Relative Activities of Radio-Products of Thorium, 319 



activities. The mean values obtained for y from observations 

 <on five films are as follows :— 5*21, 5'19, 5'29, 5\31, 5-23 ; 

 the average of these values being 5'23. 



Owing to the fact that Rt is in part carried down with 

 the mercuric sulphide precipitate, when freeing the Rt from 

 B and by this method it was thought desirable to try a 

 ■frequent reprecipitation of the Rt-aluminium by ammonia, 

 through a period of about a hundred and ten hours. By 

 continually removing the ThX as fast as it formed, the B and 

 O originally present would have practically all decayed after 

 an interval equal to ten times the period of B. This was 

 done, the precipitations of the aluminium hydroxide being 

 made every six hours during the first day and a half ; every 

 four hours for the next day ; every three hours for the next 

 day ; and finally every hour for a period of twenty hours. 

 In this solution, after the final precipitation, barium sulphate 

 was precipitated and the Rt-barium sulphate made into two 

 films. These are the two giving average values of y =5*19 

 •and 5*21. These values for y are thought to be more nearly 

 -correct as far as the accuracy of our experimental data is 

 concerned, because there was less uncertainty in this case 

 as to what was time zero. Owing to the fact that about 

 0\5 per cent, of ThX, B, and C remain in the Rt when this 

 is purified by the method first described, and that unless 

 conditions are carefully controlled the quantity may even be 

 greater, the exterpolation for the initial activity at the time 

 of the last precipitation of the aluminium by ammonia (time 

 zero) may lead to too high a value for a . This leads to 

 low values for y. However, C is not precipitated with 

 barium sulphate to the extent that B is, and B has little 

 activity as compared with the C, which it produces much 

 more rapidly than Rt produces ThX. This causes the 

 observed activity during the first few hours when the 

 activity changes as a linear function of the time, to increase 

 too rapidly. Exterpolation under these circumstances tends 

 to give too low a value for a , and this leads to high values 

 for y. Both these effects were evident in some preliminary 

 experiments. Any error in the decay constants of Rt will 

 have an effect on the value of y. As our observations have 

 not as yet been carried on over a sufficient length of time to 

 enable us to calculate with great accuracy the period of Rt, 

 we make use of the value found by Blanc *. It seems 

 probable, from the description he gives of the preparation of 

 his Rt, that his material was pure, but it is possible that it 

 may have contained some Ms 1? and so have given too great a 



* Phys. Zeitschr. viii. p. 321 (1907). 

 Phil. Mag. S. 6. Vol. 25. No. 147. March 1913. 2 B 



