418 Dr. Barkla and Mr. Sadler on Secondary 



the secondary radiation as ordinates, curves were obtained 

 for the radiation from each element. A gradual change of 

 gradient was noticed with an increase in the atomic weight 

 of the radiator. The gradient of the curve obtained for nickel 

 lay between those for cobalt and copper. Although the 

 experimental error was too large to make the results of 

 one series of experiments absolutely conclusive, this position 

 was maintained in the several series taken, again indicating 

 a strong probability of the atomic weight given above being 

 approximately correct. 



[Later experiments have shown that it is possible that this 

 result is not really independent evidence on the point in 

 question .] 



Since performing the experiments, the results of which are 

 described above, we have noticed that Prof. J. J, Thomson * 

 in experimenting on the easily absorbed corpuscular rays 

 emitted by elements subject to ^-rays, found that "in every 

 case except nickel an increase in atomic weight is accompanied 

 by an increase in the stream of radiant energy," proceeding 

 from the element. On examining the relative values of the 

 ionizations found in these experiments it is evident that if we 

 assign to nickel the atomic weight given above, it falls into 

 order and the law becomes general. 



Again, in experiments on the absorption of primary ^-rays 

 by elementary substances, the same irregularity occurs. From 

 Benoist's results connecting the transparency of substances to 

 #-rays and the atomic weights of those substances, his 

 method does not appear sufficiently delicate in this region 

 of atomic weights to indicate such an irregularity, but Hebert 

 and Reynard f found the opacity of nickel to <r-rays to be 

 greater than that of cobalt, while in all other cases examined 

 by them an increase in opacity accompanied an increase in 

 atomic weight of the substance examined. This is not an 

 isolated record of such an irregularity, for Blythswood and 

 Marchant J found that contrary to the general grouping of 

 elements of neighbouring atomic weights as producing 

 approximately the same absorption, cobalt behaves like iron, 

 whilst nickel approximates to copper and zinc in its absorp- 

 tive powers. Though with certain Rontgen beams it appears 

 that an increase in the atomic weight of the absorbing 

 substance may be accompanied by a slight decrease in opacity, 

 the change is invariably a gradual one, and quite unlike the 

 irregularity exhibited by nickel. This irregularity would 



* Proc. of Camb. Phil. Soc. xiv. pp. 109-114, Nov. 1906. 

 t Comptes Rendtis, cxxxii. pp. 408-409 (1901). 

 % Proc. of Royal Society, lxv. p. 413. 



