183 



presented by the expression: 24e-'^2d + ygg-i-ssd. xhe dif- 

 ference between these two expressions, viz., 6(e-'52d_g-r35d.) 

 is a measure, therefore, of the total emergence secondary y 

 radiation from Pb. Now it has been shown in previous ex- 

 periments in this paper that the secondary y radiation from 

 Pb can be approximately represented by a homogeneous radi- 

 ation, for which the value of A is 1*25 ; we are, therefore, 

 able to account for the results which have just been obtained 

 if we suppose that the original hard bundle of y rays, which 

 proceed from the Ra, makes all, or, at least, very nearly all, 

 the secondary radiation which comes from Pb ; this is in 

 agreement with the result obtained previously (page 169). 



In the case of substances of the nature of Zn the differ- 

 •ence between the two absorption curves, as shown in fig. ix., 

 cannot be explained so simply. The secondary radiation after 

 reaching its maximum value falls off more rapidly than we 

 should expect, had all the secondary been derived from the 

 hard set of original y rays. We must suppose that in such 

 substances the soft y rays from the Ra also produce a certain 

 amount of secondary emergence radiation, or that even 

 though such may be produced in all substances it is better 

 able to escape from such substances as Zn than from sub- 

 stances of the nature of Pb. This of course is merely an- 

 other way of saying that Pb absorbs the softened secondary 

 radiation to a much greater extent than Zn. 



Again, it has been shown earlier in this paper that the 

 emergence radiation from such substances as Zn appears, when 

 tested by Pb, to be divisible into two quantities correspond- 

 ing to a hard and soft bundle of y rays, the hard having 

 about the same penetrating power as the secondary rays 

 from Pb. As it will be shown presently that there is good 

 reason to believe that two distinct bundles of y rays — a hard 

 and a soft — are given out from the Ra, we may for the pre- 

 sent look upon the two bundles of secondary rays which are 

 produced from Zn and such substances as the products of the 

 corresponding hard and soft bundles of the original radia- 

 tion. 



It will be seen from fig. viii. that when as much second- 

 ary radiation as possible is excluded the curves for all sub- 

 stances very nearly correspond. Let us suppose that had it 

 been possible to exclude all the effects of secondary y radi- 

 ation, the curves for all substances would have become iden- 

 tical. This supposition must apparently be very near the 

 truth, for it can be seen from the geometry of the arrange- 

 ment in fig. vii. that by no means can all the secondary radia- 

 tion have been excluded in the first set of experiments. We 

 arrive, then, at this result: that, excludine: all secondary y 



