Secondary y Radiation. 231 



secondary radiation depends on the direction in which the 

 radiation emerges from the radiator, on screening the emana- 

 tion, and to a certain extent on the material of the radiator ; 

 and that the secondary radiation for any definite direction is 

 not homogeneous. Recent experiments with zinc as the 

 radiator placed at different angles, show that the absorbability 

 of the secondary radiation depends largely on the direction 

 in which it is issuing and not altogether on the fact of its 

 being " emergent " or " returned/' 



Examining the absorption coefficients included in the 

 tables, it is seen that the emergent secondary radiation from 

 lead is more penetrating than from zinc, but the " returned " 

 radiation from lead is less penetrating at least for the first 

 screen. It appears then that from lead there arises a type of 

 radiation less penetrating than that from zinc and probabty 

 characteristic of lead. 



This point had previously been investigated with radiators 

 of lead, mercury, and platinum. The experimental arrange- 

 ment is shown in fig. 3. 



Fiir. a. 



MM is a large electromagnet giving a field strength about 

 1400 gauss when the pole-pieces are 9 cm. apart. The side 

 of the electroscope consists of mica equivalent to about 6cm. 

 of air. The absorbing screens are placed at A A and the 

 radiators in the positions RiRj and lUu The electroscope 



