892 Secondary y rays excited by ft rays of Radium. 



noticed. I am not clear from the paper whether the 

 numbers given by Dr. Hughes represent the total leak, or 

 the difference in the leaks when the salt is first obscured and 

 then exposed to light. I£ the former, then the whole effect 

 may be that measured by Garrett ; while even in the latter 

 case Garrett's experiments are not without interest, as they 

 show that the salt is already in an instable condition and may 

 easily be broken down by proper means, of which apparently 

 ultra-violet radiation is one. It would be interesting to know 

 whether a current was obtained with the accelerating field 

 reversed, and, if so, how it compared with those given in 

 the paper; in Garrett's experiments the positive ions were 

 frequently in excess at low temperatures. 



Cass Institute, E.C. 



XCVII. On Secondary yrays excited by the /3 rays 

 of Radium. 



To the Editors of the Philosophical Magazine. 



Gentlemen, — 



MR. CHADWICK, in his interesting paper on secondary 

 iy-rays excited by the /3-rays of radium (Phil. Mag. 

 1912, [6] xxiv. p. 594), deduces by analogy from the 

 behaviour of the rays of radium that not more than 10 per 

 cent, of the total 7-radiation of uranium X, observed by 

 Mr. Russell and myself (ibid. 1909, xviii. p. 620), can be due 

 to the impact of the /3-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 7-rays of uranium X are not 

 secondary rays produced by the impact of the /3-rays (ibid. 

 1910, xix. p. 728). 



In this paper, which Mr. Chadwick is evidently unaware 

 of, he will find data as to the absorption of the 7-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 /^ and fi 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. 



