36 



ROYAL SOCIETY OF CANADA 



The transmitted or emergent secondary radiations at first re- 

 ceived less attention, although it was shown^ in 1904 that for vary- 

 ing thickness of the secondary radiator the intensity of the emergent 

 radiation followed a curve of the well known type whose equation 

 involves the difference of two simple exponentials. A. S. Mac- 

 kenzie' has called attention to the remarkable fact that there travels 

 in the direction of the y rays, when absorbed, a stronger stream of 

 negative secondary rays than in the reverse direction. This fact, 

 substantiated by Bragg, is one of the arguments in the chain of 

 reasoning which led him, in spite of able opposition and many diffi- 

 culties of experiment and theory, to controvert the ether-pulse theory. 



Bragg^ has proved that the emergent secondary corpuscular 

 radiation from various elements does not follow the order of atomic 

 weights, but that the curve, plotted with atomic weights as abscissae 

 and intensities of secondary radiation as ordinates, roughly resem- 

 bles a parabola with vertex downwards, such as may be seen in 

 Figure 3 of this paper. Again, there is some divergence in the views 

 and results of Bragg and Kleeman, for the latter found evidence of 

 selective absorption, while Bragg and Madsen attributed the observed 

 effects to the hardening of the rays. Also Kleeman, using both 

 primary and secondary ;- rays, divided the metals examined into 

 three well marked groups, but Bragg's work, and Madsen's, indicates 

 no such sharp fine of demarcation into groups. The present work 

 was undertaken to investigate this point also, and it will be seen 

 that the results favour the conclusions of Bragg and Madsen. 



Apparatus. 

 A rectangular brass electroscope (Fig. 1), 18 x 16 x 12 cm., 

 was used, but both ends were removed and covered with thin alu- 



AB 



7 



tpH^-r 



-J 



CO 



H f 



WmWrnmi/ 



3 



Fig. 1. 



minium foil. It requires about 12 layers of this foil to stop the 

 a rays of radium, so that it may be expected, and it was found, that 



^ Phil. Mag., Dec. 1904. ^ pyj j^g^ j^jy 1907. 



' Trans. Roy. Soc. S. Aust., 32, 1908. 



