189 
1911-12.] The Sun as a Fog Producer. 
surface one ; so that, though high humidities increase the hazing effect of 
this dust, they only do so to a slight extent. For instance, with a wet-bulb 
depression of 3° the hazing is only twice what it is with a depression of 8°. 
The other kind of nucleus, however, has an affinity for water vapour, 
and condenses it into minute drops in even unsaturated air, and so causes 
dense fogs. The first kind is the most common, and it is the one which 
generally forms haze in pure districts. The second is generally found in 
abundance in inhabited areas, and is the true fog-former, since it forms fogs 
in even unsaturated air ; and as it causes condensation in unsaturated air, 
we will call its action spontaneous condensation. As these nuclei have an 
affinity for water, each particle tends to persist — unlike cloud particles, 
the smaller drops of which tend to evaporate and hand over their vapour 
to the larger ones, so causing a decrease in their number. The fog particles 
have no such tendency, as they each tend to hold their own share of water, 
since any tendency for the particles to evaporate is checked by the 
concentration of the impurities in the smaller ones and by dilution in the 
larger, thus increasing the affinity in the smaller and decreasing it in the 
larger. Nuclei with affinity for water, therefore, tend to persist and retain 
their numbers and fogging effect. 
In a previous communication, read before this Society in February 
1890, there is described a somewhat rough method of finding the condensing 
powers of the different kinds of dust. It is shown that the dust produced 
by burning magnesium begins to condense vapour at about the same 
temperature as does a glass surface — that is, at the dew-point. Dust from 
gunpowder smoke condensed at a temperature 5° above the dew-point; 
dust from burning sodium condensed at a temperature as much as 1 7° 
above the dew-point ; and the ordinary dust in the atmosphere condensed 
at about 2 ‘3° above the dew-point. There would no doubt be water 
condensed on all these kinds of dust at all degrees of dryness, and these 
figures only show at what degree of dryness the dust began to show a 
visible change, due to a great increase in the amount of condensed vapour. 
The figure for dust in ordinary air corresponds very much with what we 
find from observations on the action of dust in the atmosphere — namely, a 
quick thickening of the air when the wet-bulb depression goes under 2°. 
From the above it is evident that it is not enough to show that the 
sun can form nuclei out of any particular impurity : we must also find 
whether or not the nuclei have any affinity for water vapour. In the 
one case the nuclei will only form haze more or less dense, and not a fog, 
in unsaturated air. Only those nuclei which have an affinity for water 
can be called true fog-formers in unsaturated air. 
