IN THR PRESEiSrCE OE ])ITST-PREE AIR AND OTHER OASES. 
803 
' 
Result. 
1-326 
No X-rays 
No condensation 
1-253 
X-rays 
Drops very few 
l-3oy 
X-rays 
Fog 
1-246 
X-rays 
No condensation 
1-259 
X-rays 
Shower 
With the X-rays on, condensation begins when vjv^ lies between i’246 and 1-253, 
the density of the condensation increasing rapidly with increasing expansion. 
Fresh hydrogen was now prepared. The bulb was 30 centims. from the cloud 
chamber. 
X-RAYS on in all Cases. 
t'o/r,. 
Result. 
1-251 
No condensation 
1-254 
Very few drops 
1-253 
Very few drops 
1-251 
No condensation 
1-258 
Drops few 
1-272 
Dense shower 
1-282 
Fog 
Thus it appears that condensation begins in hydrogen originally saturated when 
is between 1-251 and 1-253, if the gas be exposed to the action of tlie X-rays. 
Condensation therefore begins when the supersaturation reaches the same limit as is 
necessary for rain-like condensation in air, the supersatuvation required to produce 
condensation under ordinary conditions being nearly twice as great. 
As in the case of air the nuclei introduced by the Rontgen rays only last a few 
seconds. Thus immediately after obtaining condensation when was as low as 
1'253, if the expansion was made while the gas was exposed to the X-i-ays, an 
experiment was made in which the current was switched oflF half a minute before the 
expansion. Although was as great as 1-315, no condensation resulted. When, 
however, a similar expansion was made with only a few seconds’ interval a slight fog 
resulted. 
Since the X-rays make condensation begin in hydrogen, with a much smaller 
expansion than is necessary in their absence, it is much more easy to detect the effect 
of very weak radiation than in air, where only an increase in the number of the 
drops results from the action of the rays. 
It was found that the effect of the rays was quite noticeable, even when the bulb 
producing them was at a considerable distance away. For example, quite a distinct 
