368 Mr. R. K, McClung on the Relative Amount of 
of the rays produced a variation in the relative amount of 
ionization in air and oxygen, as in the case of hydrogen and 
air. In this case, however, the ionization in oxygen decreased 
relatively to that in air. As the pressure in the bulb was 
lowered, and the rays consequently became harder, it was 
necessary to let more oxygen into the cylinder containing 
oxygen in order to restore the balance. This variation of 
the relative amount of ionization with the change in the 
type of rays was quite marked in the case of these two gases. : 
A set of observations illustrating this is shown in Table III. 
As before the first column gives the length of the alternate 
spark-gap, while the second column gives the pressure in the 
oxygen cylinder necessary to balance the ionization of the 
air at atmospheric pressure in the other cylinder. 
TABLE III. 
Length of Alternate | Pressure in the cylinder 
Spark-gap in mus. : containing Oxygen. 
5:03 | 623°3 mm. 
7:01 | 63672 ,, 
101 | 652'3 _,, 
16°49 656°0_,, 
20°48 663°0 ,, 
As in the case of hydrogen and air, the tendency seems to be 
for the amount of ionization in oxygen and in air to become 
more nearly equal to one another as the rays producing the 
ionization become harder. 
Comparison of the Ionization in Carbon Dioxide and Air. 
Experiments were also made to compare carbon dioxide 
and air ina manner similar to the other gases. The ioniza- 
tion in carbon dioxide being greater than that in air, the 
pressure of the carbon dioxide was lowered so.as to balance 
the ionization in it with that in air at atmospheric pressure. 
For the softer rays the mean value of the ratio prwas found 
to be 1:46, while for the harder rays the value was reduced 
to 1°33. The change took place in the same direction as in 
the case of oxygen and air. 
In the experiments in which the pressure in the Rontgen- 
ray bulb was altered similarly to the experiments with the 
previous gases, an effect was shown similar to that in the case 
