248 Intelligence and Miscellaneous Articles. 



To the Editors of the Philosophical Magazine. 



Gentlemen, — 



In a paper published in the Phil. Mag. for May 1910, p. 725, 

 Mr. and Mrs. Soddy and Mr. Russell describe some highly inter- 

 esting experiments on the absorption of the y rays of radium bv 

 different materials. Since these experiments bear on some of my 

 work on y rays, I would like to make a few remarks upon them. 



According to a formula which they use, the y rays of radium 

 appear to be homogeneous. I would like to point out that the 

 formula used, like all the other absorption formulae, can only 

 approximately represent the facts. Besides the absorption of 

 the rays secondary radiation of a more absorbable type than the 

 primary is produced, and some primary rays probably get scat- 

 tered without any change in their nature. The formula used 

 does not take any account of this, and in fact it is impossible 

 at present to formulate one that does. I therefore venture to 

 think that the primary y rays are not homogeneous. The experi- 

 ments that have been carried out on the change in penetrating 

 power of the cathode rays ejected from substances exposed to 

 the y rays of radium by previous screening of the rays, cannot be 

 brought into harmony with the idea that they are homogeneous. 

 Thus I* have shown by means of a magnetic deflexion method 

 that the proportion of slow cathode radiation to the more 

 penetrating from lead exposed to y rays becomes less when 

 the rays are previously passed through a metal screen. This 

 was also shown by Prof. Bragg and Dr. Madsenf using a 

 method of scattering. Mr. and Mrs. Soddy and Mr. Russell think 

 that the soft cathode radiation is solely due to the soft secondary 

 y rays generated in the substance by the primary rays. If that 

 were so, the proportion between the cathode radiation of different 

 penetrating powers would be independent of screening, but 

 according to the experiments quoted it is not. 



I have carried out some experiments on the " scattering " of 

 y rays of different penetrating powers. These experiments are 

 too lengthy to be discussed in this connexion in this letter. I 

 may mention only that the scattering of "hard" rays, obtained 

 by previously passing the y rays through a lead screen, is less 

 than that of the "softer" unscreened rays. However, the experi- 

 ments of Mr. and Mrs. Soddy and Mr. Russell should throw some 

 additional light on the intricate mechanism of the absorption of 

 y rays. 



Tours faithfully, 



Cambridge, May 19. R. D- IvLEEMAX. 



* Proc. Rov. Soc. A. vol. Ixxxii. p. 128 (1909). 

 t Phil. Mag. xv. pp. 663-675, May 1908. 



