742 Dr. C. G. Barkla and Mr. C. A. Sadler on 



on the primary radiation, diminished in intensity by 

 transmission. 



All the phenomena observed may be divided into two 

 groups : — 



(1) When a radiation was transmitted through an element 

 whose characteristic secondary radiation was of an equally or 

 more penetrating type, or through an element from which 

 this type o£ radiation has not been observed, the emergent 

 radiation was identical with the incident radiation. 



(2) When a radiation was transmitted through an element 

 whose characteristic radiation was of more absorbable type, 

 the emergent radiation differed from the incident beam in 

 penetrating power. 



The following are a few examples of these types of 

 transmission. The first three exhibit no change after 

 transmission, the last three a considerable change. 



Substance Percentage Q , . A 



-p ,. .. through which Absorption ^stance usea Percent 



.Radiation. ,, b A . ,. , r to test ., ,.° 



the radiation by , , ,. v . Absorption. 



, ... , i J . . absorbability. ^ 



was transmitted, transmission. J 



Cu — Cu (-00067 cm.) 254 



Cu 74 „ 25-3 



Cu — Zn (-00262 cm.) 697 



Zn 97 „ 68-2 



Ag Al 16 Al (-0208 cm.) 13-8 



72 „ 14 



92 „ 13-8 



97 „ 14-2 



Cu 



Fe 



Fe 

 Fe 







Fe (-00315 cm.) 



30-8 



51 



5> 



37 



79-7 



» 



36-1 







Al (-0208 cm.) 



14-5 



51 



„ 



21-7 







Al (-0104 cm.) 



72 



972 



,, 



85-6 



From these and many other instances it became evident 

 that only in those cases of transmission in which the primary 

 radiation was able to stimulate a homogeneous secondary 

 radiation was there any appreciable difference between the 

 penetrating powers of the incident and emergent beams. 

 It remained, however, to show that this change was simply 

 due to the superposition of the homogeneous secondary 



