Relative Activities of Radio-Products of Thorium. 351 



with such a film. Column one gives the interval in hours, 

 column two the corresponding activities, in terms of the 

 uranium oxide standard as unity. Column three gives the 

 period. Hahn * had previously found as the period of 

 Ms 2 6*20 hours. 



Lerch t found the period of ThX to be 3*64 days. Levin J 

 found 3*65 days, and Elster and Geitel §, 3*6 days. From 

 our measurements of the activity of the ThX barium sulphate 

 films we have calculated the period of ThX. Observations 

 through an interval of seven days on one film gave a value 

 o£ 3*64 days ; on a second, through an interval of six days, 

 3 '63 days ; on a third, through an interval of eleven days, 

 3*65 days. The average of these mean values, 3*64 days, 

 agrees exactlv with Lerch's value. 



Lerch || found a value of 10*60 hours for the period B, and 

 this remains the accepted value. In preparing B for 

 measurements of its activity, a platinum plate charged to 

 110 volts was exposed to the emanation from a very active 

 film consisting of a few milligrams of basic ferric carbonate 

 containing Rt with its products. The excited activity so 

 obtained amounted in 24 hours to about 100 times that of the 

 uranium standard. After a plate so covered with B and C 

 was removed from the presence of the emanation it was kept 

 10 or 12 hours before measurements were begun in order 

 that the amounts of C and D should attain a fixed ratio to 

 that of B present. Nevertheless the periods found from the 

 rate of decay increased very noticeably toward the end of the 

 measurements. Thus for an interval of 36 hours the period 

 was found to be 10*71 hours, but after 76 hours it became 

 11*78 hours, 10 per cent, greater. This led us to think that 

 ThX was also present with the B, a small quantity collecting 

 on the platinum plate by recoil. This bears out the results 

 obtained in the " blank " experiment reported in Part II. 

 When the excited activity was obtained in a similar manner 

 from a hundred grams of thorium hydroxide, the total amount 

 of excited activity collected in the same time was only about 

 one-sixth as great, but the decay of this excited activity gave 

 a constant period, one film giving a mean value of 10*59 

 hours, for measurements extending over an interval of fifty 

 hours. When the active Rt film was covered with a sheet of 

 tissue paper during the collection of the excited activity the 



* Phys. Zeitschr. ix. p. 246 (1908). 

 t Wien. Ber. (1905). 

 % Phi/s. Zeitschr. vii. p. 515 (1906). 

 S Phys. Zeitschr. vii. p. 455 (1906). 

 H Wien. Sitz.-Ber. cxvi. (II a) p. 1443 (1907). 

 2 B2 



