Mr. J. A. Crowther on the. Secondary 

 Table IV. 



Substance. 



Percentage Absorption of 

 Secondary Eadiation. 



From Grases. 



From Solids*. 



Sulphur 



25 

 73 

 65 

 25 

 25 



39 



81-5 



435 



36-8 

 50 



Nickel 



Arsenic 



Tin 



Iodine 





Absorption of Primary ... 



25 



33 



Considering 



* Barkla, Phil. Mag. xi. p. 825 (1906). 



the differences between the two series of 

 experiments, and the difficulties of obtaining comparable 

 measurements in X-ray work, the agreement between the 

 two columns is fairly close. It seems therefore that the 

 character of the secondary radiation given out by an element 

 is independent of its physical state. 



Conclusion. 



The theory of secondary Rontgen radiation has been worked 

 out by Prof. J. J. Thomson*. He supposes that the 

 Rontgen rays consist of electromagnetic pulses radiating out 

 through the sether. When these pulses pass into a medium 

 containing corpuscles, the electric forces in the pulse act upon 

 the charged corpuscles, and if the latter are in any way free 

 to move, set them in motion with a considerable acceleration. 

 Now, on the generally accepted theory of Rontgen radiation, 

 these rays are produced by the acceleration of the motion of 

 the negative corpuscles f. Thus a medium through which 

 Rontgen rays are passing becomes itself a source of secondary 

 Rontgen rays, from the acceleration produced in the cor- 

 puscles in the medium, by the electric forces in the primary 

 pulse. 



Prof. Thomson has further shown that if 



N is the number of corpuscles per unit volume of the 

 substance, 



e is the charge on the corpuscle, and 



m its mass, 



* • Conduction of Electricity through Gases/ p. 321 (1906). 

 t Loc. cit. p. G57. 



