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XI. Condensation of Water Vapour in the Presence of Dust-free Air and other Gases. 
Bij C. T. II, Wilson, B.Sc. (Viet.), M.A. [Canted).), of Sidney Sussex College, 
Clerh-Maxwell Student in the University of Camhridge. 
Communicated hy Professor J. J. Thomson, F.R.S. 
Receiv’ed March 15,—Read April 8, 1897. 
The behaviour of air saturated with aqueous vapour and allowed to expand suddenly, 
has been investigated by Couliee,'"' Aitken,! Kiessling.| and E. v. Helmholtz.§ 
As is well known, if the moist air has been previously freed from “ dust,” no con¬ 
densation takes place except on the walls of the vessel, even if the expansion be 
sufficient to produce considerable supersaturation. For convenience, the term “dust” 
is here used to include all nuclei which can be removed either by filtering or by 
repeatedly forming a cloud by expansion and allowing it to settle. 
What is the limit, if such exists, to the degree of supersaturation which can be 
attained without condensation taking place throughout the moist air, is a question of 
considerable meteorological as well as purely physical interest. It was primarily with 
the object of finding an answer to this question that the experiments to be described 
were undertaken, such experimental evidence as already existed on the subject being 
of a very incomplete and contradictory character. 
Aitken|| observed condensation when a large quantity of steam was passed into a 
receiver containing air which had been filtered through cotton wool. Kiessltng!! 
also produced a rainlike condensation in the same way. The latter observer also 
states that if saturated filtered air be even slightly expanded, scattered drops are 
formed visible only in direct sunlight. Again, Aitken^'^ mentions that in his dust¬ 
counting experiments, in which sudden expansion of the saturated air was produced 
by means of an air-pump, a very quick stroke of the pump was found to produce a 
* ‘ Journal de Plaarmacie efc de Cliimie,” vol. 22, pp. 165 and 254, 1875. 
t ‘ Tran,s. Roy. Soc.,’ Edin., vol. 30, p. 337, 1880-81, and vol. 35, p. 1, 1890; ‘ Proc. Roy. Soc.,’ 
Lond., vol. 51, p. 408, 1892. 
t ‘Hamburger Abbandl. der Haturwissenschaften,’ vol. 8, 1884; ‘ Gbtting. Nachr.,’ p. 122, 1884, and 
p. 226, 1884. 
§ ‘ Wied. Ann.,’ vol. 27, p. 509, 1886. 
II ‘ Trans. Roy. Soc.,’ Edin., vol. 30, p. 337. 
^ ‘ Gotting. Hachr.,’ p. 226, 1884. 
** ‘ Trans. Roy. Soc.,’ Edin., vol. 35, p. 1, 1890. 
MDCCCXCVII.—A. 2 M 
24,8.97 
