870 Messrs. Gray and Wilson on the Heterogeneity o] 



ionization did not change materially with the velocity, then 

 the rise and fall in the curve would have to be explained by 

 supposing the number of /3-partieles reflected to increase 

 with the velocity up to a certain value and then decrease 

 again. Since, however, the ionization produced by /3-par- 

 ticles of different velocities may play an important part in 

 the results obtained, a satisfactory explanation of this pheno- 

 menon cannot be arrived at until we know definitely the 

 variation of the ionization due to ^-particles moving with 

 different velocities. 



In conclusion we wish to express our best thanks to 

 Prof. Rutherford for suggesting this research. 



Victoria University of Manchester. 

 July 25, 1910. 



/ f 



XCV. The Heterogeneity of the ft Rays from a Thick Layer 

 of Radium /;. By J. A. GrRAY, B.Sc, 1851 Exhibition 

 Scholar, Melbourne University, and W. WlLSON, M.Sc, 

 Hon. Research Fellow, Manchester University* . 



THE law of absorption of ft rays by matter has lately 

 been the cause of some discussion. Until the ex- 

 periments of W. Wilson f, it had generally been assumed that 

 ft rays absorbed exponentially by aluminium were homo- 

 geneous. It was shown, however, that the coefficient of 

 absorption of approximately homogeneous ft rays rapidly 

 increased with the thickness of matter traversed, which 

 suggests that the rays experience a diminution in velocity 

 as they pass through the aluminium. Orowther J obtained a 

 similar result by the same method. 



The decrease in velocity indicated by these experiments 

 has been determined directly §. It follows as a necessary 

 consequence of these results that ft rays which are absorbed 

 exponentially by aluminium are not homogeneous. 



Recently, however, v. Baeyer and Halm ||, using a photo- 

 graphic method, have shown that the ft rays from several 

 radioactive products initially possess a considerable degree of 

 homogeneity. We have no definite evidence so far that the 



* Communicated by Prof. E. Rutherford, F.R.S. 

 t Wilson, Proc. Roy. Soc, A. lxxxii. 1909, p. 612.| 

 % Crowther, Proc. Camb. Phil. Soc. xv. pt. v. p. 442. 

 § Wilson, Proc. Roy. Soc, A. lxxxiv. 1910, p. 141. 

 || v. Baeyer & Halm, Phys. Zeit. xi. 1910, p. 488. 



