720 Dr. J, A. Crowther on "J" Radiation. 



The large discrepancies between the values obtained by 

 different observers for the coefficient of scattering from 

 these substances, discrepancies which are considerably 

 greater than any possible errors of experiment, would 

 suggest that the radiation measured was not all pure 

 scattered radiation, but that it was accompanied in some 

 cases by very considerable amounts of X radiations of a 

 different origin. A value for the mass coefficient of scattering 

 for aluminium as high as 1*18 was obtained by the author 

 (Proc. Roy. Soc. A, Ixxxv. 1911), while later experiments* 

 by a different method indicated a value of about 0"27 for 

 a radiator of filter-paper and 028 for aluminium. Barkla 

 (Phil. Mag. May 1911) obtained a value 0*20 for a carbon 

 radiator, a result which is in good agreement with the theory 

 of scattering due to Sir J. J. Thomson, assuming that the 

 number of electrons in the atom of any element is equal to 

 its atomic number. Still lower values have been obtained 

 by other observers, 



The distribution of the " scattered v radiation around the 

 radiator showed similar variations. It appeared to vary 

 considerably with the quality of the primary radiation 

 employed to excite it (J. A. Crowther, Proc. Hoy. Soc. A, 

 lxxxvi. 1912, p. 478), and also in some cases with the thickness 

 of the radiator (J. A. Crowther, Proc. Camb. Phil. Soc. xvi. 

 1911, p. 365). These results are most easily explicable on 

 the assumption that the radiations dealt with are not all o£ 

 the same type. 



Other evidence of the existence of a hard fluorescent 

 radiation is available. Barkla & White (Phil. Mag. (6) 

 xxxiv. 1917, p. 270) found evidence of the existence of 

 absorption bands in the case of some of the elements of low 

 atomic weight, the wave-lengths at which these bands 

 occurred being 0'42 x 10" 3 cm. for carbon, and 0'37 X 10~ 8 cm. 

 for aluminium. These bands, presumably, correspond to 

 possible characteristic radiations of those elements of corre- 

 sponding wave-lengths, and hence of a much more penetrating 

 type than the K radiations. For these Barkla proposed the 

 name " J " radiations. 



The existence of this ' 'J " radiation does not appear to 

 have been generally admitted, partly because such radiation 

 had not been directly detected, and partly because radiation 

 of this type is not indicated by the current theory of the 

 structure of the atom. The latter objection, though probably 

 the more influential of the two in producing this suspension 



* J. A. Crowtlier, Proc. Roy. Soc. A, lxxxvi. 1912, p. 478. 



