302 
MH. C. T. R. WILSON ON CONDENSATION OF WATER VAPOUR 
seconds, a fog lasting for more than a minute was produced. If, how’ever, v. 2 ii\ was 
below I'25, no condensation resulted, whether the rays were acting or not. 
The same results were obtained when the expansion vessel was completely wrapped 
up in tinfoil, which was only removed after the expansion had been made and the 
current had been switched off. Direct electrical action was thus excluded. 
The air inside A is probably sufficiently well shielded from electrical effects by the 
film of water which covers the inner surface of the walls. For it was afterwards 
found that no effect was produced on the condensation when A was placed directly 
between the terminals of an induction coil while the expansion was made. 
The nuclei introduced by the X-rays only persist a few seconds. No effect is noticed 
if the expansion be made half a minute after the current has been turned off. 
It will be noticed that in these experiments the rays had to pass through the glass 
walls of the tube, and must, therefore, have been very much reduced in intensity. 
Richarz* has recently shown that the condensation of the steam-jet becomes dense 
under the action of the X-rays. The intensity of the radiation was probably much 
greater in his experiments, for the rays had merely to pass through aluminium. 
Hydrogen .—The action of the X-rays when hydrogen was substituted for air was 
afterwards investigated. It was considered unnecessary in this case, for the reasons 
already mentioned, to take any precautions for shielding off’ direct electrical action. 
The arrangements were in other respects exactly as in the experiments with air. 
The hydrogen was obtained as already described. 
In the first experiments the following results were obtained :— 
ij/t'l. 
Result. 
1-308 
1-296 
No X-i-ays 
X-rays 
No condensation 
Fog 
Several experiments were then made with the tube exposed to the rays, each 
expansion being made less than the preceding one. The number of cloud-particles 
was observed to diminish with diminishing expansion, till, when was made equal 
to 1'260, only a few drops were formed, and none were seen when vjvi was equal 
to 1-255. 
Further measurements were now made of the smallest expansion required to cause 
condensation when the moist hydrogen was under the inffuence of the X-rays. 
* ‘ Wit'd. Ann.,’ vol. 59, p. 592, Ib9li. 
