Aggregations produced in Gases by Sudden Cooling. 761 
phase and sublimation from the solid. We have found that 
the gas evolved from liquid air, oxygen, ethylene, methane is 
nuclei-free, confirming the ordinary view that the evaporation 
of a liquid consists in the escape of separate molecules. But 
the fact that a mass of solid C0 2 (previously condensed in a 
perfectly dust-free state) continues to give off gas containing 
enormous numbers of nuclei until the whole mass has dis- 
appeared, suggests that the sublimation of solid C0 2 (and 
possibly of other substances which can pass from the solid 
direct into the gaseous phase) consists in the escape of separate 
molecules together icith numerous molecular aggregations. 
We will in conclusion meet some possible objections to 
the above views. As the effects obtained are larger when 
the gas is allowed to rush into the cooled evacuated chamber 
it might be urged that the nuclei are simply dust particles 
dislodged from the walls of the vessel by the sudden rush of 
gas. This view, however, is quite untenable from the following 
considerations : — (1) The effect shows no signs at all of 
diminution however often the experiments are made with any 
particular tester. (2) No effect is obtained when the gas 
rushing into the evacuated tester is pure ethylene, whereas 
a considerable effect is obtained when the ethylene contains 
a small percentage of air. (3) The nuclei are rapidly 
destroyed at a temperature of about 200° C. 
Nor can it be argued that the nuclei are due to insufficient 
drying of the gases, as there is very little difference between 
the behaviour of wet gases and of those dried with the utmost 
precautions. 
Then again it might be urged that the nuclei are due to 
the presence of some impurities in the gases. This point was 
discussed in the previous paper. If the impurity be, let us 
imagine, C0 2 it is not likely that the effects in purified air 
and in air mixed with five per cent, of C0 2 would be 
indistinguishable as was found to be the case (see Table IV.). 
Supposing on the other hand the impurity to be some gas 
only slightly more easily liquefied than air, we meet with the 
same difficulty in accounting for the production of the nuclei 
in this impurity as we do in the case of air. If the effects 
are due to some impurity, then the only gas we obtained pure 
must have been ethylene. The precautions which we have 
taken, however, to obtain pure gases, especially in the case 
of oxygen and air, render this " impurity " explanation 
highly improbable, certainly not more probable than the view 
we have expressed above. 
George Holt Physics Laboratory, 
The University of Liverpool. 
