^68 Dr Barkla, Phenomena of X-Ilai/ Tra^is^^mission. 



Lastly, preliminary experiments on the secondary radiation from 

 alloys appears to indicate that the bombardment of a second 

 substance by the ejected electrons produces no change in the 

 character of the secondary radiation produced. The most difficult 

 point to explain is the fact that the primary radiation must be 

 just more penetrating than the homogeneous secondary radiation 

 characteristic of the exposed substance for that radiation to be 

 excited. This law appears to be an extension of Stokes' Law of 

 Fluorescence to which the phenomenon of the secondary chai'acter- 

 istic radiation is akin. 



The exactness with which the law is obeyed, and the striking 

 similarity between the various absorption curves indicates a de- 

 fiuiteness in the structure of the pulses, and the absence of 

 anything corresponding to variation in pulse structure. 



As absorption of energy must precede the expulsion of electrons, 

 we might expect to find the intensity of corpuscular radiation 

 connected with the absorbability of the primary radiation in the 

 same way as is the X-radiation, except that the velocity would 

 vary A^ith the penetrating power of the primary beam, as has 

 been found by Innes. 



A detailed account of these experiments and a fuller discussion 

 will be published later. 



LrvERPooii. 



