187 



In addition to these rays we have emerging per sec. a 

 number I^e-^^ which have suffered no scattering. 



We are not at present able to evaluate the expression 

 given above, from want of knowledge regarding Fd. How- 

 ever, we may proceed to make an approximate calculation of 

 the effect which might be expected in such a case as we have 

 investigated in fig. i. 



Considering as before a fine pencil of homogeneous y 

 rays falling normally upon a plate of thickness L, we may ex- 

 press the emergence radiation as — 



fl^ -\x -a\'{L-x) J^l \ I -XL -ayL\ 



e e dx = [e —e I 



where k is a constant, A the coefficient of absorption of the 

 primary set of rays, \' a similar coefficient for the secondary 

 rays, and a a constant obliquity factor, which in the present 

 case is taken as 2, a value obtained by considering the geo- 

 metrical arrangement of the radiators in fig. i. with regard 

 to the ionization chamber. 



Let us now apply this result to the case of a Pb radi- 

 ator : the values of X and A.' are '32 and 1*3 respectively. The 

 curve corresponding to e--32L_e-2-6L js shown by the dotted 

 line in fig. ii., the maximum having been adjusted to the 

 same value as for the experimental curve. 



The agreement between the theoretical and the actual 

 curve is very good. 



The maximum value is reached for about the same thick- 

 ness of radiator, and for thicknesses of radiator greater than 

 that required to give the maximum effect the curves slope 

 away to about the same extent. This, taken in conjunction 

 with previous experiments, seems to show that practically all 

 the secondary y radiation obtained in these experiments from 

 Pb may be considered as derived from the original hard y 

 rays of the Ra. 



It must, however, be remembered that the secondary 

 emergence rays have to penetrate the Pb sides of the ioni- 

 zation chamber, and allowing for obliquity this corresponds 

 to a thickness of about 2 mm., so that it is still quite pos- 

 sible that a fair amount of very soft secondary y radiation 

 may have escaped detection. 



If we now proceed in a similar manner to construct the 

 corresponding curve for Zn, viz., e-"2iL_e--64L, it is found 

 that it by no means corresponds to the experimental curve. 

 If, however, we assume that for a thickness of about 10 cm. 

 of the Zn radiator the primary soft bundle is producing very 

 little of the secondary emergence y radiation, the curve as 



