412 Dr. Barkla and Mr. Sadler on Secondary 



remarkable result; we therefore give in fig. 2 the percentage 

 absorptions of the secondary rays from copper (after trans- 

 mission through various thicknesses of zinc) by plates of 



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aluminium, zinc, and copper. These absorbing plates were 

 not, however, of the same thickness, so again the relative 

 absorptions are not shown. The curves obtained by plotting 

 percentage absorption by a plate of any one substance and 

 the percentage previously absorbed by zinc show that what- 

 ever was the absorbing substance used, the absorbability ^ of 

 the rays from copper did not change even after transmission 

 through plate after plate of zinc. The contrast between 

 the curves given in fig. 2, obtained by experiments on primary 

 and secondary beams, shows the marked difference between 

 these radiations. 



Another point worthy of notice is the fact that after the 

 transmission of the secondary rays from copper through zinc, 

 the ratio of absorptions by other substances does not change- 

 that is to say, the special powers of penetrating certain 

 substances possessed by primary beams after passing through 



