466 Dr. G\ Owen and Mr. H. Pealino- on Condensation 



measure of the expansion the pressure-fall — -that is, the 

 difference between initial and final pressure of the gas in 

 the apparatus, — then, as is well known *, there is no con- 

 densation in the body of the gas wlien the pressure-fall is 

 less than 15 cms. of mercury. When the expansion is 

 between 15 cms. and 20 cms. a few scattered drops are 

 observed (" rain-like " condensation), the nuclei in this case 

 being the few ions always present in the gas. On subjecting 

 the gas to a pressure-fall over 20 cms., a dense fog is 

 obtained. The nuclei on which this fog forms are generally 

 regarded as being minute drops of water continually being- 

 formed from the saturated water-vapour. In the present 

 paper we shall refer to the above effects as the " Wilson 

 effects." 



Results of the Tests made with Camphor, Naphthalene, 

 Benzoic Acid, and Iodine. 



With camphor, naphthalene, benzoic acid, the showers 

 or clouds obtained on expansion were identical with the 

 usual Wilson effects. Evidently, then, these substances do 

 not sublime in the form of molecular aggregations sufficiently 

 large to act as condensation nuclei. C. Barns f had 

 previously obtained a similar result with a somewhat different 

 apparatus for camphor and naphthalene. With iodine, how- 

 ever, we obtained very marked effects, as is shown in the 

 following table. The figures in the columns marked 



Table T. 



Cloud-chamber filled with pure moist 

 dust-free air. 



Cloud-chamber filled with air 

 which had passed over iodine. 



Pressure-fall 



in cms. 



Observation. 

 (Ordinary Wilson effects.) 



I 



Pressure-fall 

 in cms. 



Observation. 



15-0 







15-0 



Few drops. 



16-5 



Few drops. 



16-5 



Thin shower. 



17 5 



Thin shower. 



17-5 



Good shower. 



18-5 



Good shower. 



185 



Tinted cloud. 



19-5 



Very dense shower. 







20-5 



Fog. 







* C. T. R. Wilson, Phil. Trans. A. toI. cxxxix. (1897). 

 t C. Bams, ' Condensation of Vapor as induced by Nuclei and Ions. 

 (Carnegie Institution of Washington, May 1907.) 



