DISCUSSION 231 
That is the way you check it. That is the way 
in which we essentially measure the supersatura- 
tion over this very wide range from zero to 
300% relative to ice. 
Mr. C. E. Anderson—lf I understand, you 
have also examined the influence of low pres- 
sure, and have found no influence? 
Dr. Mason—That is right except the erystals 
grow faster; the shape is completely unaltered. 
This is in conflict with the Japanese work. 
Dr. H.-W. Georgii—Would you suggest there 
is a relation between the misfit and the special 
nucleating temperature for any given mineral? 
Dr. Mason—TVhere are really two threshold 
temperatures. There is the highest temperature 
at which ice crystals appear at water saturation, 
—4°C in the case of silver iodide, and —12°C, 
below which crystals appear in a sub-water 
saturated atmosphere providing the supersatu- 
ration relative to ice exceeds 12%. One can ex- 
press the nucleating ability in terms of the 
supersaturation or of the temperature. 
Dr. Georgii—lIs it a specific value? 
Dr. Mason—Yes, for a specific subject. 
Dr. W. Hitschfeld—I would like to ask, in 
connection with your table of ‘activation tem- 
peratures’ whether you agree that there is no 
sharply defined temperature at which a given 
nucleus becomes active, but rather there is a 
range in temperature in which activation is pos- 
sible with varying likelihood. 
Dr. Mason— Activation temperature’ means 
the highest temperature at which one gets one 
ice crystal from 10,000 particles of seeding agent. 
If you want 1 in 100 for most of those sub- 
stances, take two degrees away from those 
figures. 
Dr. Hitschfeld—Closely associated is the 
‘time of activation.’ In earher papers from Dr. 
Mason’s laboratory (for example, Bigg, Proc. 
Phys. Soc. B, 66, 688, 1953), time was accorded 
an important place. But in some of the current 
work, one feels that not enough emphasis 1s 
always placed on it. Recent experiments at 
MeGill University (Barkhe and Gokhale, Part 
Ill of Scientific Report MW-30, Stormy 
Weather Group, July 1959) have clearly shown 
that the probability of a freezing occurrence is 
an approximately lear function of the time, 
and so the possibility arises that nuclei can 
become active at relatively higher temperatures 
if you wait long enough. This is a factor which 
meteorologists need to take most carefully into 
account when they apply the nucleation infor- 
mation which is becoming available. 
Dr. R. Weaxler—You mentioned kaolinite. Is 
this common on the ground and in the at- 
mosphere ? 
Dr. Mason—It comes in very small particles, 
and, in fact, in nature is hard to find in particles 
greater than oné micron, and so it is in very finely 
divided form. It can not be very abundant. Most 
soils containing kaolinite tend to have a good 
vegetation cover, but obviously wherever this is 
disrupted it gets into the air. I would also say 
that voleanic ash when weathered produces kao- 
linite. So, I think, there is a reasonable amount in 
the atmosphere, but not too much. The only di- 
rect evidence that we have is that of the Japanese 
workers who often detect kaolinite particles in 
snow crystals (K. Isono, Jap. J. Geophys., 2, no. 
2.1959). 
Dr. H. Weickmann—I am very happy about 
your paper because it is closely connected to 
the questions which I had posed in my letter of 
invitation: Can we prove that true sublima- 
tion nuclei do not exist and that AgI acts only 
as a freezing nucleus? There remains, however, 
a problem which is still unsolved: How does 
nature achieve the formation of ice at 0°C? So 
far in controlled laboratory tests this has not 
been achieved, not even with the best known 
freezing nuclei. It appears however that our 
experiments with freshly cleaved mica come 
closest. It would be very interesting if Dr. 
Mason would repeat those using his well con- 
trolled diffusion chamber. In our experiment 
breathing against the mica plate caused the 
formation of a very thin film of water on the 
mica plate recognizable only due to the for- 
mation of Newton interference rings. The crys- 
tallization of this film is easily visible. It started 
either at the very thin edges or at steps and 
irregularities in the mica plane. Crystalliza- 
tion occurred between 0° and —1.0°C wet bulb 
temperatures, that is, at room temperatures well 
above freezing! Erroneously we had assumed 
a practically perfect match between the geo- 
metric similarity of the cleaved plane and the 
base plane of an ice crystal but later we found 
out that the lattice structure of the cleaved 
plane is that of quartz and that it should 
—12°C. This ex- 
periment seems to indicate that we have to 
differentiate between the crystallization of a 
thin liquid film and that of bulk water, and 
nucleate at best at around 
