124 MR. G. W. C. KAYE ON THE 



elements, when used as anticathodes in a discharge tube, might be repaid by the 

 discovery of some sort of relation between their atomic weights and the quantity and 

 quality of the Rontgen rays given out and transmitted under various conditions. 



Previous Work. 



It is, of course, known that the heavier metals, or rather, those of high atomic 

 weight, make the most efficient anticathodes. RONTGEN* found that the rays from 

 platinum are more intense than those from aluminium. CAMPBELL SWINTON! came 

 to the conclusion that different metals emit Rontgen rays of the same penetrating 

 power and in quantities which depend, but not very much, on the atomic weight. 

 KAUFMANN | also showed a rough relation between atomic weight and intensity of 

 radiation, and endeavoured without success to find a special connection between the 

 rays from a metal and their penetrating power for screens of that metal. R6lTl's 

 work should also receive mention. All these observers used a photographic or 

 fluoroscopic method of measuring intensities, and to their results can only be attached 

 the accuracy which such methods permit. An ionisation method offers obvious 

 advantages, and was naturally adopted in the present research. 



Among the work on the transmission of Rontgen rays should be mentioned a paper 

 by BENOIST and HUBMUZESCU,|| one by BKNOIST^ in addition to the one referred to 

 above, and one by WALTKR.* : ADAMS, ft during the course of the present work, has 

 worked on selective absorption, and TlA(4A,JJ during a research on the polarisation of 

 secondary Rontgen rays, noticed selective absorption in the case of carbon and ebonite. 

 An account of the earlier form of apparatus used by the writer, together with some 

 preliminary results on " Selective Absorption," was given in May last year to the 

 Cambridge Philosophical Society, 



The central portion of the beam of cathode rays passed down the metal tube T 

 (fig. I), and was incident on the anticathode at about 45. A pencil of the Rontgen 

 rays produced passed along the tube B, and out through the thin aluminium window 

 W, into the ionisation chamber C. Both cathode and anode were of aluminium. 



* RONTGEN, Wiirzburg, Stahel'scher Verlag, Miirz, 1890. 

 t SWINTON, ' Proc. Roy. Soc.,' LXI., p. 222 (1897). 

 I KAUFMANN, ' Ver. Phy. Ges. Berlin,' XVI., p. 116 (1897). 

 E6m, 'Roma R. Accad. Lincei Rendic.,' VI., 2, p. 123 (1897). 

 || BENOIST and HURMUZESCU, 'Compt. Rend.,' Fey. 17, 1896. 

 U BENOIST, 'Compt. Rend.,' Jan. 18, 1897. 

 ** WALTER, 'Ann. der Phys.,' XVII., p. 561 (1905). 



tt J. M. ADAMS, 'Amer. Acad. Arts and Sciences,' XLII., p. 671 (1907); 'Phil. Mag.,' XIII., p. 576 

 (1907); 'Phys. Rev.,' XXVL, p. 202 (1908). 



It HAGA, 'Ann. der Phys.,' XXIII., p. 445 (1907). 



KAYE, ' Proc. Camb. Phil. Soc.,' XIV., p. 236, May, 1907. 



