638 Miss M. S. Leslie : Comparison of the Coefficients of 



effusion coefficients found by Debierne * for radium emana- 

 tion and by the author f for thorium emanation. No other 

 data than the diffusion coefficient of the emanation are 

 available for determining the molecular weights in the 

 actinium series, and the results obtained in this direction 

 suffer from the same uncertainty as in the case of the other 

 emanations. For example, Debierne % and Bruhat § both 

 obtained '112 as the diffusion coefficient in air at 76 cm. 

 pressure and 15° C, whereas Russ || obtained '123. One 

 would expect, however, to find the same disturbing cause, 

 whatever it may be, operating in the diffusion experiments 

 for all the radioactive gases. Therefore a comparison of the 

 rates of diffusion of thorium and actinium emanations under 

 strictly identical conditions should give the ratio of their 

 molecular weights. If the value for thorium emanation is 

 taken as 220, that for actinium emanation may then be 

 calculated. 



Such a comparison was the object of the present investi- 

 gation. Unfortunately, through the discovery of an un- 

 expected source of error, no very definite conclusions could 

 be drawn from the results, but as these throw some light on 

 the cause of the discrepancies between the observed values 

 for the diffusion coefficients, it will be useful to record them. 



Experimental Arrangements. — The diffusion vessel was a 

 glass cylinder of length 50 cm. and internal diameter 5 cm. 

 It was closed at the top by a hollow ground-glass stopper to 

 which was sealed, for communication with the exterior, a 

 tube with two branches each provided with a tap. The 

 bottom of the vessel was covered by a tray over which the 

 substance providing the emanation was uniformly spread. 

 The vessel was surrounded by a water-jacket, which kept it 

 at a temperature sufficiently constant for the requirements 

 of the experiment. 



Preliminary Experiments with Thorium Emanation. — 

 Thorium hydroxide dried at a gentle heat was used as the 

 emanating source, and the distribution of emanation was at 

 first measured by the active deposit on a metal plate down 

 the centre of the tube. To both sides of the plate, across 

 the end near to the substance, strips of tinfoil of equal width, 

 about 4 mm., were attached. The exposures were from 15 



* A. Debierne, Comjjtes Rendus, juin 1910, p. 1740. 



t M. S. Leslie, Comptes Rendus, juillet 191], p. 328. 



X A. Debierne, Le Radium, 1907, p. 213. 



§ Bruhat, Le Radium, 1909, p. 67. 



I! S. Russ, Phil. Mag. xvii. p. 412 (1909). 



