70 
me. j. c. Mclennan on electeical conductivity in gases 
Table VIII.—Summary of Measurements. 
Gases compared. 
Pressures. 
Ionizations. 
millims. 
Air. 
. mean of 30 readings 
746-7 
1-31 
Oxygen. . . 
,, 30 ,, 
675-1 
1-32 
Air. 
. mean of 25 readings 
734-3 
1-11 
Nitrogen . . . 
25 
• 5) 
757 
1-09 
Air. 
. mean of 30 readings 
772-7 
1-20 
Carbon dioxide . 
,, 30 ,, 
505-4 
1-18 
Air. 
. mean of 18 readings 
53-2 
1-70 
Hydrogen . . . 
„ 18 „ 
770-9 
T79 
Air. 
. mean of 23 readings 
759 
1-09 
Nitrous oxide 
23 
. ,, -jO ,, 
499-3 
1-10 
A summary of complete sets of observations on the different gases is given in 
Table VIII. This statement includes the number of readings made in each case and 
the pressures at which these were taken. The ionizations quoted are the averages of 
the several sets of readings. 
The close agreement exhibited by the numbers corresponding to each comparison 
fully bears out the conclusion deduced from the earlier experiments. It not only 
forms a striking corroboration of Lenard’s absorption law, hut also shows that the 
ionization follows an analogous one, which may he stated thus :—When cathode rays 
of a given strength pass through a gas, the number of ions produced per second in 
1 cub. centim. depends only upon the density of the gas, and is independent of its 
chemical composition. 
The similarity in the laws of absorption and ionization, holding, as it does, with so 
many gases over such a wide range of pressures, is a clear indication that when 
cathode rays are absorbed to a certain extent, the positive and negative ions produced 
by these absorbed rays are of a definite amount, which bears a constant ratio to the 
quantity of the rays absorbed; that is to say, the absorption of a definite amount of 
radiant energy is always accompanied by the appearance of a fixed amount of potential 
energy in the form of free ions. 
This granted, it follows that in order to ascertain the relative ionizations pro¬ 
duced in any two gases by cathode rays of the same intensity, it is sufficient to 
determine the absorbing powers of the two gases for the same rays. In other words, 
