1888-89.] Mr John Aitken on Dust Particles. 161 
conclude that all atmospheric haze is due to dust ; yet these 
indications are worth keeping in view for future consideration. 
It seems possible that the “ alpine haze,” about which so much 
correspondence has lately appeared in Nature , may be due to dust. 
This, however, can he determined only by actual observation. There 
is another point of vast importance in the economy of nature, to 
which attention ought to be directed, which is this. If further 
investigation should prove that the transparency of the air is due 
to its freedom from dust, then we would he inclined to think that 
the diathermancy of the air will also have a close connection with 
the quantity of dust in it. These two points seem worthy of future 
investigation. 
Our interest in the matter, however, does not end here. If dust 
should really prove to he a good absorber of the sun’s heat, then it 
will also be a most important factor in the formation of fogs in 
another way than has been already pointed out. If the atmosphere 
was perfectly diathermanous, then probably we should have no 
fogs. Because the cooling of the air at night would he done entirely 
by the cold radiating bodies at the earth’s surface ; the air passing 
over these cold surfaces would he robbed of its moisture as well as 
of its heat, and would thus he prevented from becoming saturated. 
As fogs are formed during calm weather, they cannot at all situations 
he caused by the mixture of hot and cold saturated airs, hut must fre- 
quently he produced by radiation from the atmosphere itself. Now, 
if dust is a good absorber, it will also he a good radiator ; and, as a 
consequence, an abundance of dust in the atmosphere will at night 
cause it to he rapidly cooled to the dew-point, when a fog will begin 
to form, and by its formation increase the radiating power of the air. 
The radiating power of dust is probably one of the causes of the 
greater frequency of fogs in towns than in the country, there being 
far more dust in town than in country air. After condensation 
begins, there is plenty of evidence of the radiating power of the 
particles. A good example of this is given in a letter by Mr E. J. 
Lowe, in Nature , vol. xxxvii. p. 319. In this letter is described a 
remarkable rime. The peculiarity was that the thickness of the 
rime was greater high up than low down. At 5 feet the length of 
the crystals was -J inch, and they gradually increased to 1 J inch at 
25 feet. This increase in the deposit upwards was due to the fog 
vol. xvi. 16/5/89 l 
