Boltwood — Radio-Activity of Thorium Salts. 97 



technical products supplied by the Welsbach Company and 

 obtained by them in the process of preparing pure thorium 

 nitrate from monazite. 



Table. 



I II III 



Number Source of oxide Decrease Time Decrease 



% days. calc. 



1 Mantle dust 18 500 38$ 



2 Welsbach nitrate 32 489 38 



3 " " 15 34V 28 



4 " " 22 365 30 



5 Thorianite 30 489 38 



6 " --- 26 428 34 



7 Monazite. 26 428 34 



8 Miner's oxide ..: 30 365 30 



9 Thorianite. 30 408 33 



10 Welsbach residue No. 3 23 331 21 



11 Welsbach residue No. 4 19 331 27 



It will be noted that none of the oxides has lost its activity 

 at a rate greater than that, corresponding to a fall to half -value 

 in 737 days, while most of the oxides have lost their activity 

 at a lower rate. The behavior of the oxides indicates that at 

 the time they were first measured they each contained much 

 less mesothorium than the quantity in equilibrium with the 

 thorium present. Oxides numbered 2, 5, 8, 9 and 10 in par- 

 ticular must have contained but a small proportion of their 

 equilibrium amounts of mesothorium since the rate of fall of 

 their activity has so closely approached that of radiothorium 

 itself. 



The results which have been obtained clearly indicate that 

 the low specific activity of the thorium series in the salts pre- 

 pared by the Welsbach Company can not be attributed to any 

 special peculiarity in the chemical methods employed in the 

 Welsbach works. It also appears that the chemical separation 

 of thorium from mesothorium can be effected without difficulty 

 by a variety of reactions. 



A portion of the Welsbach nitrate from which the three 

 oxides Nos. 2, 3 and 4. had been indirectly prepared has been 

 preserved in its original crystalline form. A small amount of 

 this nitrate was converted directly into oxide by ignition, and 

 the specific activity of this oxide was determined a few days 

 later. This was done shortly after the second series of meas- 

 urements of the older oxides had been carried out. It was 

 found that the specific activity of the thorium series in the 

 original nitrate is now practically the same as it was at the time 

 when the first series of measurements was made. The obser- 

 vation of this fact at once suggested the possibility that even 



