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XII. The Supersaturation and Nuclear Condensation of Certain Organic 
Vapour's. 
By T. H. Laby, B.A., Emmanuel College, Cambridge, Exhibition of 1851 Scholar 
of the University of Sydney, Joule Student of the Royal Society. 
Communicated by Professor J. J. Thomson, F.R.S. 
Received April 10,—Read April 30, 1908. 
Condensation in Water Vapour. 
The first precise measurements of the conditions causing condensation when dust-free- 
air saturated with water vapour is expanded were made by C. T. R. Wilson.* 
He calculated the cooling due to the adiabatic expansion, and then the super- 
saturation, which is the ratio of the actual pressure of the water vapour at the end of 
the expansion to the saturated vapour pressure of water at the temperature at the 
end of the expansion. 
Mr. Wilson obtained the results given in the following table, in which v 2 / i \ is the 
ratio of the initial to the final gas pressure in the expansion apparatus :— 
Gas. 
Rain-like condensation. 
Cloud-like condensation. 
Expansion 
v-ijvi. 
Supersaturation. 
Expansion. ! Supersaturation. 
i 
Air. 
1-252 
4-2 
1*38 A 
Oxygen. 
1 • 257 
4-3 
1 • 383 
>1-375 7-9 
Nitrogen .... 
1-262 
4-4 
1-38 
Hydrogen .... 
— 
— 
1-38 j 
Carbon dioxide . . 
1 • 365 
4-2 
1-535 
7-3 
Chlorine. 
1-3 
3-4 
. 
1-45 
(5-9) 
It was concluded from the size and number of the nuclei producing the cloud-like 
condensation that they were small aggregates of water molecules. 
VOL. ccviii. —-A 438. 
* ‘Phil. Trans.,’ A, 189, p. 265 (1897). 
24.8.08 
