G34 Mr. H. G. J. Moseley and Dr. K. Fajans on 



greater than that of the second. This was determined by 

 continuing the deposition of actinium A until by its decay 

 a. large quantity of active deposit had been iormed on the 

 disk. The actinium was then removed and the currents in 

 the two boxes compared, allowance being made for the decay 

 in activity of the deposit. The observed difference in sensi- 

 tiveness may in part have been due to imperfect adjustment 

 of the positions of the boxes, so that a little activity may 

 hai^e escaped the second box but not the first. It will be 

 seen that in correcting for sensitiveness, thus determined, 

 error due to faulty adjustment is at the same time eliminated. 

 In the case of thorium A, the correction for sensitiveness 

 was made in an entirely different manner. Sets of experi- 

 ments were made alternately with the disk rotating in oppo- 

 site directions, the ratios of the activities then giving ke ±Kt , 

 where k is the sensitiveness ratio. That value of k was then 

 chosen which made the mean value of X from all the " clock- 

 wise " experiments the same as the mean from all the 

 "counterclockwise." In other respects the experiments on 

 thorium A were similar to those on actinium A, but in the 

 iormer case the calculation was simpler, since the growth of 

 active deposit was usually inappreciable. Unexpected irregu- 

 larities were occasionally observed, but, owing to the departure 

 from England of one of the authors, it was not found possible 

 to inquire into their cause. The mean value found for the 

 half-value period of thorium A was 0*145 second, and this 

 value is thought to be correct to within 10 per cent. A more 

 careful determination will, however, be made as soon as work 

 on this subject is resumed. The result is in agreement with 

 the deduction of Geiger and Marsden*, that an a-ray product 

 exists in thorium emanation with a period of the order of a 

 fifth of a second. 



Experiments icitli Actinium C. 



The active deposit from actinium is known to consist of 

 three successive products, actinium B (36min.), (2*lmin.), 

 and D (4*7 min.)< Mile. Blanquies, believing the a radiation 

 of actinium C to be complex t, has suggested that it may 

 have come from more than one product, and a product named 

 actinium B 2 has obtained a provisional place in Mme. Curie's 

 ' Radio-activity.'' The results of experiments made by 

 Mile. Blanquies to test this question! were not conclusive, 

 but irregularities were observed in the decay of actinium C, 



* Rhys. Zeit. vol. xi. p. 7 (1910). 

 t Le Radium, vol. vi. p. 230 (1909). 

 \ Le Radium, vol. vii. p. 159 (1910). 



