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LIIT. Note on the Production of Nuclei in Air by Intense 

 Cooling. By GrWILYM Owen, M.A., D.Sc, University of 

 Liverpool*. 



IF a sealed glass bulb containing moist dust-free air be 

 plunged into liquid air, a dense cloud may be seen 

 forming inside the bulb. On removing the liquid air and 

 allowing the bulb and contents to warm up to atmospheric 

 temperature, the drops evaporate and the cloud disappears. 

 This effect is, of course, the cloud-like condensation which 

 C. T. R. Wilson f has shown to take place when a moist 

 dust-free gas is subjected to a sudden expansion of magnitude 

 1'38, the nuclei of condensation being accidental aggregations 

 of water-molecules which appear to be always present in 

 saturated water-vapour. If a similar experiment be tried 

 with a dry gas (such as is obtained from boiling liquid air), 

 then no visible cloud can be observed on cooling the bulb in 

 the way described. It has, however, been shown + that even 

 in the dry gas large numbers of small particles are produced 

 by the cooling process. These particles are not visible 

 directly, but their presence can be shown by admitting some 

 of the gas into a AVilson expansion apparatus § and subjecting 

 it to a sudden expansion (of magnitude 1*15 about). The 

 water-vapour in the expansion apparatus thus condenses 

 round the invisible particles, and a dense cloud is obtained. 

 It may also be remarked that so low a temperature as that 

 of liquid air is not necessary for the production of these nuclei. 

 A temperature of —130° C. is sufficient. 



Now it is obvious that no definite conclusion can be drawn 

 regarding the origin and nature of these nuclei produced in 

 a gas by intense cooling if there is any doubt as to the gas 

 being really dry. For, if the gas contains only traces of 

 moisture we may explain the effect as being due to the forma- 

 tion (at the very low temperature) of aggregations of water- 

 molecules, too small to be themselves visible, but yet 

 sufficiently large to be in equilibrium and to act as conden- 

 sation nuclei. 



On the other hand, if we supposa that the precautions 

 taken by Owen and Hughes to obtain dry air warrant the 

 assumption that the cooled gas was as free from water-vapour 

 as it can possibly be obtained, then it is necessary to consider 

 the view tentatively . suggested by them, namely, that the 



* Communicated by the Author. 



t C. T. R. Wilson, Phil. Trans, clxxxix. p. 265 (1897). 



t G. Owen & A. LI. Hug-hes, Phil. Mao-. Oct. 1907, June 1908. 



§ C. T. R. Wilson, Proc. Camb. Phil. Soc. p. 333 (1897). 



