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XXX. Note on the Scattering of X-ray s and Atomic Structure. 

 By C. Gr. Baekla, F.R.S., Professor of Natural Philosophy i 

 and Janette G. Dunlop, M.A., B.Sc, Carnegie Research 

 Scholar, University of Edinburgh * '. 



^HE results of experiments made by one of the writers 

 on the radiation proceeding from substances containing 

 only light elements when these were exposed to Rontgen 

 radiation led to the conclusion that this radiation, as observed 

 under ordinary experimental conditions, was almost purely a 

 scattered X-radiation and that the scattering particles were 

 not the ions or atoms, but the constituent electrons f . It 

 was shown that the intensity of the scattered radiation over 

 a wide range of penetrating powers was approximately 

 independent of the penetrating power of the radiation 

 scattered ; and, with the possible exception of hydrogen 

 among the light elements, the intensity was proportional 

 merely to the quantity J and independent of the quality of 

 matter traversed by the primary radiation. 



From measurements of the intensity of Rontgen radiation 

 scattered from light elements, applied to the theory as given 

 by Sir J. J. Thomson, the number of electrons in the atom 

 was determined. With the data for e/m and e for an electron 

 at that time available, it appeared to be several times the 

 atomic weight of the element considered. Applying the 

 more recently determined values of e/m, e, and n — the number 

 of molecules per c.c. of gas — to the same measurement, the 

 number of electrons was later § found to be almost exactly 

 half the atomic weight, for elements of atomic weight 

 greater than that of hydrogen and not greater than that of 

 sulphur || . This determination accounted for the previously 

 observed anomalous behaviour of hydrogen, for only if 

 hydrogen had possessed one electron for each pair of atoms 

 could it have scattered only as much mass for mass as any 

 other light element. Hydrogen had been found to scatter a 

 little more than twice as much, mass for mass, as the other 



* Communicated by the Authors. 



t Barkla, Phil. Mag. June 1903, May 1904, and later papers. 



j Measured by mass. 



§ Barkla, Phil. Mag-. May 1911. 



j| Sir E. Rutherford about the same time (Phil. Mag. May 1911) 

 arrived at the complementary conclusion : that the central (positive) 

 charge of an atom is about half the atomic weight times the charge of 

 an electron. 



