Secondary y Radiation. 443 



practically all the secondary 7 radiation obtained in these 

 experiments from Pb may be considered as derived from the 

 original hard 7 rays of the Pa. 



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 possible 

 that a fair amount of very soft secondary 7 radiation may 

 have escaped detection. 



If we now proceed in a similar manner to construct the 

 corresponding curve for Zn, viz., e~' 2lL — e~' 6iL , 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 7 radiation, the curve as 

 calculated above may be placed in the position shown by C, 

 fig. 3. The curve I) is now drawn to represent the difference 

 between the curves B and 0. We may now see how closelv 

 this remainder corresponds to what might be expected if 

 the soft bundle of original 7 rays produced a correspondino- 

 softer secondary. The maximum in the case of this re- 

 mainder curve is further to the left than in the case of curve 

 C, and after reaching its maximum value the curve falls 

 away much more rapidly than does C. It corresponds, in 

 fact, very well with what we might reasonably expect to be 

 produced by the soft bundle of y rays coming from the Ra. 

 Again, although for any given thickness of radiator the ratio 

 of the ordinates to the curves ( ! and D is by no means constant, 

 it is of the order indicated in fig. 5 by the curve A". 



If, now, the curves C and D represent the effects due 

 respectively to the hard and soft 7 rays from the Ra, it 

 should be possible to show experimentally that the quality 

 of the radiation from a Zn radiator depends to some extent 

 upon the thickness of the radiator. No serious attempt has 

 so far been made to carry out this investigation. However, 

 we may check the result in another way. The quality of 

 the radiation from Zn should show some change if we absorb 

 some of the soft 7 rays coming from the Pa by means of a 

 Pb screen or plug before allowing the radiation to fall upon 

 the Zn radiator. As we have already seen, it is possible in 

 this way to reduce the softer radiation without cutting down 

 the hard to such an extent as would be necessary if a plug 

 of material such as Zn were used. 



Using 17 mm. of Al as radiator, with no plug, the reading 

 with no dome was to the reading with a 6 mm. dome as 



