35 



An Experimental Investigation of the Nature 

 OF y Rays.- No. 2. 



By W. H. Bragg, M.A., F.R.S., Elder Professor of Mathe- 

 matics and Physics in the University of Adelaide ; and J. 

 P. V. Madsen, D.Sc, Lecturer on Electrical Engineering. 



[Read May 4, 1908 : a preliminary summary was given on 

 April 7.] 



In a previous paper (Trans. Roy. Soc. of S.A., 1908, p. 1) 

 we have given a preliminary account of an investigation of 

 the properties of the secondary radiation due to y rays, and 

 discussed the evidence thus afforded as to the nature of the 

 rays. The first section of the present paper contains an ac- 

 count of further experiments, and the second a list of the pro- 

 perties of the secondary radiation, derived in part from the 

 work of other observers, and in part from our own. In the 

 third we have tried to show that the properties are readily 

 explained if the y rays are supposed to be material, but are 

 not easily to be reconciled with the ether-pulse hypothesis. 



§ ^- 



In the former paper we showed that on the neutral-pair 

 hypothesis the connection between the amount of secondary jS 

 radiation emitted from the front side of a plate struck by y 

 rays and the atomic weight of the material of the plate should 

 be approximately the same as for the ^ rays. It is, of course, 

 known that this is actually the case. Also, we showed that 

 the /8 radiation emitted from the other side of the plate, the 

 side from which the y rays emerge, should be thte same for all 

 substances, provided three things were true, viz. : — 



(1) The y rays were homogeneous; 



(2) The y rays were absorbed according to a simple den- 

 sity law ; 



(3) The i8 rays were also absorbed according to such a 

 law. 



If these laws did not hold, and to the extent to which they 

 did not hold, the ''emergence" radiation would not be the 

 same for all substances. 



The experimental evidence which we submitted showed 



that the emergence radiation was not connected with the 



atomic weight of the material by the same law as that which 



held for the incidence radiation and for /3 rays: that it was 



b2 



