66 
mr. j. c. Mclennan on electrical conductivity in gases 
chamber, the electrometer showed no leak until the critical pressure was reached, 
when it immediately began to charge up. 
10. Ionization in a Gas independent of its Chemical Composition. 
An important result in connection with these experiments is the agreement 
exhibited between the number given in Table I. for the ionization in hydrogen at 
atmospheric pressure and that given in Table II. for the ionization in air at a 
pressure of 53 millims. 
Here two gases, hydrogen and air, were introduced in succession into the same 
measuring chamber and adjusted to the same density. Cathode rays of the same 
intensity were projected into this chamber in the two cases, and these rays, after 
traversing a certain length of the gas, reached a point where the ionization they 
there produced was measured. The values obtained show that under the circum¬ 
stances the same number of ions was produced in both gases. 
Since the rays issuing from the window were in both cases of the same intensity, it 
follows from Lenard’s absorption law that the disposition of the rays, their actual 
intensities, and the quantities of them absorbed from point to point in the chamber, 
were precisely the same in both gases. Under these circumstances, therefore, the 
equal ionizations obtained in hydrogen and in air at the same density not only form 
a confirmation of Lenard’s absorption law, but also show that where equal absorption 
occurs equal ionization is produced. 
In the case of Rontgen radiation, R,utherford # has made a determination of the 
relative absorbing powers of a number of gases. Taking I to denote the intensity 
of the rays on entering a particular gas, and Ie~ u ' their intensity after traversing a 
length x, he has found that the values of the coefficient of absorption for the different 
gases practically represented the relative conductivities produced in these same gases 
by Rontgen rays. It is thus interesting to note that with cathode rays, just as with 
Rontgen rays, equal absorption gives equal ionization. 
To test still further the accuracy of this conclusion a detailed examination was 
made of the ionization produced in a number of different gases. Throughout the 
experiments air in the chamber B, fig. 5, was taken as the standard. In some 
comparisons this air was kept at atmospheric pressure, while in others lower 
pressures were taken, the pressure selected being maintained through each complete 
determination. In making a comparison the chamber A was filled in turn with the two 
gases to be examined, and their pressures were adjusted so as to reduce them to the 
same density. Two ratios were in this way found for the ionizations in the chambers 
A and B, and as the influence of absorption was eliminated on account of the equal 
densities, these ratios represented the relative ionizations in the two gases under 
cathode rays of the same intensity. 
* ‘ Phil. Mag.,’ April, 1897, p. 254. 
