[eve] 



PRIMARY AND SECONDARY GAMMA RAYS 



39 



centimetres thick it would, of course, send back through C second- 

 ary Y rays, the additive effect of which could be detected. 



In the actual experiments, keeping at A and B any plates of a 

 definite material, and using at S screens of iron 2 cm., or lead 0.6 

 cm., or lead 2 cm., the effects of varying C are shown in the fol- 

 lowing table and in curve I., Fig. 2. 



Table I. 



There is some evidence of a slight change due to the hardening 

 of the rays by screening. The results given, the mean of many 

 experiments, might have been expected and predicted from the 

 known character of the incident secondary radiation due to ;■ rays, 

 which like that due to /? rays, follows the order of atomic weights. 



Effect of Reveksing Plates A and B. 



If a plate of any given substance is kept constantly at C, and 

 if readings of the electroscope are taken when two different metal 

 plates are at A and B, and also for those same plates in reverse order, 

 then the two readings are not the same. The change observed is 

 not due to an alteration of the intensity of the ;- rays traversing 

 the electroscope, but to the negative electrons, or corpuscles, which 

 constitute the emergent secondary radiation from plate B. Hence 

 the advantage of the reversal method, using two plates; for with 

 single plates there is an uncertain decrease in the 7- rays depending 

 on thickness and density. Nor is it necessary to use a magnetic 

 field to remove negative rays approaching the face of A from the 

 screens, for these are absorbed by the plates. 



It has been shown by many observers that when very thin 

 plates are placed in the path of the ;- rays near an electroscope the 

 effects due to such thin plates are small, but on increasing their 

 thickness the ionization current in the electroscope rises quickly 

 to a maximum, owing to the increase of the emergent corpuscular 



