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[ 383 ] 

 XXXVII L Intelligence and Miscellaneous Articles. 



To the Editors of the Philosophical Magazine. 

 Gextlemei?, — 



I AGREE with Dr. Kleeman (Phil. Mag. 1910, xx. p. 248) 

 that evidence can be cited from his own work and that of 

 others against the view that the y-rays are homogeneous. The 

 question, as he points out, is an intricate one, and there exists a 

 real inconsistency in the experimental results, different lines of 

 work leading to opposite conclusions. The alterations of the pro- 

 perties of the y-rays, and the diminution of their absorption- 

 coefficients (" hardening ") by previous screening of the rays, are 

 certainly opposed to the view that the rays are homogeneous. 

 Dr. Kleeman, however, scarcely does justice to the evidence we 

 advanced in Part II. of our paper (Phil. Mag. 1910, xix. p. 725), 

 for the belief that the y-rays are homogeneous. According to 

 him this evidence depends on the use of a formula " which like 

 other absorption formulae can only approximately represent the 

 facts," and which does not take any account of the production of 

 secondary radiation or of scattering of the primary without change 

 of nature. This criticism is singularly at fault. The formula we 

 used for the absorption of the y-rays of radium over a semicircular 

 arc from a point source placed at the centre of a truncated hemi- 

 sphere of absorbing metal, as we employed it in our experiments, 

 is, unlike other absorption formulae, mathematically exact and is 

 deduced from the three definite postulates, (1) that the y-rays are 

 homogeneous and exponentially absorbed, (2) that scattering of the 

 primary radiation does not take place, (3) that no (penetrating) 

 secondary radiation is produced in the metal. The formula could 

 not hold true if either of the three postulates were false. 

 Nevertheless the theoretical formula agreed nearly perfectly with 

 the experimental results for the case of lead, with the same value 

 for the absorption coefficient as had been found in numerous other 

 experiments. For zinc the formula also held for thicknesses 

 greater than 2 cm., the results clearly indicating that here a 

 penetrating secondary radiation was produced. When the com- 

 plicated character of the theoretical expressions, involving as 

 they do two exponential integral terms, is borne in mind, the 

 almost perfect agreement between the theoretical and experimental 

 results for lead cannot be dismissed as lightly as Dr. Kleeman 

 indicates. 



I take this opportunity of putting right au error arising out of 

 a reference we made (p. 730) to some work of Bragg and Madsen. 

 "We deduced theoretically that the transformation of a fraction, 

 varying from zero to unity, of the absorbed primary into a pene- 

 trating secondary radiation by different metals would result in 

 variations in the ionizations observed with great equivalent thick- 

 nesses of different metals in the ratio of rather more than 2:1, 



