•404 Prof. C. G. Barkla on the Spectra of 



penetrating power of the primary is accompanied by a special 

 absorption connected with the emission of fluorescent radia- 

 tion. The emission of fluorescent radiation implies the special 

 absorption of the primary radiation, and vice versa. The 

 absorption curves published by Barkla and Sadler *, drawn 

 from the results of experiments on absorption by metal 

 sheets, thus indirectly exhibit the accuracy of this law of 

 fluorescence. Similar curves have been got by Barkla and 

 (Jollier from experiments on absorption by gases and vapours, 

 and with fewer observations near the critical point in the 

 case of absorption by gold and platinum, so that the generality 

 of the law has been well tested. 



Similar relations hold between the ionization in R when 

 in the gaseous state and the ionization in another gas or 

 vapour whose fluorescent radiation is not near the range of 

 penetrating power experimented upon on its more absorbable 

 side. The writer showed this in a paper on " Phenomena of 

 X-ray Transmission " f, and later published the relation in 

 detail in the case of ionization in ethyl bromide. A similar 

 relation holds in the case of every gas upon which experiments 

 have been made. 



Now the definiteness of the turning point in these absorp- 

 tion and ionization curves, and the coincidence of this 

 penetrating power with that of the fluorescent radiation 

 characteristic of the absorbing and ionized substance is further 

 evidence not only of the homogeneity of the fluorescent 

 radiations, but of the accuracy of the law of X-ray fluorescence 

 analogous to Stokes's law. 



Unlike the scattered radiation the fluorescent radiation is 

 uniformly distributed around a polarized beam of X-rays +. 

 It is also uniformly distributed in a plane containing the 

 direction of propagation of the primary radiation §. Thus 

 the intensity is entirely independent of the direction of electric 

 force or of propagation of the exciting primary beam. 



The total energy of the secondary fluorescent radiation is in 

 some cases certainly very great. The variation of intensity 

 with the penetrating power of the primary is shown by the 

 curve in fig. 2 ||, and at its maximum the copper radiation 

 carries away — if we measure energy by total ionizing power 



* Phil. Mag. May 1909. 



t Proc. Camb. Phil. Soc. May 1909. 



% Phil. Trans. A. vol. ociv. 1905, pp. 467-479 ; Proc. Roy. Soc. A. 

 vol. lxxvii. 1906, pp. 247-255. 



§ Phil. Mag. Feb. 1911. 



|| Fig. 2 was given by Barkla and Sadler in a paper on " Absorption 

 of Rontgen Rays " (Phil. Mag. May 1909). 



