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weight from those contained in filter papers. Consequently 

 primary rays, and secondary rays which differ very little from 

 the primary, are very penetrating to air, and cause relatively 

 small ionizations therein. But secondary rays from Cu and 

 Fe are softened so much as to bring them within reach, so 

 to speak, of air, which rapidly converts them into cathode 

 rays, so that there is a very large ionization. For the cathode 

 rays produced from these secondary rays have probably but 

 little less energy than those produced from the primary ; the 

 speed of the cathode ray does not differ very greatly with 

 the penetration of the primary X-ray, so far as experiments 

 have shown. The very large secondary radiations, which some 

 substances appear to give, therefore owe their magnitude 

 largely to the fact that the air in which they are measured 

 is sometimes ten to twenty times as favourable to them as 

 to the primary rays which produced them. In this way we 

 may account to some extent for the startling results obtained 

 by Crowther in the case of arsenic and bromine ('Thil. 

 Mag.," Nov., 1907). 



