Aggregations produced in Gases by Sudden Cooling. 755 
mixture behaves just the same as i£ the ah* were absent, the 
nuclei being produced only when condensation occurs. 
The same result is borne out by experiments performed 
with a mixture containing 90 per cent. o£ C0 2 . 
That the effects in pure air and air mixed with 5 per cent. of 
C0 2 are practically indistinguishable is sufficiently accounted 
for by the fact that only very few nuclei would in any case 
be produced by the complete condensation and subsequent 
sublimation of the small quantity of C0 2 present in this 
mixture. 
Air and Ethylene. 
As pure ethylene itself gives no effect at all when cooled 
to every temperature we have tried, the effect in air mixed 
with ethylene was investigated. 
The following table shows the results obtained in three 
different mixtures of air and ethylene. 
Table V. 
Temp. 
Air. 
5 per cent. 50 per cent. 
Ethylene. Ethylene. 
90 per cent. 
Ethylene. 
-125° C. ... 
-127 
-128 
-131 
-133 
-143 
-190 


Very thin shower. 
Thin shower. 
Good shower. 
Heavy shower. 
Tinted rain-cloud. 

Few drops. 
Thin shower. 
Good shower. 
Good shower. 



Very thin shower. 
Heavy shower. 
Tinted rain-cloud. 
Fair shower. 
Tinted rain-cloud. 
Here again it will be noticed that the effects in pure air 
and in air mixed with 5 per cent, of ethylene are practically 
identical. The "nucleating temperature " in the mixture, it 
is true, is a few degrees higher than for pure air, but the 
difference is not sufficiently marked for any great importance 
to be attached to it. On the other hand, the results in a 
50 per cent, mixture are readily understood, the effects 
obtained being simply those which would be obtained in 
air at a pressure equal to its partial pressure in the mixture. 
The nucleating temperature in the 90 per cent, mixture 
bears out the same view. It will be seen, however, from the 
table that when the 90 per cent, mixture is cooled by liquid 
air just as many nuclei were produced as in the case of pure 
