1132 
arrests virulence), because of the increase in the con- 
centration of ammoniacal substances in the nuclei, 
whereas in Berlin the mean pH value is forced almost 
to the neutral point (which promotes virulence). Con- 
versely, the mean pH value on the Island of Norderney 
again lies in the weakly acidic range, because of the lack 
of ammoniacal substances. In this case, however, the 
weakly buffered acids are not nitrites but hydrogen 
compounds of the halogens originating from the sea. 
WORKING HYPOTHESES 
Significance of Molecular Forces for Condensation. 
The unexpected results of quantitative analyses ob- 
tained by the condensation method gave the first stimu- 
lus to the theoretical consideration of condensation [12]. 
If gaseous substances subsequently dissolve in sus- 
pended drops of pure water, an equilibrium must occur 
between the gaseous and the dissolved portion of the 
gas. This precludes a total quantitative determination 
by analysis of the dissolved portion only. The adequacy 
of a quantitative determination rests therefore on the 
assumption that prior to the condensation a certain 
orderly arrangement exists between the corresponding 
gaseous chemical substances and the water-vapor mole- 
cules. The hygroscopic substances accordingly repre- 
sent an integral component of numerous condensation 
nuclei, owing to their attractive forces which are in- 
herently stronger than those which the water molecules 
exert upon one another. Wilson [59] and Wegener [57] 
were aware that the laws of thermodynamics are in- 
sufficient to explain cloud formation. According to con- 
temporary concepts, as treated by Wolf [60], a gas 
(water vapor) is not condensable at finite temperatures 
if only elastic impact forces exist between its molecules 
in accordance with the Boyle-Mariotte law. To explain 
condensation (cloud formation), the chemist must there- 
fore have recourse to intermolecular forces and, in the 
last analysis, to processes of atomic energy. This ap- 
proach was initiated by Lenard [87] prior to 1914. He 
was followed by Volmer [56], who, however, did not 
recognize as clearly as Lenard the great significance of 
the short-range forces manifest in the nuclei formation 
before water-vapor saturation (¢.g., fog formation by 
NH;NO; produced by O; and other factors, or even by 
artificial acidic fogs Cl-SO2-Cl). He believed that it was 
possible to explain water-vapor aggregation and con- 
densation into droplets by relatively strong supersatura- 
tion alone without the aid of nuclei. The four types of 
short-range molecular forces will now be discussed in 
the light of our present knowledge. 
Polar Forces. These forces, which are of electrostatic 
nature, give rise to mutual orientation of the molecules, 
particularly that between electrical charge carriers and 
the dipole. As in the case of the ion atmosphere in 
solutions treated by Debye [20], it is obvious to assume 
that neutral dipoles of the water vapor become attached 
in a more or less oriented manner to charged gas mole- 
cules and form a “‘cluster.”” At any rate, this clustering 
can be considered as the origin of the so-called “‘small 
ions” in air (water-vapor molecules, r = 1.4 X 1078 
em: small ions,r = 7 X 10-8 cm). Spatial considerations 
BIOLOGICAL AND CHEMICAL METEOROLOGY 
may require a certain supersaturation before the small 
ions can grow sufficiently large to become visible. There 
is room around the ion for only a restricted number 
of oriented water molecules. Only when supersaturation 
occurs, or in the course of an increased number of collisions, 
is it possible for additional, similarly oriented dipoles to 
penetrate this configuration. In this process the centers of 
the molecules approach each other and as a result the 
induction forces, discussed below, become effective and 
further increase the attractive forces of the molecules. 
The free rotation due to the thermal motion is thereby 
considerably diminished and is even nullified during 
sufficient, say adiabatic, cooling. If charged gas mole- 
cules were to adhere to varyingly large groups of water- 
vapor molecules scarcely, if at all, affected by molecular 
forces, the so-called ‘“‘small ions”? would increase in size 
irregularly and at a much lower supersaturation. In the 
free atmosphere the majority of the small, charged 
nuclei do not grow into large ones, since small ions 
vanish by recombination and by association with me- 
dium and large nuclei which are produced by more 
strongly hygroscopic substances (tangling of chains and 
rings of dipolar substances with water-vapor mole- 
cules, resulting in condensation—see below) and which 
possess an electrical double-layer. 
Dipole Forces. These forces, which are also of elec- 
trostatie nature, orient the neutral, permanently di- 
polar water-vapor molecules with respect to each other. 
This orientation is achieved without the aid of extra- 
neous ions and solely by the polar attractive forces 
between dipoles. As the temperature of the system is 
lowered, the free rotation ceases here also. Depending 
on whether the water dipoles come in contact with each. 
other or with different dipoles (1) a simple super mo- 
lecular formation may occur with dipoles nonpolarly 
parallel to each other, (2) a rmg formation may occur, 
or (8) a chain formation may occur when the dipoles 
are in polar arrangement with subsequent tangling. 
Tangling phenomena involving dipoles of supercooled 
liquids (rubber) were observed by Debye. In a conver- 
sation with the present author, Hiickel and Eucken 
advanced the idea that water-vapor molecules unaf- 
fected by an ion and not in combination with hygro- 
scopic substances occur principally in groups of eight 
(rings), which are in equilibrium with groups of four 
(rings) and with single molecules. Because of their 
closed form, the pure water rings, in the present author’s 
opinion, are probably less suited to formation of nuclei 
than are the chains having admixtures of foreign sub- 
stances. In the latter, a more rapid growth and tangling 
can occur as a result of the greater proximity of the 
centers of the molecules. Furthermore, for the same 
reasons, these chains probably resist dissolution from 
an addition of heat somewhat more strongly than do 
the ring-shaped, readily dissolvable, pure water rings. 
Naturally, dipoles of the most varied types, such as 
NH;, HCl, H2SOs, and HCO, unite with the dipole 
HO. The decisive factor for the foregoing process is the 
dipole moment (the unit of which is 1 “debye” (D) 
= 10~® electrostatic units) of the two substances, 
since the attraction at sufficient proximity is propor- 
