274 
MR. C. T. R. WILSON ON CONDENSATION OF WATER VAPOUR 
the filter was found to be without effect upon the appearance of the rain-like 
condensation, or the expansion required to produce it. 
3. When no cotton-wool filter was present the air could be passed from the one 
vessel to the other and back without any effect, so long as it was not allowed to 
bubble through the distilled water in either vessel. If, however, the air had to 
bubble through water on being driven back, quite a small expansion v/as sufficient 
to cause a shower even some minutes later. 
4. As already stated, the air may be allowed to expand considerably more than is 
necessary to produce condensation without the drops becoming very numerous. 
With very great expansions, however, if, for example, the increase in volume be 
made twice as great as is necessary for condensation to result, a dense fog showing 
colours and settling slowly is produced. 
Second Form of Apparatus. 
'I'he apparatus already described was not suitable for experiments upon pure gases, 
on account of the large volume of water present. 
To remove all the dissolved gases from so large a quantity of water would have 
been very difficult. 
Another reason for changing the form of the apparatus was that I wished to 
investigate in what way the number of drops produced depended upon the degree of 
supersaturation reached. It appeared, from the experiments already made, that the 
drops remained comparatively few with expansions considerably greater than that 
required to cause condensation to begin, and over a considerable range there was no 
appreciable increase in the number with increasing expansion. Yet, with very large 
expansions, the number was very great, and the drops sufficiently small to produce a 
coloured fog which settled very slowly. 
The first apparatus was not convenient for making measurements with very large 
expansions, so no attempt was made to investigate with it whether there was a 
sudden transition from the one form of condensation to the other. It was thought 
that another form of apparatus would be more suitable for the purpose. 
A very rapid expansion is evidently required for this investigation. For, let us 
consider one cubic centimetre of saturated air which is expanded rapidly. If we 
suppose the effect of the walls to be negligible, the ordinary equation for the cooling 
of a gas by adiabatic expansion may be applied to find the lowering of temperature 
and the resulting supersaturation till the volume amounts to 1'252 centims. x4t 
this stage, as we have seen, condensation begins upon a comparatively small number 
of scattered nuclei. There must at once result from the ijiitial condensation a 
simultaneous loss of vapour and rise of tempei’ature in the region immediately 
surrounding each incipient drop. The subsequent growth of the drop must be more 
or less gradual, being the result of the comparatively slow processes of diffusion and 
heat conduction. 
