XCVII. On Secondary [gamma] rays excited by the [beta] rays 
of Radium. 

To the Editors of the Philosophical Magazine. 

Gentlemen, — 

MR. CHADWICK, in his interesting paper on secondary 
[gamma]-rays excited by the [beta]-rays of radium [citation redacted], deduces by analogy from the 
behaviour of the rays of radium that not more than 10 per 
cent, of the total [gamma]-radiation of uranium X, observed by 
Mr. Russell and myself [citation redacted], can be due 
to the impact of the [beta]-rays on the platinum trays in which 
our preparations were contained. He seems not to have 
noticed that I had already shown by actual experiment with 
the rays themselves that the [gamma]-rays of uranium X are not 
secondary rays produced by the impact of the [beta]-rays [citation redacted]. 

In this paper, which Mr. Chadwick is evidently unaware 
of, he will find data as to the absorption of the [gamma]-rays of 
uranium X in iron up to 0'92 cm. and lead up to 0'61 cm., 
together with the extrapolation of the absorption curves for 
greater thickness after they have become approximately 
exponential (fig. 1, plate xii.). He will, however, see from 
these curves that the values he assigns to his so-called 
absorption coefficients [mu]1 and [mu] 2 have no meaning whatever 
apart from information, not provided, as to the material and 
the thickness of the material constituting the base of his 
ionization chamber A. 

Frederick Soddy.