XI] AND OF ORDINARY MATERIALS 377 
body would lose its charge extremely slowly. This is in agreement 
with an observation of Crookes, who found that a pair of gold- 
leaves retained their charge for several months in a high vacuum. 
Wilson! at a later date investigated the leakage for different 
gases. The results are included in the following table, where the 
ionization produced in air is taken as unity: 





ae 
: Relative Relative ionization 
Gas ionization density 
Air fe th 1:00 1:00 
Hydrogen ai 0184 27 
Carbon dioxide ... | 1:69 1:10 
Sulphur dioxide... 2°64 2 
Chloroform ale 4:7 1:09 





With the exception of hydrogen, the ionization produced in 
different gases is approximately proportional to the density. The 
relative jonization is very similar to that observed by Strutt 
(section 45) for gases exposed to the influence of the « and @ rays 
from radio-active substances, and points to the conclusion that the 
lonization observed may be due either to a radiation from the 
walls of the vessel or from external sources. 
Patterson? examined the variation of the ionization of air 
with pressure in a large iron vessel of diameter 30 cms. and length 
20 cms. The current between a central electrode and the cylinder 
was measured by means of a sensitive Dolezalek electrometer. 
He found that the saturation current was practically independent 
of the pressure for pressures greater than 300 mms. of mercury. 
Below a pressure of 80 mms. the current varied directly as the 
pressure. For air at atmospheric pressure, the current was inde- 
pendent of the temperature up to 450°C. With further increase 
of temperature, the current began to increase, and the increase 
was more rapid when the central electrode was charged negatively 
than when it was charged positively. This difference was ascribed 
to the production of positive ions at the surface of the iron vessel. 
The results obtained by Patterson render it very improbable that 
the ionization observed in air is due to a spontaneous ionization of 
the enclosed air: for it would be expected that the amount of 
this ionization would depend on the temperature of the gas. On 
1 Proc. Roy. Soc. 69, p. 277, 1901. 2 Phil. Mag. Aug. 1903. 
