TRAVERSED BY CATHODE RAYS. 
73 
to mechanical difficulties, however, the former method is generally impracticable, 
while the action of the applied electric field in breaking down the insulation of the 
gas precludes the use of the latter artifice. 
It is then open to measure the ionizations produced by rays traversing layers of 
gas of considerable thickness. But before any relation connecting ionizations and 
pressures can be deduced from such measurements, it is necessary to have definite 
information regarding the absorptive powers of the gases at different pressures, and 
to know exactly the form and dimensions of the region from which the ions are 
drawn. 
Although the absorption laws for cathode rays have been fully developed by 
Lenard, and are quite definite and clear, it is scarcely possible to define even 
approximately the region in the ionizing chambers (fig. 5) from which the ions go to 
make up the saturation current. 
On this account a direct verification of the proportionality law is not possible ; 
but, as already pointed out, the results of the experiments described in Section X. 
strongly support the conclusion that, in the case of a gas subjected to increasing- 
pressure, the ionizations produced by rays of constant intensity bear the same ratio 
to each other as the coefficients of absorption corresponding to these pressures. 
If, then, the ionization in a gas varies with the pressure, it follows at once that if 
rays of the same intensity were allowed to traverse thin layers of different gases at a 
constant pressure, the ionizations produced would be directly proportional to the 
densities of these gases. 
Take, for example, carbon dioxide and air. It has been shown that the ionization 
produced in carbon dioxide at a pressure of 504'7 millims. of mercury is the same 
as that produced in air at 7 72 '7 millims. by rays of the same intensity. 
According to the proportion law the ionization produced by these same rays in 
C0 2 at 772'7 would then be just 1*53 times that obtained at the lower pressure ; 
that is, with rays of the same intensity the ionizations in carbon dioxide and in air 
would be to each other as 1-53 to 1 when these gases were subjected to the same 
pressure. 
A similar conclusion may be deduced from a consideration of the other gases 
examined. Hence, on this view, the relative ionizations produced by rays of 
constant intensity in a series of gases subjected to the same pressure would be 
expressed by the numbers which under these circumstances give their relative 
densities. 
These numbers are given for the gases examined in Column I., Table X., while in 
Column II. are given the values found by J. J. Thomson # for the relative ionizations 
produced by Rontgen rays of constant intensity in the same gases. 
* ‘Proc, Camb. Phil. Soc.,’ vol, 10, Part I., p. 12. 
VOL. CXCV. — A. 
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