DISCUSSION 
and I agree with nearly everything he says; I 
just want to raise the problem of contamination, 
to see how important that is. 
There is some advantage of living in a filthy 
atmosphere! If one takes an absolutely clean 
silver surface and puts it in the London air, for 
a few minutes, it absorbs enough iodine or sulfur 
to form patches of silver iodide of silver sulfide 
on which oriented ice crystals may be formed. 
If one irradiates silver iodide with ultraviolet, 
one finds in the first five minutes that its nucleat- 
ing properties actually improve because its 
irradiation removes certain impurities; but 
thereafter its nucleating effectiveness decays log- 
arithmically. So, this shows that impurities can 
work in either direction, and I think it is im- 
portant to bear that in mind. 
Dr. H. Weickmann—I would like to call at- 
tention to the observation regarding the increase 
of nucleating ability when sub-cooled droplets 
are in the state of evaporation. This may apply 
to Georgii’s observations in radiation fogs as well 
as to Vonnegut’s observation in the exterior 
layer of a thunderstorm cloud. Two processes 
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seem to act: (1) the one which we mentioned 
before (see discussion of Dr. Mason’s paper) 
and which seems to indicate that a thin film of 
water crystallizes easier than bulk water, and 
(2) an observation which I made in the labora- 
tory when making ultramicroscopic studies of 
the melt water of cirrus crystals. Insoluble par- 
ticles in the interior of these drops migrated dur- 
ing the evaporation, driven by Brownian motion, 
to the edge of the drop. Here the water sur- 
rounding the nucleus may only have the thick- 
ness of a thin film and nucleation starts. 
Dr. H.-W. Georgii—The mechanism of evap- 
oration effect is not quite clear to me yet. The in- 
crease of the concentration of freezing nuclei is 
also present if the fog is at temperatures above 
freezing without any supercooled droplets in the 
outside air, 
Dr. C. J. Todd—On one occasion we were 
counting freezing nuclei and watching rain with 
a vertical radar, and as the rain changed from 
warm-cloud precipitation to ice precipitation, 
there was a very marked increase in the nuclei 
count that we measured on the ground. 
