746 Dr. C. G. Barkk and Mr.0. A. Sadler on 



But the ionizations produced by these beams are not pro- 

 portional to their energies, as they have different penetrating 

 powers. I£ i 2 and i 2 represent the relative numbers of ions 

 produced by the two beams, if of equal intensity and cross- 

 section, in a thin layer of air, 



,i Secondary Ionization _ mk z 2 |~ ~0 2 — K)*"] 



Primary Ionization — l7r(\ 2 — ^1) h L J 



All possible cases can be divided into three classes : — 



(1) When a homogeneous primary radiation passes through 

 a plate of an element whose characteristic radiation is of 

 equal or greater penetrating power, no appreciable secondary 

 radiation of this type is emitted, i. e. & = 0. The secondary 

 radiation characteristic of the absorbing element is not then 

 present in the transmitted radiation, and this exhibits no 

 special powers of penetration. 



This type of transmission occurs when the radiation from 

 Cr passes through C, Mg, AI, Fe, Ni, Ou, &c, or when Gu 

 radiation passes through Cu, Zn, Ag, &c. 



(2) When a homogeneous primary radiation passes through 

 a plate of an element whose characteristic radiation is of more 

 absorbable type and more absorbable in the plate itself than 

 is the primary radiation, then the secondary radiation is 

 excited, k is finite and \ 2 — A^ i g positive. 



In this case, as the thickness of the absorbing plate is 

 increased, the beam received by the electroscope gradually 

 approaches a final constitution in which the ratio of the 

 energies of secondary and primary radiations is 



to 



4?r (\ s — XJ 



Such conditions are obtained when a radiation mucli more 

 penetrating than the radiation characteristic of an element 

 passes through that element, as when radiation such as is 

 emitted by Ag is transmitted through Fe, Ni.Cu, Zn, &c. 



(3) When a homogeneous radiation passes through an 

 element whose characteristic radiation is of more absorbable 

 type, but more penetrating to the element emitting it, k is 

 again finite and (X 2 — ^i) is negative. 



In this case, as the thickness of the absorbing plate is 

 increased, the ratio of the energy of secondary radiation to 

 that of the primary radiation increases indefinitely ; that is, 

 the primary radiation becomes ultimately transformed into 

 radiation characteristic of the absorbing substance. 



