Critical Phenomena of Ether. 809 



in the bell, being supplied with heat less rapidly than that 

 above, maintained equilibrium of temperature by partial con- 

 densation. By suitable manipulation it was possible near 

 the critical temperature to have two menisci in the tube at 

 different levels — one in the bell and one above the plunger. 

 It was thus unnecessary to bring the meniscus down from 

 above as originally contemplated. If the temperature was 

 then raised a little above C , sensibly the same phenomena 

 occurred in both menisci as have been described in 

 Section A (4). Hence there were two levels differing by 

 about 6 cm. at which zones of transition appeared and per- 

 sisted for several minutes until the opalescence became 

 uniform. Nor did the zone in the bell show any perceptible 

 tendency to rise. 



These phenomena favour the conclusion that the part played 



by hydrostatic pressure in the production of the transition 



zone is quite a subsidiary one. According to Gouy's pre- 



dv 

 diction there may be one level in the tube at which -=- is 



dp 



very great, but there can be only one such level. The exist- 

 ence of temperature differences in the vapour jacket, it is 

 true, makes this conclusion somewhat uncertain. If the 

 variation of temperature with height is uniform, the only 

 effect will be a tendency to annul the effect of hydrostatic 

 pressure and to prevent the formation of the transition zone ; 

 but if, owing to the separation of the tube into two sections 

 by the plunger, each portion of the ether tends to assume a 

 uniform temperature which is slightly higher for the lower 

 portion than for the upper, the portions may be considered 

 as representing short lengths of two isothermals in Andrews's 

 diagram separated by a very small temperature difference ; 

 and in this case it is conceivable that each column may 



dv 

 include the level at which — is a maximum for the given 



isothermal. It is probable, however, that, as in Villard's 



experiment (v. page 794), this level of maximum value for 



dv 



-r- was not contained within the limits of the tube. The 



dp 



behaviour of the meniscus at its appearance during slow 

 cooling, or its disappearance during very slow rise of tem- 

 perature, seems to show that the mean specific density was 

 so much less than the critical density that the level in 

 question would be somewhat below the bottom of the tube. 

 Phil. Mag. S. 6. Vol. 20. No. 119. Nov. 1910. 3 H 



