1889-90.] Mr J. Aitken on Dust Particles. 239 
expect, as fogs are formed when the air is still, and we have seen 
that when this happens the quantity of dust increases. The ex- 
planation of the formation of fogs would appear to he, — calms tend 
to increase the quantity of dust and moisture in the air near the 
ground, and as the dust increases the radiating power of the air it 
soon gets chilled to the condensing point, when fog forms. The 
density of the resulting fog depends in part on the quantity of dust 
present, as town fogs are thicker as well as blacker than country 
ones. These remarks only apply to radiation fogs, and not to fogs 
resulting from the mixing of hot and cold air, nor to fogs formed 
by reduction of pressure. Fogs seem to be much more frequent in 
town than in country air. This possibly may be due in part to the 
greater amount of dust in the city air increasing its radiating power, 
so causing it to become colder than country air, when there is no 
protection above it. If the air was clear and perfectly diather- 
manous, then the radiation would be only from bodies on the earth’s 
surface. The air would then be cooled by these surfaces, and 
deposit its vapour on them as dew ; but when there is much dust 
in the air, these particles acting as radiating surfaces get cooled, and 
the vapour being deposited on them, fog is the result. 
Speculations. 
Another question which may be asked is, Is there any relation 
between the amount of dust and the temperature of the air ? It 
has been shown that there is a distinct connection between the 
amount of dust in the air and its transparency. We are, there- 
fore, led to expect that the dust will have some effect on the 
temperature, by the alteration it will effect on the diathermancy 
of the air. In order to see if any information can be gained on this 
point from these observations, there are entered in the diagrams, 
Plates I, II and III, the maximum and minimum temperatures 
of the air on the days on which the dust observations were taken. 
It is evident that there are two distinct effects of the dust which 
we must distinguish. First, its effect on the heat received by the 
earth from the sun ; and second, its effect on the heat radiated into 
space by the earth. We may expect the former of these effects will 
be best seen in summer, when the sun is high, while the latter will 
be best studied during night temperatures and during winter. 
