Secondary Rontgen Radiation from Gases and Vapours. 653 



would, therefore, not seriously affect the applicability of 

 Oroya's method. 



Quite otherwise were the results obtained when comparing 

 a mercury-vapour lamp against a carbon -filament glow-lamp. 

 The luminosity of this lamp was concentrated in three isolated 

 bright lines in the yellow, green, and violet respectively, 

 practically no red rays being present. 



It was found that the depth of solution utilized in the 

 experiments described above not only failed to very notice- 

 ably improve the colour difference between the sources but 

 also seriously affected the photometric results obtained. 



The use of yellow screens in order to facilitate the photo- 

 metric comparison of heterochromatic sources of light is 

 certainly only justifiable in the case of illuminants yielding 

 continuous spectra. 



Even in this case it is essential that the yellow screen 

 should exhibit absorbing properties similar to those of Crova's 

 solution, namely minimum absorption in the neighbourhood 

 of \ = 0*58/a, and a uniformly increasing absorption on either 

 side of this point. 



LXIV. On the Secondary Rontgen Radiation from Gases and 

 Vapours. By J. A. Crowther, B.A., late Scholar. 

 St. Johns College, Cambridge ; Open Research Student, 

 Emmanuel College, Cambridge*. 



Introduction. 



THAT gases through which Rontgen rays are passing give 

 off some secondary rays was first discovered by Rontgen 

 himself |, who noticed that a photographic plate, placed near 

 a beam of X-rays, was gradually affected even though shielded 

 from the direct action of the primary rays. Sagnac % also, 

 during his researches on the secondary radiation from 

 metals, observed a similar effect. The subject has been more 

 thoroughly investigated by Barkla §, who studied very ex- 

 haustively the secondary Rontgen radiation from air, and 

 also made some experiments on four other gases, hydrogen, 

 carbon dioxide, sulphur dioxide, and hydrogen sulphide. 



* Communicated bv Prof. J.J. Thomson. 



t Rontgen, Ann. Phys. Chem. Ixiv. pp. 18-37 (1898). 



% Sagnac, Comptes Rendus, cxxvi. pp. 521-523 (1898). 



§ Barkla, Phil. Mag. y. pp. 685-698 (1903) ; vii. p. 543 (1904). 



Phil. Mag. S. 6. Vol. 14. No. 83. JSfov. 1907. 2 X 



