304 
MR C. T. R. WILSON ON CONOENSATIOR OF WATIOR VAPOUR 
shower was produced with the bulb 120 centims. olf, when was equal to 1'326, 
wliile an expansion of the same amount made immediately afterwards wdth the 
current switched off from the coil caused no condensation. 
Wlicn the bulb was as far away as this it was found necessary to make rather 
greater expansion to bring about condensation,, than wdien stronger radiation fell on 
the expansion apparatus. With the bulb 120 centims. off condensation w^as first 
detected when was equal to 1'278, none being visible with smaller expansions. 
Since every nucleus, capable of promoting condensation, in vapour supersaturated 
to the extent reached at the completion of the expansion, becomes visible Iw the 
growtli of a dro23 around it, it is not surprising that even weak radiation should have 
a visible effect. 
I have not yet made any experiments* to see if X-radiation, which has not been 
weakened by passing through glass, makes the condensation begin at a lower super¬ 
saturation or not. The experiments of Richarz, to which reference has already 
been made, do not give us any information on this ])oint, as the extent of the super¬ 
saturation reached in a steam-jet is unknown. 
Interpretation of the ResnJts. 
The view here taken as to the meaning of the phenomena described in this paper 
is lu'iefiy as follows :— 
In aqueous vapour in the presence of air, oxygen, nitrogen, or carbonic acid, there 
always exists at any moment a small number of nuclei, capable of acting as centres 
of condensation when the density of the vapour reaches a certain value amounting at 
— 6° 0. to about 4’2 times that of the vapour in equilibrium over a flat surface of 
water at the same temperature. 
The nuclei capable of acting as centres of condensation when the supersaturation 
lies between this low^er limit and another amounting at 16° to 7'9 are comparative!}' 
few, and their number depends on the nature of the gas, for they appear to be 
absent in moist hydrogen. No attempts have yet been made to count the drops 
produced when the supersaturation lies between these limits, but from the appear¬ 
ance of the resulting shower they almost certainly do not amount to nearly 100 in a 
cubic centimetre. 
When the X-rays, or such components of the radiation as are able to pass through 
glass, act upon moist air or hydrogen, similar nuclei are produced in much greater 
numbers, those of them which are the most effective in helping condensation again 
requiring exactly the same minimum supersaturation in order that condensation may 
take place upon them. 
The number of nuclei capable of acting as centres of condensation when tlie super¬ 
saturation, even slightly, exceeds the upper limit mentioned, is, whatever gas be 
present, very large, and the number which come into play increases with enormous 
* See uote al the eiul of the paper. 
