produced hy Cooling Gases to Low Temperatures. 529 



was admitted. This peculiar effect we have investigated in a 

 series of experiments extending over several months, and the 

 present communication contains an account of the main 

 results obtained hitherto. 



It was at once found that it was not necessary to cool air 

 to a temperature so low as that of liquid air in order to 

 produce these condensation nuclei. Different gases have 

 been tried ; and with all those experimented upon (except 

 hydrogen) we have found that there is a more or less definite 

 critical temperature below which each gas has to be cooled 

 before the production of these nuclei can be detected. 

 Further, the smaller the pressure of the gas while undergoing 

 cooling, the lower is this critical temperature. The investiga- 

 tion thus resolved itself into an attempt to determine these 

 critical temperatures for various gases at different pressures, 

 and thereby possibly to obtain some information regarding 

 the origin of the nuclei. 



Pi 



A 



i 



TO EXPANSION 



APPARATUS 



& ASPIRATOQ* 



ID ASPIRATOR 



The final form of the apparatus used is represented in the 

 figure. Q is the cloud chamber of a Wilson's expansion 



