184 



radiation and its effects, we may consider the Ra to give 

 out two distinct sets of y rays, one more penetrating than 

 the other, each practically homogeneous, with values of 

 A./ A— '028 and '012 respectively, these values being practi- 

 cally independent of the nature of the absorbing substance. 



When the secondary y radiation is allowed to produce its 

 effect in addition to that produced directly by the original 

 ■y rays, this law may be modified to a very considerable ex- 

 tent. In the usual arrangement of apparatus used for de- 

 termining the quality of a radiation the absorbing plates are 

 as a rule placed close to the ionization chamber, and the 

 values of A deduced from the tangent to the absorption curve 

 — that is to say, it is generally with the dotted line curves 

 of fig. viii. that we are dealing, and the effects of secondary 

 radiation may, as has already been shown, be in some cases 

 very nearly as important as the effects produced directly from 

 the original y radiation. 



This, for example, affords a ready explanation of the 

 effect observed by Eve (Phil. Mag., Aug., 1908), who states: 

 — "It is noteworthy that the Ra in 2'2 cm. of nickel-steel 

 gives an effect about 1*5 times as great as when the Ra is 

 in 1 cm. of Pb. From the relative densities we should ex- 

 pect 2'2 cm. of steel to be equivalent to 15 cm. of Pb, and 

 therefore the Ra in the steel cylinder should give, by the den- 

 sity law, two-thirds of the effect of the Ra in Pb. It actu- 

 ally gives one and a half times as much ; thus the primary 

 y rays traverse steel much more readily than Pb, but the 

 rays passing through iron are subsequently absorbed more 

 readily by Pb than if the Ra were in Pb." 



From the results of the present paper we can say that 

 in going through 2'2 cm. of steel or 1*5 cm. of Pb, the soft 

 primary y rays will have suffered in each case the same num- 

 ber of collisions, but that the effect of a collision with an 

 atom of Pb, or rather with the constituent of such an atom, 

 is much more definite than in the case of substances of lower 

 atomic weight ; that in the case of Pb the collision has broken 

 the original y ray up, immediately sending out a )8 ray, or 

 has so shattered the original y ray as to make it easy for Pb 

 to subsequently complete the process. 



In the case of Zn, etc., the effect of the collision has 

 not been so definite : much scattered y radiation is produced, 

 somewhat softened by the effects of collision, but still able to 

 suffer further modification as a y ray before being eventually 

 broken up and sending out the jS ray. 



It must be noticed that in experiments upon the second- 

 ary incidence y rays, such as have been carried out by Klee- 

 man and by Eve, the nature of the secondary effect may be 



