438 Dr. J. P. V. Madsen on 



Returning to fig. 8 it will be found that the ordinary 

 absorption curve, corresponding to the dotted curve Pb 1, 

 i. <?., in which as much secondary radiation as possible is 

 included, can be represented very accurately by the ex- 

 pression : 30e-' S2d + 10e- yr » d . 



Again, the full-line curve Pb 2, from which as much as 

 possible of the secondary radiation has been excluded, is 

 represented by the expression : 24:e~' 32d + 76e~ l ' 35d . The dif- 

 ference between these two expressions, viz., 6(e~' S2d — e~ 1,35d ) 

 is a measure, therefore, of the total emergence secondary 

 7 radiation from Pb. Wow it has been shown in previous 

 experiments in this pnper that the secondary 7 radiation 

 from Pb can be approximately represented by a homogeneous 

 radiation, for which the value of A, is 1*25 ; w r e are therefore 

 able to account for the results which have just been obtained 

 if we suppose that the original hard bundle of 7 rays, which 

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

 the secondary radiation which comes from Pb ; this is in 

 agreement w T ith the result obtained previously (p. 425). 



In the case of substances of the nature of Zn the difference 

 between the two absorption curves, as shown in fig. 9, 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 7 rays. We must suppose that in such 

 substances the soft 7 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 

 another way of saying that Pb absorbs the softened secon- 

 dary 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 corre- 

 sponding to a hard and soft bundle of 7 rays, the hard 

 having about the same penetrating power as the secondary 

 rays from Pb. As it will be shov\n presently that there is 

 good reason to believe that two distinct bundles of 7 rays — 

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

 the present look upon the two bundles of secondary rays 

 which are produced from Zn and sucli substances as the 

 products of the corresponding hard and soft bundles of the 

 original radiation. 



It will be seen from fig. 8 that when as much secondary 



