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XLIV. 
ON HAZE, DRY FOG, AND HAIL. 
By W. N. HARTLEY, D.Sc., F.B.S., 
Royal College of Science, Dublin. 
[Read Novemprr 20; Received for Publication, Drcemprr 13, 1901; Published 
Frsruary 27, 1902.] 
Tue work of Mr. John Aitken, F.R.S., on Dust, Fogs, and 
Clouds’, led him to the conclusions which here follow :—first, that 
wherever water vapour condenses in the atmosphere, it always 
does so on some solid nucleus; secondly, that dust particles in 
the air form the nucleus on which vapour condenses.? The 
sources of atmospheric dust, besides ocean spray which becomes 
converted into a fine dust of salt, are the the products of com-. 
bustion, together with almost all substances which are strongly 
heated, since these have been shown to contribute minute solid 
particles to the atmosphere. ‘The fewer the dust particles, the 
more moisture condenses upon them, and they thus become 
heavier and fall. The more numerous they are, the less is the 
moisture condensed upon each of them, so that they remain 
suspended as fog, instead of falling as rain, hail, or sleet. 
These islands are seldom free from cloud, rain, or fog, and 
minute quantities of dust, in a more or less moist condition, are 
nearly always present in the upper atmosphere. Observations on 
the number of dust particles in measured quantities of air have 
been made for some time past on the summit of Ben Nevis. 
The prevailing winds with us are the west, south-west, and 
south ; and the warm moist-laden air of the Atlantic is the cause 
of the condensation of aqueous vapour upon the dust particles. 
1 Abstract, Proc. Roy. Soc. Edin., vol. xl., p. 14 (1880-1881). 
* More recent experiments have shown that when air is filtered or washed so that 
it may be believed to be free from dust, a sudden expansion of saturated air causes the 
formation of a rain or mist. The expansion required being in the ratio V2/V1 = 1:252, 
where V1 is the initial and V2 the final yolume. This does not however affect the 
question of dust-laden air. C. T. R. Wilson, Phil. Trans. vol. 189, p. 265, 1897; also 
vol. 193, p. 289, 1900. 
SCIENT. PROC. R.D.S., VOL. IX., PART V. el 
