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XXXVIII. On the Condensation Nuclei produced in Air 



and Hydrogen hy heating a Platinum Wire. By GwiLYM 



Owen, B.Sc. {Vict.), 1851 Exhibition Science Scholar of 



University College, Liveiyool; Christ's College, Cambridge^, 



[Plate XT.] 



Introduction. 



IF ordinary air saturated with water-vapour be suddenly 

 expanded, a dense fog is produced, owing to the con- 

 densation o£ the water-vapour on the numerous dust-particles 

 presnt in the air. 



These particles o£ dust can be removed from the air by 

 filtering the latter through cotton-wool, or by repeated ex- 

 pansions, by which j^i'ocess the dust-particles are carried 

 down to the bottom of the vessel. 



Aitken f found that by heating a glass tube containing 

 pieces of iron or brass wire, or by making a platinum wire 

 red-hot by means of an electric current, he could produce 

 sufficient nuclei to give dense fogs with a very small super- 

 saturation of the air, which was originally dust-free. He 

 was unable to detect any decrease in the weight of the wires, 

 even after nuclei sufficiently numerous to give many dense 

 fogs had been produced by the heating. Aitken did not 

 come to any definite conclusion as to the nature of the nuclei, 

 but suggested that they were very small particles of matter 

 emitted by the wire, also that some of them might be formed 

 by the condensation, during the expansion, of some vapour 

 given off from the wire when heated. 



The present investigation was undertaken at the suggestion 

 of Prof. Thomson in the hope of obtaining some further 

 information with regard to the production of condensation- 

 nuclei by the heating of a platinum wire. 



It will be convenient to mention here some results got by 

 C. T. R. Wilson J, as reference will be made to them in the 

 paper. Wilson found that when dust-free air, originally 

 saturated with water-vapour, is suddenly expanded, then 

 condensation takes place if the maximum degree of super- 

 saturation thus produced exceeds a certain limit. Measuring 



the expansion by — , the ratio of the final to the initial volume, 



he found that when -^ is less than 1*25, no condensation 



takes place in dust-free air. When — is greater than 1*25 



* Communicated by Prof. J. J. Thomson, F.R.S. 

 t Trans. Roy. Soc. Edin. xxx. p. 348 (1883). 

 i Phil. Trans. A. vol. clxxxix. p. 265 (1897). 



