Tonization produced in Gases by Réntgen Rays. 367 
rays has a very marked effect on the relative ionization in 
these gases. 
Later on some absolute determinations were made for these 
two gases, using much harder rays than those employed in 
the earlier experiments. Bulb No. 3 was used, and before 
making the determinations it was run for a long time, using 
a mercury interrupter so that it might give out very hard 
rays. When making the experiments, a sheet of lead about 
0-3 mm. in thickness was placed between the bulb and the 
cylinders to cut off all the soft rays that might be given out 
and only allow the hard rays to enter the cylinders and pro- 
duce the ionization. When these hard rays were used, the 
relative value for the ionization in air and hydrogen was quite 
different from that obtained for the softer rays, as was to be 
expected from the previous results. The value of the ratio 
/ 
= was considerably increased. The mean value of three 
experiments which were in fair agreement was 0°177, which 
is much higher than any of the values obtained for the 
softer rays. This is quite in agreement with the results 
obtained from the experiments in which the hardness of the 
rays was varied by directly altering the pressure in the bulb. 
Comparison of the Ionization in Oxygen and Air. 
Other gases besides hydrogen were also investigated to 
ascertain whether this peculiar behaviour observed in hydrogen 
and air was common to these other gases as well. Among 
other gases oxygen was examined. The experiments were 
performed in an exactly similar manner to those with 
hydrogen, the ionization in oxygen being in every instance 
compared with that in air. The ionization in oxygen of 
course is higher than that in air, and in this case the pressure 
of the oxygen had to be reduced in order to obtain a balance. 
When the comparatively soft rays given out by bulb No. 1 
were used, fairly steady results were obtained as in the case 
of hydrogen. The mean value for the ratio ae for these 
rays was 1°30. When, however, the hard rays given out by bulb 
No. 3 were used, the value of this ratio was reduced to 1°17. 
This lower value is about the same as those obtained by 
previous observers, but the value for the softer rays is some- 
what larger. 
Hxperiments were also made with oxygen similar to those 
made with hydrogen to test the effect of varying the pressure 
in the bulb. It was found that an alteration in the hardness 
