240 Mr. C. T. R. Wilson. Condensation of Water Vapour 



front of or behind the th rower ; in each of these cases success has 

 been attained. An explanation is also afforded of the returning of 

 a boomerang without " twist," made by Mr. 0. Eckenstein, and of the 

 wonderfully long, straight trajectories of some of the native non- 

 returning implements. 



" Condensation of Water Vapour in the presence of Dust-free 

 Air and other Gases." By C. T. R. Wilson, B.Sc. (Vict.), 

 M.A. (Cantab.), of Sidney Sussex College, Clerk-Maxwell 

 Student in the University of Cambridge. Communicated, 

 by Professor J. J. Thomson, F.R.S. Received March 15, 

 Read April 8, 1897. 



(Abstract.) 



In a note read before the Cambridge Philosophical Society (May 

 13, 1895) I stated, as the result of some preliminary experiments, 

 that when air, originally saturated with aqueous vapour, undergoes 

 sudden expansion exceeding a certain critical amount, condensation 

 takes place in the form of drops throughout the moist air, even in 

 the absence of all foreign nuclei. 



The present paper contains an account of the measurements which 

 were afterwards made of this critical expansion in air and other 

 gases, as well as of further phenomena wdiich have since been 

 observed in connection with the condensation of aqueous vapour from 

 the supersaturated state. 



Two different forms of expansion apparatus have been used. Both 

 were designed to enable a given sample of the saturated gas to be 

 suddenly expanded as often as might be desired without any risk of 

 foreign nuclei entering. All such nuclei originally present were 

 removed by repeatedly forming a cloud by expansion and allowing it 

 to settle, till expansions of moderate amount ceased to cause any 

 visible condensation. In both forms of apparatus a definite expansion 

 of any desired amount could be produced. They were designed to 

 give an exceedingly rapid expansion, the rate at which the volume 

 was increasing being greatest, too, just before the expansion was 

 completed, when the temperature was lowest and the influx of heat 

 from the walls most rapid ; so that, as indeed appears from the con- 

 stancy of the results obtained, the theoretical lowering of temperature 

 must have been very nearly reached. 



The two machines, in spite of the fact that the volume of the air in 

 the first was twenty times as great as that contained in the second, 

 gave identical results. The larger machine was only used in the 

 experiments on air. 



