Absorjrtion and Scattering of X-Rays. 211 



lost by scattering, gives with remarkable precision the number 

 of electrons per atom now accepted. This method was in 

 fact the first to lead to the present conclusion*. All these 

 results verify not only the general laws of scattering which 

 one of us arrived at many years ago, but they indicate also 

 the accuracy of the energy measurement made at that time. 



It will, however, be noticed that the coefficient of absorption 

 in aluminium sinks quite appreciably below 2 to a value *15, 

 and that there is a tendency to lower values still. Equally 

 low values have been obtained by Rutherford, Richardson, 

 and Barnes |, and by Hull and Rice J in experiments with 

 the Coolidge tube. 



In addition the values of the coefficients of absorption of 

 7 rays obtained by Soddy and Russell, Ishino §, and others 

 indicate that the simple law of scattering (approximate inde- 

 pendence of wave-length) must break down for very short 

 waves, for values of fijp as low as '04 have been obtained. 



We have already shown || that it does not hold for much 

 longer waves, and that this is to be expected theoretically; 

 that although in light elements the scattering varies 

 little with wave- length, yet there is a slight increase with 

 an increase in wave-length of the radiation ; that this 

 increase becomes decidedly marked when the wave-length 

 becomes comparable with the size of an atom ; that in heavy 

 atoms the variation is most pronounced, the scattering in- 

 creasing many fold ; but that with shorter waves the mass 

 scattering even in the heavier elements approaches equality 

 with that in the light. 



In view of the facts stated above, it certainly looks as 

 though we have in the absorption by aluminium, evidence 

 of the beginning of a second marked deviation from the 

 simple laws — that is, evidence that a diminution of scattering 

 and absorption is setting in when the wave-length becomes 

 small. This appears to be the link connecting the absorption 

 of X-rays with the absorption of <y rays. In this connexion 

 it may be observed that the absorption of X-rays in aluminium 

 appears to cross that in paraffin, paper, water, &c, and to 

 reach a lower value. This we should expect it to do as the 



* As the values of e/m and e were obtained more accurately, re- 

 determinations of the number of electrons per atom were made. See 

 Barkla, Phil. Mao-. May 1904 ; Jahrbuch der Radioaktivitat and Elektronik, 

 April 1908; Phil. Mag. Mav 1911. 



t Phil. Mag. Sept. 1915. 



j Phys. Review, Sept. 1916. 



§ Phil. Mag. Jan. 1917. 



|| Barkla and Dunlop, Phil. Mag. March 1916. 



