TRAVERSED BY CATHODE RAYS. 
63 
to that produced in air at an equal pressure under the action of cathode rays 
entering the chamber with the same intensity in both cases. 
The results obtained from this method for hydrogen, air, and carbon dioxide are 
given in the first column of Table I. In the second column are given the relative 
ionizations found by J. J. Thomson* for these same-gases when ionized by llontgen 
rays of constant intensity. 
Table I. 
Name of gas. 
Column I. 
Column II. 
Ionization by 
cathode rays. 
Ionization by 
Rontgen rays. 
Hydrogen. 
2-65 
•33 
Air. 
1-00 
1-00 
Carbon dioxide . . . 
•34 
1-40 
These numbers, it will be seen, present a very marked difference. In the one case 
the ionization decreased as the density of the gas traversed increased, while in the 
other a law directly the reverse of this was followed. 
One explanation of this difference in the results is that the character of the 
ionization under cathode rays may be essentially different from that produced by 
llontgen rays. Apart from these numbers, however, there seems to be but little 
ground for this view T . Strong experimental evidence now exists to support the 
assumption that the cathode rays consist of small particles of matter carrying nega¬ 
tive charges of electricity. We may therefore regard the ionization they produce as 
being due to their impinging on the molecules of a gas, and to the consequent 
breaking up of the latter. On this hypothesis it is not clear that the resulting ions 
should differ in character from those produced under the influence of llontgen 
radiation. 
It appeared rather that the true explanation was to be found in the varying 
absorbing powers of the different gases. Lenard,| who studied these rays by the 
fluorescence they excited, found that the absorption of cathode rays by gases at 
atmospheric pressure was considerable. He was also led by his experiments to 
propound the law, that while different gases at the same pressure absorbed the rays 
to different degrees, yet their absorption depended only upon the densities of the 
gases, and not upon their chemical composition. 
In the apparatus here used, the distance traversed by the rays after they left the 
discharge tube until they reached the centre of the field where the ionization was 
* ‘Proc. Camb. Phil. Soc.,’ vol. 10, Part I., p. 12. 
t 1 Wied. Ann.,’ vol. 56, p. 255 (1895). 
