Energy of Scattered IL-radiation . G51 



fraction of the whole absorption. Now the quantity x which 

 the writer has found from the equation 





x 

 ~r = constant 



(-) - 



[suffixes denoting the absorbing substance] varies in different 

 cases from '16 to "25. 



It is very probable that with care the absorption coefficients 

 will be determined with greater accuracy, and consequently 

 x brought within narrower limits. The results are, however, 

 sufficiently accurate to give the order of magnitude. This 

 agrees very well with the value directly determined by the 

 writer. It should also be pointed out that a value for x 



I i.e. -I as high as 1*18 would involve complete violation in 



place of otherwise close agreement with simple laws of 

 absorption. 



(5) The penetrating power of the fluorescent radiations 

 from various elements shows no tendency with increasing 

 atomic weight of the radiator to approximate to the limit 



suggested by a scattering coefficient ( -) as large as 1*18. 



On the other hand, there is such an approximation in the 



absorption in carbon to an inferior limit for ( — J equal to 

 something of the order of *2. \P ' 



There thus appears abundant, consistent, and apparently 

 conclusive evidence that the intensity of Rontgen radiation 

 scattered by light elements is of the order of magnitude 

 found directly by the writer in early experiments, and that 

 Mr. Crowther's result is several times too great. 



The theory of scattering as given by Sir J. J. Thomson 

 leads to the conclusion that the number of scattering electrons 

 per atom is about half the atomic weight in the case of light 

 atoms*. 



* This applies to atomic weights not greater than 82, with the possible 

 exception of hydrogen. Accurate results have not been obtained lor the 

 intensity of radiation scattered from heavier elements owing to the 

 difficulty in many cases of getting rid of the fluorescent X -radiations 

 superposed on the scattered radiation. Barkla & Sadler estimated the 

 intensity of the radiation scattered from silver to be about 6 times that 

 from an equal mass of the light elements. There is also indirect, but 

 by no means conclusive evidence that still heavier atoms scatter to a 

 greater extent. Measurements might easily bo made in a number of 

 cases with a fair degree of accuracy. The subject is worth further 

 investigation. 



2 U 2 



