472 
Proceedings of the Royal Society of Edinburgh. [Sess 
expansion is increased the number of drops increases slightly. This 
increase goes on slowly till a certain degree of expansion is reached, when 
the appearance of the condensation changes somewhat suddenly, the rainy 
condensation changing to dense fog ; that is, with the lower expansions 
the particles are few, large, and fall quickly, while in the second stage they 
are extremely numerous, very small, and fall very slowly. This sudden 
change in the nature of the condensation takes place when vjv 1 = l‘ 38. 
With still higher expansions the number of particles becomes greater ; that 
is, the fog gets denser. 
In Wilson’s apparatus, as the expansion is extremely rapid, the degree 
of expansion may be taken as a measure of the supersaturation, and as the 
different instruments used by him give almost the same values for vjv v 
we may take it that they are correct. In the instruments here to be 
described the values of vjv 1 are different from Wilson’s, and they differ 
with each other owing to the conditions not being so simple as in Wilson’s. 
But any difficulties one may feel about this will disappear if we will keep 
in mind that it is the degree of supersaturation and not the amount of 
expansion which determines the result. Supersaturation appears to depend 
greatly on the quickness of the expansion. A very slight slowing of the 
velocity of expansion, even though to the eye there may seem to be no 
change, requires a decided increase in the expansion. Part of this is due to 
heat exchanges between the air and the walls of the vessel and to radiation ; 
but in these experiments with high supersaturations a greater part is due to 
the manner in which the nuclei relieve the vapour tension. If the expan- 
sion be not instantaneous, then some of the nuclei, being better centres of 
condensation than others, come into action before the others : even in these 
infinitesimally small particles there seems, as elsewhere, to be no equality. 
The result of this is that the best nuclei begin condensing before the others, 
and if there is time given they rob the surrounding air of its vapour owing 
to the high supersaturation, so that if the other nuclei are to come into 
action time must not be given for this condensation to take place. We see 
from this that the quicker the expansion the greater the number of nuclei 
that will become active, while with a slow expansion an odd nucleus here 
and there will be able to relieve the tension. This suggests that the gradual 
increase in the number of drops observed with increase of expansion during 
both rainy and foggy stages may be as much due to increase in rapidity of 
expansion as to increase in its amount. 
These experiments on condensation in supersaturated air refer to the 
condensation which takes place on nuclei always present in saturated 
air. When X-rays and radioactive bodies acted on the air in Wilson’s 
