Tonization produced in Gases by Réntgen Rays. 87 
TABLE ].—<Air. 
Amount of Ionization as represented by Deflexions 

of Electrometer Needle. Temperature 
| of 
| Observed Deflexion in | Corrected for Change in Air. 
) Seale-Divisions. Density of Air. 
| 267 267 oe, 
232 279 74° 
199 | 212 120° 
| 179 266 155° 
) 157 258 201° 
| 131 248 212° 


The correction for the change in density of the air is made 
so that the deflexions at the higher temperatures are all 
reduced to the same standard as that at the lowest temperature 
observed. 
Although the numbers in the second column are not 
perfectly constant yet the variation is quite within the limits 
of experimental error, especially when we consider the 
difficulty of keeping the intensity of the source of ionization 
constant. The maximum variation in these numbers from 
the mean is only about 6°5 per cent., which is really very 
good when working with Réntgen rays. The agreement is 
quite close enough to show that the apparent variation in the 
ionization when the temperature is raised is due entirely to 
the change of density of the air and not to the change in 
the temperature directly. 
Experiments on Air at Constant Density. 
After concluding the experiments just described I attacked 
the question by the second method mentioned at the beginning 
of this paper, measuring the amount of ionization produced 
in a volume of gas inclosed in an air-tight vessel and kept at 
a constant density. This method is much more definite than 
the first one. If the air is inclosed in an air-tight vessel 
then the only thing that can affect the ionization in it when 
the air is heated is the change of temperature itself. The 
question is consequently simplified by getting rid of the 
change of density. 
Although this method simplified matters in one respect 
it complicated them in another, for at the very outset the 
difficulty arose of constructing a suitable vessel which would 
