532 Messrs. Owen and Hughes on Condensation Nuclei 



minute. When the number o£ nuclei produced is large, as 

 is the case when the gas is cooled well below its " critical 

 temperature/'' some nuclei can be detected quite half-an-hour 

 after their formation. In all the experiments to be described 

 we adopted a uniform interval o£ 90 seconds for the warming- 

 up process, this interval being considered sufficiently long 

 for the gas to acquire normal temperature and yet not long- 

 enough for an appreciable number of the nuclei to have 

 disappeared. 



(4) The number of nuclei " caught " is independent of 

 the value of the expansion used. On using large expansions 

 (greater even than an expansion of 1'25 which causes con- 

 densation on ions) no nuclei were detected in addition to the 

 ordinary Wilson effect, where none had been detected with 

 small expansions. On many occasions it was noticed 

 that the nuclei caused condensation in consequence of the 

 very slight supersaturation produced when adjusting the 

 pressure-drop in the expansion apparatus preparatory to 

 making an expansion. The small degree of supersaturation 

 thus required to cause condensation on the nuclei proves 

 them to be of considerable size. It is therefore unimportant 

 what expansion is employed. We found it convenient to use 

 a small expansion of 1 # 10 corresponding to a pressure-drop 

 of 7 centiins., and this was used throughout all the experi- 

 ments described below. 



Determination of the " Critical Temperature." 



In contradistinction to the results given above, the follow- 

 ing depend upon the nature and pressure of the gas cooled. 

 The values of the " critical temperatures " given represent 

 the mean estimates arrived at after a large number of experi- 

 ments. The gases tried were ordinary air, air from boiling 

 "iquid air, hydrogen, oxygen, nitrogen, and carbon dioxide. 



Air from Boiling Liquid Air. 



Liquid air was poured through two thicknesses of filter- 

 paper into the reservoir B, (fig. 1) until the latter was half- 

 full. The mouth of the reservoir was then closed by a small 

 rubber stopper. When the tap A was closed the air as it 

 boiled away escaped through the mercury at M. The rate of 

 boiling was regulated by surrounding R with a tall Dewar 

 vessel also containing liquid air. Before commencing the 

 experiment, the air escaping from R was allowed to stream 

 for half an hour or longer through the apparatus escaping 

 through D and E into the aspirator. While this was going 



