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XIII. Polarised Kdntgen Radiation. 



By CHARLES G. BARKLA, D.Sc. (Liverpool), M.Sc. (Viet.), B.A. (Cantab.), King't 

 College, Cambridge ; Oliver Lodge Fellow, University of Liverpool. 



Communicated by Professor J. J. THOMSON, F.R.S. 

 Received January 21, Read February 16, 1905. 



THOUGH many attempts have been made to produce a beam of polarised X-radiation 

 and to detect the polarisation by such methods as are applicable to ordinary light, 

 the experiments have proved unsuccessful, and no evidence of polarity has been 

 obtained. An arrangement of molecules such as occurs in crystals does not appear 

 to affect a beam of this radiation transmitted through the crystalline sultance. 



The experiments here described were suggested by the results of an investigation 

 of secondary radiation proceeding from gases and certain solids subject to X-rays,* 

 for it was found that the gases experimented upon were the source of a radiation 

 differing little in character from the primary radiation which produced it. In some 

 res{>ects the difference was inappreciable, as, for instance, in the absorbability of the 

 radiations by aluminium. The primary and secondary radiations differed slightly, 

 however, in their ionizing powers in air.t The energy of this secondary radiation 

 was found to be proportional to the mass of gas through which the primary beam of 

 definite intensity passed, and to be independent of the nature of the gas. 



This led to the conclusion that this radiation is due to what might l>e called a 

 scattering of the primary X-rays by the electrons constituting the molecules of 

 the gas. 



More recent experiments have shown that from light solids which emit a secondary 

 radiation differing little from the primary, the energy of this radiation obeys the 

 same law. 



The phenomenon of secondary radiation from metals, however, is apparently much 

 more complex, for in addition to secondary X-rays differing enormously in character 

 from the primary, the metal radiator emits negative corpuscles. The total energy of 

 these secondary radiations and the energy of the secondary X-rays alone are subjects 



* C. O. HAUKI.A, 'Phil. Mag.,' pp. 685-698, June, 1903, and pp. 643-560, May, 1904. 

 t See note, 'Phil. Mag.,' p. 549, May, 1904. 

 (384.) 3 O 3 31.5.05 



