430 Dr. J. P. V. Madsen on 



from a Pb radiator ; we may therefore assume without 

 much possibility of error that the value of X by Zn for this 

 radiation is *34. 



It is now possible to apply the necessary correction to the 

 curve A, fig. 3. A quantity 570(1 — e -^' 5x ' u)d ) has been 

 added to the previous readings ; the result is shown in the 

 full-line curve B. The last two readings require a slightly 

 less correction on account of the Zn plug cutting off some of 

 the original y radiation which produces the effect from the 

 sides of the cone ; the necessary correction in this case was 

 determined by a separate experiment. 



The quantity of the emergence radiation from Zn and 

 from Al was now tested by Zn and by Al domes. The 

 results are given in Table I., proper correction being applied 

 in each case for the absorption of the radiation from the 

 sides of the cone. 



The Zn radiator was 12 mm. thick, the Al 17 mm. 

 Measurements were also made with radiators of other thick- 

 nesses, but the results were not sufficiently accurate to enable 

 one to say whether the quality of the emergence radiation 

 varied with the thickness of radiator. 



The results are plotted in fig. 5, from which it will be 

 seen that the radiation from Zn and Al is far from homo- 

 geneous when testecMby Pb domes. 



The emergence radiation from Zn and Al appears to 

 consist of two sets of y rays — one of much the same quality 

 as those which are produced from Pb, the other a very much 

 softer bundle. A rough analysis of either the Zn or Al 

 curve, if such is legitimate, would suggest that it may be 

 derived from a hard and soft bundle of y rays, for which the 

 values of \ by Pb are 1*3 and 5 respectively, the effect of 

 the hard bundle being initially almost twice that of the soft. 



It will be seen from the figure that mass for mass Al and 

 Zn domes are not able to bring out so clearly as Pb domes 

 the distinction between the quality of the secondary radia- 

 tions from Zn and Al as compared with that from Pb. 



This, again, is much the same as is found when one 

 attempts to sort out the original hard and soft y rays of Pa 

 by such screens. 



However, the radiation from Pb always appears somewhat 

 harder than that from Zn and Al, no matter what screen is used. 



This result has been checked on different occasions with 

 care, and is of importance, as the results obtained by Klee- 

 man from the secondary incidence y rays, and the theory of 

 selective absorption founded thereon, and extended to the 

 original y radiation, should apparently apply also to the 

 secondary emergence rays. 



