236 Mr Kaye, The Selective Absorption of Rontgen Rays. 
The Selective Absorption of Rontgen Rays. By G. W. C. Kaye, 
B.Sc., A.R.C.Sc. (Lond.), Trinity College, Cambridge. (Com- 
municated by Prof. J. J. Thomson, F.R.S.) 
\Read 6 May 1907.] 
M c Clelland* has shown that there is a close connection between 
the secondary radiation given out by a substance exposed to the 
/3 and 7 rays from radium, and the atomic weight of the substance. 
More recently Prof. J. J. Thomson f has obtained a similar rela- 
tion between secondary Rontgen radiation and atomic weight. 
With the exception of nickel an increase in atomic weight was 
always accompanied by an increase in the amount of secondary 
ionization. 
In view of these interesting results, an attempt was made to 
derive a relation between primary Rontgen radiation and the 
atomic weight of the substance producing the rays. A special 
form of Rontgen bulb was employed, and the quantity of rays 
emitted was measured by an ionization method. 
There were one or two things to be considered. Firstly, some 
of the substances it was proposed to use, have comparatively low 
melting points, and therefore it was not advisable to focus the 
cathode rays on the anticathode by using a concave cathode. A 
plane cathode was accordingly employed. Secondly, this being so, 
only a central pencil of the cathode rays should be used, so as to 
restrict them to the anticathode area. 
It follows that the quantity of Rontgen rays produced, would 
never be very large, and might in unfavourable circumstances be 
small. So it was advisable to keep down the capacity of the 
ionization chamber system, and also to allow the emergent 
Rontgen rays to pass out of the bulb through as thin an 
aluminium window as would withstand the atmospheric pressure 
from without. Such a window would of course stop all the 
secondary cathode particles which are produced along with the 
Rontgen pulses when the primary cathode particles are arrested 
by the anticathode. 
* McClelland, Sci. Trans. Roy. Dublin Soc., 1905 and 1906. 
t J. J. Thomson, Proc. Gamb. Phil. Soc. xiv. 1, p. 109, Nov. 1906. 
