fy-Rai/s of Thorium and Actinium. 

 2. Zinc. Brass base 1'73 cm. thick. 



14: 



[ 



Range (cm.). ... 



to 0-66 



066 to 162 



1-62 to 6 00 



A (cm.) 



0-226 



0-233 



0255 



3. Lead. Lead base 1 cm. thick. 



Range (cm.). ... 



to 025 



0-25 to 0-62 



0-62 to 60 



X (cm.) 



0608 



0-545 



0-500 



4. Zinc. Lead base 1 cm. thick. 



Range (cm.). ... 



to 0-34 



034 to 10 



1 -0 to 6-0 





A (cm.) 



0-238 



0251 



0-270 





But it is possible to find a disposition in which zinc absorbs 

 the Y-rays strictly according to an exponential law with the 

 normal value of \ from thicknesses sufficient to absorb all 

 the /5-rays up to the greatest thickness it was possible to use 

 with the disposition. This was briefly referred to before 

 (II. p. 754) and may now be more fully described. A prepara- 

 tion of radium (7 rag.) was placed at the apex of a cone of 

 length 11*5 cm. and base 3 cm. diameter cut out of a cylinder 



Tier. 1. 











/ 









.l r 



l l ■ 





..... 



J ■■ -- 



of lead (length 12*7, diameter 10*5). 2 cm. from the mouth of 

 the cone was placed a short cylindrical ionization chamber of 

 lead, 9*2 cm. inside diameter and 4*? em. long, an electrode in 



