238 
the inactivation of the particles is irreversible, 
meaning that the trace substance is not only 
loosely adsorbed but more firmly bound to the 
particle by chemisorptive forces. 
The parallel measurements of condensation 
nuclei and freezing nuclei carried out at different 
places prove that besides the importance of ch- 
matic conditions the threshold temperature of 
ice nucleation of a given particle is not only a 
function of the constitution and size of the par- 
tiele but is also influenced by environment con- 
ditions. The presence of certain trace substances 
and their concentration will effect the activation 
of the particle within a wide range of tempera- 
ture. Preactivation of the nucleus extends the 
temperature range for ice formation to higher 
values than normal while surface poisoning will 
drop the threshold temperature to lower values. 
The factors will most certainly affect the op- 
eration of the mechanism of the rain formation 
process as postulated by Bergeron and Findeisen. 
The investigations reported in this paper have 
been sponsored in part by the Geophysics Re- 
search Directorate, ARDC under contract AF 
61 (514)-927 through the European Office. 
REFERENCES 
Biaa, E. K., A new technique for counting ice-form- 
ing nuclei in aerosols, Tellus, 9, 394-400, 1957. 
Birste1n, 8S. J., Adsorption studies of heterogeneous 
phase transitions, Geophysical Res. Papers 32, 
1954. 
DreissacH, K., Die vertikale Verteilung der grossen 
Kkerne in der unteren Troposphire und ihr Zu- 
sammenhang mit dem elektrischen Potential- 
DISCUSSION 
gefille, Archiv Met. Geophys. Biokl. A 9, 36-53, 
1956. 
Georcu, H-W., Eine neue Mischwolkenkammer 
fiir wolkenphysikalische Untersuchungen, Be- 
richte Met. Geophys. Inst. Frankfurt, 6, pp. 25- 
30, 1956. ; 
Groreu, H.-W., Uber die Eiskeimbildung in unter- 
kiihlten Wolken in ihrer Abhdangigkett vom at- 
mosphdrischen Aerosol, Habilitationsschrift Uni- 
versitat Frankfurt, 110 pp., 1959. 
Georcit, H-W., ann A. L. Merninks, An investiga- 
tion into the properties of atmospheric freezing 
nuclei and sea-salt; nuclei under maritime condi- 
tions at the west coast of Ireland, Geofis. Pura e 
Appl. 41, 159-176, 1958. 
Junar, C., Gesetzmissigkeiten in der Gr6dssenver- 
teilung atmosphirischer Aerosole tiber dem Kon- 
tinent, Berichte Dt. Wetterd. US Zone, 35, 261- 
277, 1952. 
Ikassanper, A. R., L. L. Sims, anp J. McDownatp, 
Observations of freezing nuclei over the South- 
western United States, The artificial stimulation 
of rain (H. Weickmann and W. Smith, eds.) 
Pergamon Press, pp. 392-403, 1957. 
LanpsperG, H., Atmospheric condensation nuclei, 
Ergebnisse Kosm. Physik, 3, 155-252, 1938. 
Mason, B. J., Recent developments in the physics 
of rain and rainmaking, Weather, 14, 81-98, 1959. 
Mason, B. J. anp D. J. Moorr, The concentration, 
size distribution, and production rate of large 
salt nuclei over the oceans, Q. J. R. Met. Soc., 
80, 583, 1954. 
Miutersen, R., Elektrische Ladungen auf Kon- 
densationskernen bei Wasseraufnahme und Ab- 
gabe, Naturwissenschaften, 45, 34-35, 1958. 
Rerter, R., Ergebnisse luftelektrischer Messungen 
auf dem Zugspitzplatt, Geofis. Pura e Appl., 30, 
155-169, 1955. 
Weickmann, H., Recent measurements of the verti- 
cal distribution of Aitken nuclei, Artificial Stim- 
ulation of Rain (H. Weickmann and W. Smith, 
eds.) Pergamon Press, 81-88, 1957. 
Discussion 
Dr. Bernard Vonnegut—I am very much in- 
terested in the observation that the effectiveness 
of ice-forming nuclei tends to increase with con- 
vective activity suggesting that these particles 
may be brought down from aloft. I think this 
observation might shed some light on the ex- 
perience of my colleague, Charles Moore in bal- 
loon flights in thunderstorms over New Mexico. 
He observed on two flights that the exterior of 
large thunderstorm clouds was covered by a 
very thin mass of ice crystals that were not 
present in the interior. This observation, I think, 
suggests that air on the outside of the clouds, 
being pulled down from aloft, is high in nuclei 
content. In connection with ammonia and its ef- 
fect on nuclei, it is worth mentioning Reynold’s 
work of some years ago, in which he found am- 
monia had a very strong effect on silver iodide 
in the reverse direction; it appeared to increase 
the activity of silver iodide quite remarkably. 
Dr. 8. Birstein—With respect to inhibiting, it 
is gratifying to find that Dr. Georgii reports 
these effects. I gave a paper at the last Woods 
Hole Meeting, and I have another paper this 
afternoon on this subject. I think there is a lot 
more that can be said about it after that paper. 
Dr. B. J. Mason—We all agree that Dr. 
Georgii’s investigation is a very welcome one, 
