Ionization produced in Gases by Réntgen Rays. 361 
aa 
then I, =P” 
compared, in every instance, with air. The ionization of the 
given gas at atmospheric pressure was compared in each case 
with air at the same pressure. 
This was found to be a very delicate method of comparison. 
The electrometer used was one of the Dolezalek pattern, 
which gave a deflexion of about 2000 scale-divisions for a 
difference of potential of 1 volt between the quadrants when 
the needle was charged up to a potential of 120 volts. It 
was possible by this method and apparatus to detect an 
extremely slight change in the ionization in either of the 
cylinders. When a balance was once obtained, an extremely 
small alteration in either of the cylinders would produce a 
deflexion of the electrometer-needle in either one direction 
or the other. When any gas was being compared with air, the 
pressure in the partially exhausted cylinder had to be 
adjusted very accurately, as a very slight alteration in the 
pressure would destroy the balance. ‘This delicacy of the 
method made it possible to detect very slight changes in 
the ionization which, with a less delicate apparatus, “night 
very easily escape notice. 
In the actual experiments the gases were 
Comparison of the Ionization in Hydrogen and Air. 
In the results obtained by previous experimenters hydrogen 
was the gas which showed the greatest discrepancy. In the 
present series of experiments hydrogen was, therefore, the 
first gas experimented upon. It was in every case of course 
compared with air. Several comparisons were made, using 
as the source of the Rontgen rays a bulb with an automatic 
vacuum-regulator attached. By means of this regulator the 
pressure inside the bulb was kept very constant, and the 
quality of the rays given out therefore remained very uniform 
from time to time. This was one of the regular type of 
automatically regulating bulbs made by the firm of H. W. Cox; 
and since it proved so satisfactory as a source of rays it may 
perhaps not be out of place to give a brief description of it 
here. A diagram of the bulb and the attached regulator 
is shown in fig. 2. The bulb itself and its electrodes are 
of much the same form as in the ordinary simple form of 
focus-tube, but attached to the bulb is a side tube R com- 
municating with the vacuum-regulator S. This is just a 
small discharge-tube, and the anode is connected to the anode 
of the main bulb. The cathode is connected to a wire which 
terminates at a spark-gap P. The terminal on the other 
