Cases of Ionization by X-Rays. 373 



A similar relationship is found in the case of C 2 H 5 Br 

 for radiations not more penetrating than the secondary 



X-radiation characteristic of Br. |- in Al = 17'4. 



As the radiation is made more penetrating, the relative 

 ionization rises rapidly at first and more slowly afterwards. 

 It probably approaches a constant though much higher value 

 than was obtained with the more absorbable radiations. 



Similar features have been observed in CH 3 I, Sn01 2 , and 

 Se 2 01 2 . 



These variations in relative ionization are similar to the 

 variations in absorption of the ionizing radiation *. For the 

 absorption in a given element is approximately proportional 

 to the absorption in other elements until the radiation 

 becomes of more penetrating type than that characteristic of 

 ihe absorbing element, when it begins to rise rapidly and 

 finally approaches a higher proportionality. 



Similar changes take place in the intensity of the secondary 

 homogeneous radiation f emitted by an element when subject 

 to a primary radiation whose penetrating power is gradually 

 increased, except that primary radiations softer than that 

 characteristic of the element exposed, excite no secondary 

 X-radiation of this particular penetrating power. 



Mr. Sadler % has recently shown that as the penetrating- 

 power of the primary radiation increases, secondary cor- 

 puscular radiation begins to be emitted at the same critical 

 point by the particular element subject to the radiation. 

 The intensity increases rapidly with an increase in penetratino- 

 power at first, then much more slowly. 



ms . ,. £ ionization in C0 2 , ionization in CJLBr 



Ihe variations or -. — v — r . — ■— — — and — : — : : ~ — 2 — 



ionization in air ionization in air 



are shown in fig. 1, in which these ratios are plotted as 



ordinates — (on different scales for convenience) — and the 



absorbability of the radiations I — in Al J as abscissae. It 



will be noticed that owing to crowding of very penetrating- 

 radiations at one side, the curve for C 2 H 5 Br turns upward 

 again. 



As the ionization in air diminishes with increased pene- 

 trating power of the radiation, the variations in ionization 



* Barlria and Sadler, Phil. Ma?. May 1909, pp. 739-700. 

 t Barkla and Sadler, Phil. Mag. Oct. 1908, pp. 550-584 ; Phil. M«e. 

 May 1909. • * ' D 



X Sadler, Phil. Mag. March 1910, pp. 337-35G. 



