Condensation of the Radioactive Emanations. 561 



The average values of N e and of a, for all the observa- 

 tions in any series of experiments are found directly from 

 figs. 3, 4, and 5, being the values deduced from the position 

 of the centroid in the three cases. 



The centroids are given by 



III. IV. V. 



p = 13-0 p = 5-42 p = 3-63 



^= + 9-88 7i=+l*2 7 1= -0-54 



y 2 =-19-5 7 2 =-8-55 72= -6-8 



Hence we find 



ff *=4*8 III. and « = -168 III. 



= 3-7 IV. =175 IV. 



= 3-7 V. =-168 V. 



The values of N e obtained show that the effect of putting 

 an aluminium plate over B (and practically doubling the 

 distance travelled by the corpuscles before they reach the 

 place where the ionization was measured) was to reduce 

 the Becquerel ray current in the ratio of 4*8 to 3*7. 



The tilting of the radium also reduced this current as we 

 should expect, for the surface exposed was thereby decreased. 

 This accounts for N e being less in fig. 4 than in fig. 3. 



The effect on « is hardly appreciable, for the difference 

 only amounts to about 4 per cent, which is inside the limit 

 of possible error. 



From the values of a given above it may been that the 

 mean free path of the Becquerel ray corpuscles in air at 

 1 mm. is about 6 cms. This very large value for the mean 

 free path accounts for the very small absorption of the de- 

 flectable Becquerel rays. 



In conclusion, my thanks are due to Prof. J. J. Thomson 

 for many suggestions given during the progress of the work. 



Cavendish Laboratory, Cambridge. 



L1X. Condensation of tlte Radioactive Emanations. By 

 E. Rutherford, M.A., D.Sc, Macdonald Professor of 

 Physics, McGill University, Montreal, and F. SoDDY, M .A. 



(0X071.). 



[Plate XIV.] 



IN a previous paper (Phil. Mag. 1902, iv. p. 581) we have 

 shown that the radioactive emanation from thorium passes 

 in unchanged amount through a white-hot platinum tube and 

 through a tube cooled to the temperature of solid carbon 

 * Communicated bv the Authors. 



