458 3Ir. 0. T. E. Wilson [Feb. 19, 



WEEKLY EVENING MEETING, 



Friday, February 19, 1904, 



His Grace The Duke of Northumberland, K.G. D.C.L. F.E.S., 

 President, in the Chair. 



C. T. E. Wilson, Esq., M.A. F.E.S., 

 Fellow of Sidney Sussex College, Cambridge. 



Condensation Nuclei. 



A FAMILIAR experiment was first shown illustrating the action of 

 ordinary dust particles as condensation nuclei. From a large globe, 

 which had been allowed to stand for some hours, some of the air was 

 removed by opening communication with an exhausted vessel. Only 

 a very few drops were formed as a result of the expansion. On 

 allowing air to enter the globe through a cotton-wool filter, so that 

 the pressure was brought back to its original value (that of the 

 atmosphere), and allow^ing the air to exjmnd as before, the drops 

 formed were again very few. The ordinary air of the room was now 

 admitted ; an expansion of the air in this case resulted in the pro- 

 duction of a thick fog. 



When air has been freed from dust by filtering, or by repeatedly 

 forming a cloud by expansion, and allowing it to settle, the vapour 

 which, in the presence of the nuclei would have separated out in 

 drops, must be in the " supersaturated " condition immediately after 

 the expansion is comi^leted. 



Another method of producing clouds was now shown. Air was 

 allowed to escape through a fine orifice into an atmosi3here of steam ; 

 the mixed air and steam were then passed through a Liebig's con- 

 denser, where the greater part of the steam was condensed, and then 

 into a large glass globe, where the clouds were observed. From this 

 vessel the air was drawn off by a pump which maintained the 

 pressure in the globe and condenser at a considerable number of cms. 

 of mercury below that of the atmosphere. Before reaching the jet 

 the air of the room had to pass through a cotton- wool filter, and then 

 through a long tube containing water ; finally it was led through an 

 aluminium tube to the orifice. The latter was about half a mm. wide. 

 The fall of pressure in passing through the orifice was about 15 or 

 20 cms. In the absence of the filter, the air being admitted 

 directly to the water tube through a tap turned just sufliciently to 

 give the same flow as with the filter, a dense fog poured out from the 

 end of the condenser tube ; on closing the tap and letting the air 



