900 Messrs. J. C. Chapman and S. H. Piper on 



silver plate by 7 per cent. On testing the radiation coming 

 through this thickness its penetrating power was found to 

 differ only slightly from that of homogeneous copper radiation. 



Thus :■ 



Percentage absorption by aluminium ~\ 

 sheet ('00163 cm. thick) after 70 per I 

 cent, absorption of homogeneous [ 



Copper. Alloy. 



325 



! 

 29-9 





The slight decrease in penetrating power was fully accounted 

 for by the presence of radiation scattered from the silver in the 

 alloy, this scattered radiation having the same penetrating 

 power as the primary beam, which was of course harder than 

 the radiation characteristic of copper. 



This experiment showed that the electrons leaving the 

 copper and bombarding the silver atoms did not themselves 

 cause the latter to give out an appreciable homogeneous 

 secondary X radiation. This result indicates that it is im- 

 probable that the homogeneous radiation from a metal is the 

 result of bombardment of atoms, for it has been shown by 

 Campbell Swinton * and by Kaye f that the intensities of 

 X radiation produced at anticathodes of various metals when 

 subject to the same cathode stream are at any rate com- 

 parable. Usually the X rays from the heavier metals were 

 more intense (as measured by the ionization produced) than 

 those from light metals. If then an appreciable X radiation 

 were produced in copper by the bombardment of the cor- 

 puscular secondary radiation, we should expect a radiation 

 of comparable intensity to be set up in the silver by the 

 bombardment of electrons from copper atoms in the alloy. 

 The alloy experiment also shows that the silver atoms when 

 separated only by molecular distances from atoms which are 

 emitting homogeneous radiation are not stimulated so as to 

 emit their own characteristic radiation. It is still just 

 possible that the corpuscles from the copper excite neigh- 

 bouring copper atoms, but have not sufficient velocity to 

 stimulate the silver atoms. That is, a critical velocity higher 

 in silver than in copper may be necessary to the production 

 of the characteristic X radiation. This point will need 

 further investigation. 



* Proc. Roy. Soc. 1897, pp. 222-226. 



f Camb, Phil. Soc. Proc. xiy. Oct, 15th, 1907. 



