nants. One part per million of the contaminant 
will lower the freezing point of supercooled wa- 
ter droplets in the presence of lead iodide to 
—15°, and one part per hundred thousand to 
—20°. 
Conclusions—W hile the above data were taken 
in the laboratory on lead iodide, an artificial nu- 
eleating material, certain conclusions can be 
drawn which are directly applicable to the at- 
mosphere. The concentration of ethyl amine used 
in these experiments was purposely kept low in 
order that the lower end of the curve of partial 
pressures would approximate that of atmos- 
pheric pollutants. If one thinks in terms of con- 
centrations up to one part per hundred thou- 
sand, the nucleating temperature is lowered by 
14°. 
Although these data were taken on lead iodide, 
work done in the past [Birstein, 1957] has shown 
that it is possible to inhibit ice crystal formation 
by natural nuclei through treating the air sam- 
ple with methyl amine before it is brought into 
a cold box. These experiments showed that with 
a sufficient amine concentration it is possible to 
prevent so-called ‘spontaneous nucleation.’ We 
were able to reach —52° in one experiment be- 
fore ice-crystal formation took place. Ethyl 
amine acts in a manner similar to methyl amine 
but is slightly more effective because of the 
longer carbon chain on the molecule [Brueheman 
and Verhoek, 1948}. 
If we accept the following type of reaction as 
being the cause of the loss of effectiveness of the 
lead iodide nucleus [Biltz, 1922] 
PbI + 2C.H;N He os Pb(C2H;N Hs)oI5 
DISCUSSION 
we see that this is the simple formation of coordi- 
nation complexes on a metal salt. It is an ex- 
tremely common type of reaction and would be 
expected to occur with many compounds other 
than amines. Hundreds of other type reactions 
are also possible with other particulate matter 
which may be responsible for ice crystal forma- 
tion in the atmosphere. 
While these experiments were not designed to 
pinpoint the exact mechanism by which ice- 
crystal formation may be inhibited in the at- 
mosphere, they do show that the concentrations 
of industrial wastes present in an air mass are 
certainly sufficient to make marked changes in 
the ice-nuclei spectrum in the air mass. 
REFERENCES 
Birtz, W., Ueber die Ammoniakate der Bleihalo- 
genide, Stammverbindungen und Mischverbin- 
dungen, Zs. anorg. allgem. Chem., 124, 230-247, 
1922. 
Brrstern, 8. J., Adsorption studies of heterogene- 
ous phase transitions, Geophysics Research Pa- 
per No. 382, Air Force Cambridge Research 
Center, December 1954. 
Brrster, S. J., Studies on the effect of certain 
chemicals on the inhibition of nucleation, Arti- 
ficial Stimulation of Rain, Pergamon Press, pp. 
376-385, 1959. 
BirsteIn, S. J., AnD C. E. Anprrson, The mecha- 
nism of atmospheric ice formation, I, the chemi- 
cal composition of nucleating agents, J. Met., 
12, 68-73, 1955. 
BruEHEMAN, R. J., AnD F. J. VerHork, The basic 
strength of amines as measured by the stabili- 
ties of their complexes with silver ions, J. Amer. 
Chem. Soc., 70, 1401-1404, 1948. 
Discussion 
Dr. James P.. Lodge, Jr—yYour use of very low 
temperatures to obtain low partial pressure of 
contaminants is a very difficult one. We have 
used the apparatus of Stephan to obtain low con- 
centrations by means of diffusion through tubes 
(A. P. Altshuller and I. R. Cohen, “The Apph- 
cation of Diffusion Cells to the Production of 
Known Amounts of Gaseous Hydrocarbons,” pa- 
per presented at the 136th National Meeting, 
American Chemical Society, Atlantie City, Sep- 
tember 18, 1959). In this manner we can obtain 
concentrations of one part per million and less in 
a dynamic system. 
Mr. 8S. J. Birstein—I agree that this approach 
is a rather difficult one. 
Dr. F. W. Van Straten—It would seem that 
there is something of a conflict between the re- 
sults of this work and the work of Dr. Mason, 
because according to Dr. Mason’s paper there 
seemed to be a preferential nucleation on, shall 
we say, the active lines of the host crystal. If 
this is true, in that case, you could not expect 
poisoning to be a linear function of the concen- 
tration of the poison. It should first affect the 
host crystal on the most efficient sites, then drop 
off very sharply to the point where the other 
surfaces of the erystal begin to nucleate. 
Dr. B. J. Mason—I think it is important that 
you saw the ice crystals, in my cases, growing 
on the dislocation steps. It is at these that the 
