228 Mr. D. C.'H. Florance on 



In the paper an investigation has been made on the 

 following questions : — 



1. The quality of the secondary radiation or its variation 



in penetrating power with 



(a) position of electroscope; 



(b) position of radiator; 



(c) material of radiator; 



(d) screening of emanation. 



2. The absorption of the primary beam and its consequent 



effect on the secondary radiation. 



3. The distribution of secondary radiation depending on 



(a) thickness of radiator; 

 (6) material of radiator; 



(c) equal masses of different radiators ; 



(d) material of the side of the electroscope; 



(e) screening of the emanation; 



(/) use of air or methyl iodide in the electroscope. 



4. The energy of the scattered radiation. 



5. Evidence of a characteristic radiation and reason for 



assuming that the secondary 7 radiation is a modifica- 

 tion of the primary radiation. 



Experimental Arrangement. 



A diagrammatic sketch of the electroscope and experi- 

 mental arrangement is given in fig. 1 and fig. 2. The direct 

 ionization effect could be reduced to any extent by a suitable 

 adjustment of lead screens and by a balance method. As far 

 as possible screens were chiefly employed, since with a large 

 direct effect combined with a very active source on the 

 balance-chamber, the results were uncertain and it appeared 

 that under these conditions the sulphur insulation became 

 conducting. The secondary radiation was examined in four 

 positions of the electroscope which are denoted by the symbols 

 E x , E 2 , E 3 , E 4 . The electroscope was 12 cm. cube externally 

 and the sides were of lead 1 cm. thick, except the front face 

 which was either of aluminium *202 cm. thick or of lead 

 •084 cm. thick. The radiators were all 11 cm. square. The 

 distance between the centre of rotation and the front face of 

 the electroscope was approximately 25 cm. 



The emanation could be screened by lead of any definite 

 thickness. The secondary radiation could be absorbed 



