Diffusion of Thorium and Actinium Emanations. 637 



interfere. For instance, on the following dates, wet weather, 

 unaccompanied by wind, is followed by a rise in the amounts 

 of emanation at 50 cm.:— Sept. 4. 12. 23-29, Oct. 3, 14, 23, 

 Nov. 8, Dec. 2$, Jan. 5, 17-21, 25, Feb. 8, 18. The only 

 marked exception occurs on Oct. 27-29. 



An examination of the barometric chart for the period 

 covered by the experiments does not reveal any influence of 

 atmospheric pressure on the results, except in so far as its 

 fluctuations are accompanied by storm, rain, &c. 



The average amounts of emanation found in these experi- 

 ments should be of interest. They are as follows : — 



Ground-gas (102 experiments) 163 X 10~ 12 curie per litre. 



Exhalation (98 experiments) 2673 x 10~ 12 curie per sq.m. per 



hour. 



In the course of the previous experiments referred to *, 

 38 observations on ground-gas were made, at the same place 

 as the present ones, but at depths varying from 20 to 150 cm. 

 The average of these was 180xl0~ 12 curie per litre, which 

 agrees fairly well with that given above. 



The previous observations on exhalation were not made at 

 the same spot. Ten of them were made about 30 ft. from 

 the present ones, and 3 or 4 ft. from the wall of the labo- 

 ratory, and tw r el\ e were made at a spot three miles away. 

 These two sets gave averages of 839 x 10" 12 and 4087 x 10~ 12 

 respectively. 



Geological Laboratory, 



Trinity College, Dublin, 

 July 1, 19i2. 



LXI. A Comparison of the Coeffcieirts of Diffusion of Thorium 

 and Actinium Emanations, with a Note on their Periods of 

 Transformation. By May Sybil Leslie, M.Scj\ 



npHE values hitherto found for the diffusion coefficients 

 _L of radium and thorium emanations differ considerably 

 from one another. They are in general, how r ever, except 

 in the case of Perkins's experiments with radium emanation J, 

 higher than would be expected from the probable molecular 

 weights calculated on the basis of the disintegration 

 theory, and practically confirmed by the values of the 



* Loc. cit. 



t Communicated by Prof. E. Rutherford. F.R.S. 



J P. B. Perkins, Amer. Journ. of Sci., June 1908 xxv. p. 461. 



