646 Diffusion of Thorium and Actinium Emanations. 



continuation of the readings gave the leak. The activity 

 due to the active deposit formed in the 25 or 30 seconds 

 elapsing since the introduction of the emanation was prac- 

 tically zero. After corrections had been made for the leak 

 and for variations in the sensitiveness along different parts 

 of the scale, the difference between the final reading and the 

 reading at any given moment would be proportional to the 

 total amount of emanation present at that moment. The 

 logarithms of these values were plotted against time. In 

 nineteen experiments the values for the period varied between 

 3*70 and 4*00 seconds, giving a mean of 3*86 seconds. 



In the hope of attaining a greater degree of accuracy a 

 modification of tbe above method was devised. The pendulum 

 was insulated and kept charged to a constant voltage, and as 

 in swinging it passed its lowest point, it touched the exposed 

 tip of a fine insulated wire passing from the gold-leaf support 

 to the outside of the electroscope. Thus at definite intervals 

 of time (about 2 seconds) the leaf could be re-charged, its 

 fall representing the average activity of the emanation during 

 that time. When no emanation was present, there was 

 always a certain amount of oscillatory movement of the leaf, 

 due partly to induction effects produced by the charged 

 pendulum on the electroscope, and partly to the leak. Cor- 

 rection was made for this. In twelve experiments, of which 

 five gave 3*95 seconds, the variation was between 3*90 and 

 3*98 seconds, the mean being 3*95 seconds. 



A further change was then introduced. The outside of 

 the electroscope was charged and the pendulum earthed. 

 An earthed metal plate screened the electroscope from the 

 pendulum except at the small hole through which passed 

 the wire in connexion with the leaf. In this way si 11 dis- 

 turbance due to induced charges was prevented. Sixteen 

 experiments gave values varying between 3'81 and 4*06 

 seconds, the mean being 3*92 seconds. 



The period of thorium emanation was also determined by 

 this method. Of twelve experiments eleven gave values 

 varying between 53*5 and 55*1 seconds, mean 543 seconds: 

 the twelfth, 55*8 seconds, varying considerably from the 

 others, was neglected. 



As the variations of the mean values from the period of 

 3*9 seconds found by Debierne * by the air-current method 

 for actinium emanation, were much less than the variations 

 between the actual experimental values obtained above, this 

 value, 3*9 seconds or A,= l*8 x 10" 1 sec. -1 was retained in the 



* Debierne, Comptes Rendus, 1904, p. 411. 



