432 II M. Dadourian — Radio-activity of Thorium. 



which thorium nitrates number (1) and number (4) were pre- 

 pared. No appreciable amount of radio-thorium was lost in 

 the preparation of thorium nitrate number (•!). Therefore 

 thorium and radio-thorium were in equilibrium in this salt. 

 This accounts for the fact that its specific thorium activity was 

 about the same as those of the minerals. 



About half of the equilibrium quantity of the radio-thorium, 

 on the other hand, must have been separated from thorium 

 nitrate number (1) during the process of its chemical prepara- 

 tion. The few years' time which had elapsed between its 

 preparation from the mineral and these experiments was too 

 short for the thorium and radio-thorium, in the salt, to come 

 into radio-active equilibrium, hence the low specific thorium 

 activity. 



The more important conclusions arrived at by the above 

 experiments may be summed up as follows : 



.First. The amount of radio-thorium in minerals is propor- 

 tional to the quantity of thorium present. Therefore radio- 

 thorium is a transformation product of thorium. 



Second. If thorium and its successive products radio-thorium 

 and thorium X are in radio-active equilibrium in a substance, 

 the amount of thorium emanation evolved by the substance is 

 proportional to the quantity present of any and all of them. 



Third. The difference in the specific thorium activities of 

 substances which are in radio-active equilibrium with regard 

 to thorium X, is due to the separation of some of the equili- 

 brium amount of the radio-thorium from the substance. 



Fourth. The fact that thorium nitrate number (3), which 

 was prepared over three years ago, was notably deficient in 

 radio-thorium, indicates that the rate of recovery and conse- 

 quently the rate of decay of radio-thorium is very slow, and 

 that the half-value period cannot be below two years. 



In conclusion, I wish to express my hearty thanks to Pro- 

 fessor H. A. Bumstead for suggesting these experiments and 

 for the kind interest he has shown during the progress of the 

 work. 



Sheffield Scientific School of Yale University, 

 New Haven, Conn. 



