96 Bolt wood — Radio- Activity of Thorium Salts. 



preparation of pure thorium nitrate from monazite by the firm 

 of Dr. O. Rnofler and Co., in Berlin. He found only insig- 

 nificant differences in the specific activity of the thorium series 

 in these samples and no differences indicating the separation 

 of any considerable proportion of the radiothorium.* 



On testing samples of the pure thorium nitrate which had 

 been prepared by the same firm some years before, he found 

 that these showed a conspicuously lower activity than the 

 freshly prepared salts, and this decrease in activity appeared to 

 continue for about three years, after which the activity 

 remained fairly constant for some time. He also states that he 

 has been able to obtain preparations which are free from thor- 

 ium, but which show with the passage of time a marked 

 increase in their activity and in their power to produce thorium 

 emanation. These preparations must contain the intermediate 

 product between thorium and radiothorium. He therefore 

 reaches the conclusion that it is this intermediate product, for 

 which he suggests thename " mesothorium," and not the radio- 

 thorium, which is separated from the thorium in the technical 

 process of preparing pure thorium nitrate. 



The films of thorium oxide which were originally used for 

 the determination of the activities given in my earlier paper, 

 and also a number of others which had been prepared and 

 measured at the same time or shortly afterward, have been 

 carefully preserved. The activity of these films has been 

 recently re-measured in the larger of the two electroscopes 

 described in the earlier paper and it has been possible to com- 

 pare their present activities with their activities at the time of 

 the first measurement. It was found that the specific activities 

 of all the oxides has decreased by an amount equal to from 15 

 to 30 per cent, of their former values. 



The results obtained are shown in the following table. In 

 column I the decrease in the activity of each oxide is given in 

 terms of its activity when first measured. In column II is 

 given the approximate time which elapsed between the two 

 measurements. In column III is given the decrease to be 

 expected in the activity of each oxide if the activity had been 

 falling at a rate corresponding to half-value in 737 days (the 

 rate of decay of radiothorium). Oxides numbered 1 to 8 are 

 the oxides given corresponding numbers in my earlier paper. 

 Oxide No. 9 was prepared from thorianite by a chemical pro- 

 cess differing only slightly from that used in preparing oxide 

 No. 5. Oxides No. 10 and 11 were separated from certain 



* During the past year I have made an examination of similar chemical 

 products kindly supplied by the Welsbach Company. The results obtained 

 were similar to those obtained by Hahn, and no marked differences were 

 noted in the thorium specific activity of the freshly prepared materials. 



