DISCUSSION 
poison will be absorbed first. It is because the 
nucleation occurs only on these steps that such a 
small amount of poison ean do the trick, because 
if you put a monomolecular layer of poison all 
around the steps, it still covers only very little of 
the surface. As Dr. Van Straten says, the poison 
affects the steps first, and the amount of material 
required to poison the steps is very small mdeed. 
We certainly have evidence that the small traces 
of poison do, in fact, kill nucleation sites at 
these very steps. 
Dr. Walter Hitschfeld—I think this is ilus- 
trated quite well on Figure 1. Here the relation- 
ship of the poison concentration to temperature 
is shown to be linear. But obviously it cannot re- 
main linear down to zero concentration. So if one 
goes beyond Mr. Birstein’s data to zero concen- 
tration, one knows he has to get to the tempera- 
ture usually called ‘activation temperature.’ 
There is thus necessarily a non-linear region m 
the relationship. This is precisely what Drs. Van 
Straten and Mason require. 
Dr. H.W. Georgii—lt is right to assume from 
your results that the process of the poisoning 1s 
not only absorption but a chemisorption ? 
Mr. Birstein—On the lead iodide, I would call 
it reversible chemisorption. On the silicate it 
would probably be physical absorption of some 
sort. However, what it does in the activity, I 
could not begin to say. 
Dr. Georgii—Can the poisoned particles re- 
cover their activity ? 
Mr. Birstein—It can recover its activity, but 
not easily. 
Dr. Roscoe R. Braham, Jr—Perhaps some of 
you did not see a paper that bears on this, al- 
though not critically, when it was published. 
Back in 1949 or 1950, in work we were doing at 
251 
the Institute of Mining and Technology, we felt 
that thunderstorm electricity had its origin in 
freezing of water, according to Workman and 
Reynold’s work on freezing potentials. It had 
been found in the laboratory that water very 
shghtly contaminated with ammonia (1 to 10° to 
1 to 10") gave freezing potentials which were just 
reversed from those of normal water. We felt we 
had a straight-forward experiment that was 
ideal. All one had to do was release quantities of 
ammonia into potential thunderstorms; and it 
would be ‘flipped upside down’ and simultane- 
ously we would have proven the Workman-Rey- 
nolds effect. But it turned out when we loaded 
a B-17 with all the ammonia it could carry and 
stocked tanks of ammonia on the ground, so we 
could release it up underneath the thunder- 
storm, we were completely unable to observe any 
effect whatsoever on the electrical structure of 
this storm. We certainly were releasing quantities 
of ammonia into these particular storms, but 
Nature has a way of disposing of this material 
as though it were never released. This was un- 
doubtedly real, and I would like to suggest that 
many times Nature does not know about many 
of the things we do. 
Mr. Birstein—We have been doing some work 
in clouds inhibiting ice-crystal formation, and 
the results thus far have not been too great; but 
we seem to have gotten limited success in de- 
creasing the ice-crystal concentration. We hope 
to do some work of this sort in Flagstaff in the 
summer of 1959. 
Dr. M. Neiburger—Did you stop the rain 
from falling? 
Mr. Birstein—This is one thing we are inter- 
ested in determining. 
