substances which do not have a quantity of the 

 emanating component proportionate to their content of the 

 remaining components. Thus the difference among the specific 

 activities is accounted for, but the, agreement, which is of 

 greater significance, remains to be explained. On account of 

 its improbability and insufficiency this assumption, with some 

 others which will not be considered here, was abandoned in 

 favor of another. 



* The spontaneous ionization in the testing vessel was 0'45 cn]S . 



