Absorption and Scattering of X-Rays. 271 



a radiation was looked for and found, as already briefly 

 announced. The evidence obtained from absorption expe- 

 riments is given below*. 



But in addition, there are in these results a number of 

 other features which are of considerable theoretical interest, 

 and to which attention should be drawn. 



Experiments. 



Of the experiments little need be said. The precautions 

 essential to the accurate determination of an absorption 

 coefficient have frequently been stated f . The principal 

 danger is that of allowing secondary rays of any type from 

 the absorbing substance to enter the ionization chamber in 

 which the intensity of transmitted radiation is measured. 



The absorption coefficient //, of a homogeneous radiation is 

 denned by the equation L = I e~^ x , where I is the intensity of 

 the primary radiation of initial intensity I still proceeding 

 as primary radiation after traversing a thickness x of absorbing 

 material {. 



In these experiments the radiation employed was almost 

 invariably a primary radiation from which the more easily 

 absorbed constituents had been previously eliminated. In the 

 case of the most penetrating radiations, the remaining radiation 

 upon which the experiments were made was remarkably homo- 

 geneous. In order to reduce any error due to heterogeneity 

 to a minimum, the thickness of the absorbing sheet was 

 adjusted to absorb approximately the same fraction (50 per 

 cent.) of the incident radiation. 



A few experiments were also made with characteristic 

 radiations (K series) as primary radiations. The results 

 obtained agreed almost perfectly with those obtained with 

 the primary radiations direct from an X-ray tube §. 



* Indications of the existence of such a radiation had been obtained 

 by one of us many years before. Some of these were recorded; others 

 not. But the evidence was insufficient to show either that the radiation 

 emitted was really a characteristic radiation or that it could be in any 

 way identified or classified. Indeed the probable significance of these 

 results had been lost sight of until associated irregularities became a 

 serious hindrance to the development of the theory. It seems highly 

 probable that these J radiations account in part for some of the discre- 

 pancies between the results obtained by some experimenters and those 

 of the writer on the scattered radiation. By adjustment of the primary 

 radiation care had been taken b) r the writer to keep the scattered radiation 

 free from admixture with such a characteristic radiation. 



t Barkla and Sadler, Phil. Mag. May 1909. 



+ /x is a measure of the rate of loss of intensity of the transmitted 

 beam, whether by scattering or any other process. 



§ Except at a " discontinuity " where absorptions are exceedingly 

 sensitive to small variations of wave-length. In this case the discrepancy 

 was a regular one. 



U2 



