NUCLEI OF ATMOSPHERIC CONDENSATION 
erowth of the nuclei, regardless of their size and origin, 
starts only after a relative humidity of approximately 
70 per cent has been attamed, and that this growth 
accelerates as the humidity approaches saturation 
value. 
The interpretation of this principle of growth is not 
devoid of difficulties. A comparison of the curves of 
growth of different-sized solution droplets, computed 
in the usual way [25], shows reasonably good agreement 
90 
185, 
tion to the observations with mixed nuclei, that is, 
droplets of solution with a more or less large content of 
insoluble substances. Figure 3 gives a few computed 
curves of growth with hatched markings indicating the 
radii of the solid portions of the nuclei. Below a rela- 
tive humidity of 70 per cent, the solid nucleus is covered 
by only a thin solution film, with the radius remaining 
almost constant. 
Wall [40] pointed out that insoluble but wettable 
{oo} 
fe) 
xy 
(e) 
oO 
(e) 
RELATIVE HUMIDITY % 
BSS 
(eo) 
30 
4 5 6789106 2 3 
4 5 67 89109 2 3. 
RADIUS IN CM 
4 56789104 
Fig. 3.—Curves relating the growth of various nuclei to humidity. 
1. Measured values of the growth of newly formed gas-flame ions [19]. 
2, 5a, 8b. Calculated curves of the growth of pure droplets of solutions whose radii at 100 per cent relative humidity have 
values of 2 X 10-5, 10-°, and 10~4 cm, respectively. 
3. Growth by adsorption of a solid nucleus with the surface properties of glass and r = 10° em [40]. 
4, 6, 7, 9 and 10. Curves for the growth of mixed nuclei whose radii at 100 per cent relative humidity are successively 2 X 
10-5, 10-*, and 10-* cm and whose solid portions have the radii indicated by the hatched lines. 
5b. Modification of the growth curve 5a in the presence of 250 X 10-6g m-* of HCl and a nuclear count of 10* cm7*. HCl was 
chosen, since we have precise physical-chemical data for it. 
8a. Crystallization (with decreasing humidity) and solution (with increasing humidity) of a NaCl nucleus (schematic). 
K. Point of condensation, boundary between the growth of the nucleus, and droplet condensation. 
above a relative humidity of 70 per cent. Wright [48] 
attributes the deviation at humidities below this value 
to the crystallization of NaCl which he believes to be 
predominant. However, Simpson [37] and Wall [39] 
correctly point out that in the case of crystallization the 
size of the droplets (Fig. 3, curve 8a) is bound to de- 
crease in a discontinuous manner until all the water is 
evaporated. Dessens [8] has been able to observe this 
process directly; he noted that the solution droplets 
may show a supersaturation which is greater the 
smaller the droplets, and that the ensuing violent con- 
densation sometimes results even in a shattering of the 
erystal. The renewed dissolution of these crystals will 
then take place at higher degrees of humidity. There is, 
however, no evidence for such abrupt size changes of 
the nuclei at intermediate humidities. 
On the other hand, there is a fairly close approxima- 
surfaces may adsorb multimolecular layers of water at 
relatively high humidities, a process that may become 
significant considering the original smallness of the 
nuclei. An example of such a growth of a solid particle 
having r = 10-® em and the surface properties of 
glass, is given by curve 3 in Fig. 3. Comparison with the 
solution droplets of curve 2 shows that growth by ad- 
sorption becomes decisive for sizes below that value. 
Evidently at these dimensions the borderline between 
solution and adsorption becomes confused [39]. This is 
in accord with the observation [19] that inscluble but 
flaky nuclei require a higher degree of humidity for 
their first condensation than for their second; thus they 
seem to disintegrate at the first condensation, and the 
finely divided substance goes, so to speak, “into solu- 
tion.” 
As yet, no attention has been paid to the influence of 
