654 



Mr. J. A. Crowther on the Secondary 



His conclusions may be briefly summarized as follows : — 

 (i.) All gases, when subjected to X-rays, are a source of 



secondary radiation, 

 (ii.) The absorbability of the secondary radiation is, within 



the limits of experimental error, the same as that of 



the primary producing it. 

 (iii.) For a given primary radiation, the intensity of the 



secondary radiation is proportional to the density of 



the gas from which it proceeds, 

 (iv.) The ratio of the intensities of the primary and 



secondary beam is independent of the hardness and 



intensity of the primary rays. 

 In support of the third conclusion he gives the following 

 table : — 





Relative Intensity 

 of Secondary Radiation. 



Relative Density 

 of G-as. 



Air 



1-0 



0-17 

 1-07 

 1-45 

 2-11 



1-0 



0-17 

 M8 

 1-53 

 2-19 



H, 



H 2 S 



co„ 



S0 2 





Barkla's experiments were carefully performed, and, as far 

 as relates to the five gases upon which he experimented, there 

 can be no doubt that his conclusions are substantially correct. 

 As will appear later, they have, in the main, been confirmed 

 by experiments made on the same substances during the 

 course of the present work. 



Barkla's experiments, however, included only a very few 

 gases, and these not of a very varied type. There were no 

 gases of very great density, no gases with a very complex 

 molecule, and no gases containing elements of high, or even 

 moderately high atomic weight. Having regard to the 

 importance of the subject, it seemed desirable that more 

 accurate experiments involving a far greater number and 

 variety of substances should be made before drawing any 

 general or final conclusions. 



Experimental. 

 Until some means shall have been devised for maintaining 

 a constant stream of X-rays of a definite character, accurate 

 measurements on X-radiation will only be possible by com- 

 parative or compensation methods. Barkla measured his 



