238 



Mr. D. 0. H. Florance 



on the character of the y radiation, A few experiments 

 were therefore performed under the following conditions : — 



A. The side of the electroscope was lead '084 cm. thick. 



B. The side of the electroscope was aluminium '202 cm. 



thick. 



C. A combination of these two plates with lead inside. 



1). A combination of these two plates with aluminium 

 inside. 



In Table V. the values, in arbitrary units, of the ionization 

 currents are given for equal weights of the four standard 

 radiators when the radium was screened with *084 cm. of 

 lead, and the conditions were those of A and B. 



Table V. 



Radiators. 



E r 



E, | 



E 



| E,. j 



Al. 



168 

 170 



159 

 | 83 



Pb. 



150 

 156 



150 



82 



Al. 



88 

 80 

 64 

 25 



Pb. 



Al. 



69 



! 63 



Pb. 



39 

 38 



32 



11 



Al. 



78 



80 

 I 

 75 



35 



Pb. 



33 

 37 



36 

 16 



2'26 cin. carbon 



I'd cm. aluminium ... 



0'5 cm. zinc 



0-326 cm. lead 



65 

 66 

 59 

 24 



These numbers are only approximate but they illustrate 

 the general distribution in these four positions of the 

 electroscope. 



The effect of the lead screen becomes more marked in the 

 returned part of the secondary radiation. Although this 

 was to be expected on account of the greater absorption of 

 returned radiation, it was at the same time thought there 

 should have been a corresponding increase in the production 

 of emergent j3 rays from the lead. For instance, in case C, 

 when there was a combination of the two plates and the lead 

 was innermost, the distribution of secondary radiation was 

 similar to that obtained when lead alone "was used and 

 allowance made for the absorption of the radiation in the 

 aluminium plate. On the other hand, when aluminium was 

 innermost, the distribution was again relatively similar to 

 that obtained with lead alone, but with the values of the 

 ionization currents about 20 per cent. less. It is therefore 

 clear that more ft rays are set free in the lead than in 



