Scattering of X-rays and Atomic Structure. 



223 



light elements * ; Crowfcher f later found something a little 

 less than twice as much. This higher value simply meant 

 that the number of electrons in the hydrogen atom was equal 

 to the atomic weight. The anomalous behaviour of hydrogen 

 is thus strong evidence of the applicability of the theor} r . 



Measurements of the scattering from elements of higher 

 atomic weight than sulphur, however, led to a different 

 experimental conclusion ; for copper was found to scatter 

 about twice as much and silver about six times as much as 

 an equal mass of one of the light elements J. This result 

 was in general confirmed by the later experiments of 

 Crow^ther §. In Table I. are given the relative values of the 

 energy scattered from equal masses of eleven elements. 



Table I. 



Showing relative intensities of the X-radiation scattered 



from equal masses of various elements. 



Element. 



H 



Pie 



0.. 



N 



O 



Al 



S... 



IS T i 



Cu 



H 



Sn 



Atomic Weight. 



Eelative energy 

 scattered. 

 (B arid a.) 



Eelative energy 



scattered. 



(Crowther.) 



1 



4 

 12 

 14 



16 



27 



32 



59 



63-5 

 108 

 118 



2-4 ? 



1-0 

 10 

 1-0 



i'-b 

 2-bfl 



6-5|| 



17 



11 



10 



10 



1-0 



1-05 



1-4 



3-5 



3-5 



5 V 5 



|| Barkla & Sadler. 



If we applied the theory unmodified to each of these 

 results, we should conclude that 



(1) A hydrogen atom possesses one scattering electron ; 



(2) An atom of helium, carbon, nitrogen, oxygen, 



aluminium, or sulphur possesses w/2 electrons 

 approximately ; 



(3) An atom of nickel, copper, silver, or tin possesses wto 



3io electrons ; 



where w is the atomic weight of the element. 



* It was thought at the time that the discrepancy was due to impurity 

 of the hydrogen. Crowther's determination indicated that this was not 

 the case. 



t Phil. Mag. Nov. 1907. 



X Bavkla & Sadler, Proc. Lond. Phvs. Soc. June 1908 ; and Phil. Mao-. 

 Oct. 1908. § Proc. Camb. Phil. Soc. May 1911. ° 



