Relative Activities of Radio-Products of Thorium. 345 



the last HgS precipitation as equal to the true activity, a , 

 at time zero. 



The activity measurements were made with the gold-leaf 

 electroscope used by McCoy and Ashman *. The gold-leaf 

 system was charged initially to 500 volts, and the potential 

 fell about 100 volts during the observed discharge. The 

 standard of activity was a thick film of U 3 8 , as described in 

 the same article by McCoy and Ashman. By using barium 

 sulphate, a loss of any considerable amount of the emanation 

 and thus of C was avoided. This error is compensatory f ; 

 since the emanation escaping in the electroscope with the 

 A which it produces makes the observed activity greater than 

 it would be if the emanation did not escape. But, owing 

 to a loss of emanation, the activity of C is too small, and a 

 calculation shows that the observed activity is approximately 

 the true activity which would be observed if the emanation 

 were not escaping and thereby reducing the quantity of C 

 in the film. In our barium sulphate film not over 0'5 

 per cent, of the total emanation escaped. 



In Table I. are given the results obtained with one of the 

 ThX-BaS0 4 films. The first column gives the time (in 

 hours) elapsed since time zero, the time of the last HgS 

 precipitation. The second column gives the observed activities 



Table I. 



Interval, 



Observed 



Calculated Activity 

 on basis of 



Value of x calcu- 

 lated from 



Hours. 



Activity. 



a- = 0427. 



observed Activity. 



o-ooo 



13-65 



1-000 

 1121 



1-000 

 1120 





0-430 



15-70 



1-130 



1-124 



0437 



1752 



1-127 



1-127 



0-428 



18-15 



1124 



1-127 



0-422 



1982 



1127 



1127 



0-427 



29-08 



1102 



1-104 



0-424 



42-22 



1-034 



1-033 



0-429 



70-62 



08483 



08466 



0-430 



87-52 



07473 



0-7430 



0-434 



113-68 



0-6057 



0-6049 



0-429 



165-25 ' 



0-4035 



0*4017 



0433 



220-93 



0-2613 



0-2585 



0-440 



286-0 



0-1547 



01542 



0-431 



332-9 



0-1061 



0-1063 



0424 



381-4 



00723 



00724Q 



0-426 

 Mean... 0-429 



* Le Radium, v. p. 362 (1908) ; Amer. Jour. Sci. xxvi. p. 521 (1908). 

 -f McCoy & Koss, Amer. Jour. Sci. [4] xxi. p. 433 (1906). 



