144 



MR. G. W. C. KAYE ON THE 



once more display the homogeneity indicated when radiator and screen have the same 

 atomic weight, and the lack of it when they are remote. 



As previously mentioned (p. 123), BENOIST,* working with a definite beam of 

 Rontgen rays, obtained a smooth curve, hyperbolic in form, by plotting atomic 

 weights of screens against " coefficients of transparency " (i.e., the mass of a prism of the 

 substance of unit cross-section, which produces the same absorption as a standard 

 prism, when the rays travel along the axis). Obviously the transparency (M) is 

 inversely proportional to a mean X/p for the region of absorption, for log e (I /I) = 

 const, = \d = XM/p. 



BENOIST, using a platinum anticathode, obtained curves which yield the general 

 result that X/p increases with the atomic weight of the screen,t and more rapidly in 

 the region of low atomic weights : in other words, heavy atoms are more absorbent 

 than light, weight for weight. The relative mean values of X/p below are taken from 

 his curves to illustrate the point. 



If we examine fig. 12 (Pt radiator), BENOIST'S result appears at once from the 

 relative slopes of the curves. But, from a general comparison of figs. 10, 11, and 12, 

 it is obvious that the shape of BENOIST'S transparency curve, besides depending on the 

 extent and region of the absorption, varies considerably with the radiator. As an 

 illustration, the relative mean values of X/p for aluminium, copper, and platinum 

 screens are obtained from the curves in figs. 10, 11, and 12, and tabulated below for 

 the two cases when 50 per cent, and 10 per cent, of the radiation are transmitted. 



* BENOIST, ' Journ. de Phys.' (3), X., p. 653 (1901). 



t The statement is also true for the soft y rays from radium. With hard y rays a " density law " holds 

 and \/p is constant for different elements. 



