On Secondary Rontgen Radiation. 323 



If the plate is thick enough to make /3h large, we have 



T — - * ° c-*fc 

 2/3-\ ' 



„ 1 tfAJn 



2/3+X* 



If $ is the ionization produced by the secondary rays in a thin 

 layer of gas close to the front layer of the plate S = Ryt, where t 

 is the thickness of the layer, and y the coefficient of absorption 

 of the secondary rays in the gas. If P is the ionization produced 

 by the primary rays in the same layer P = I St, & being the co- 

 efficient of absorption of the primary rays in the gas ; hence 



S _R y 1 /c.\ y 



P~ T 8 = 2l3 + \'8' 



As the secondary rays are, in general, much more easily absorbed 

 than the primary, /3 will be large compared with X ; when this is 

 the case, we have approximately 



8 1 k . \ y 

 P = 2"~0~'B' 



Now \/S is the ratio of the coefficients of absorption of the 

 primary rays in the plate and gas respectively, while /3/y is the 

 ratio of these coefficients for the secondary rays, and for Rontgen 

 rays the softer the rays the greater is the absorption of dense 

 media in comparison with gases ; hence, as the secondary rays are 

 softer than the primary rays, we should expect /3/y to be greater 

 than \/S, and in this case 



8_ x 



p < 2 K > 



or k > 2S/P. 



When the plate is made of a dense substance like lead S is 

 much greater than P. I have lately measured S for a large number 

 of different elements, and found that taking any two elements 

 the one with the greater atomic weight gives the greater value 

 of S, so that the values of S for the different elements are in 

 the same order as their atomic weights. If, however, S is greater 

 than P, k is greater than unity ; this implies that unless the 

 proportion of energy spent in ionization is much larger in the 

 case of the secondary radiation than in that of the primary, more 

 energy is given out as secondary radiation than is supplied by the 

 primary. We may regard the primary radiation as causing an 

 explosion in some of the atoms resulting in the emission of 

 secondary radiation, and suppose that part of the energy of the 



