Critical Phenomena of Ether. 811 



was judged by the degree of uniformity of the opalescence ; 

 this method is justified in Section F. 



The temperature of the tube was raised a little above 6 C 

 and maintained for a few minutes. The plunger was 

 vigorously raised and lowered several times in order to 

 ensure complete uniformity. The ether was then uniformly 

 opalescent throughout the tube. A current of about 

 0*5 ampere was then passed through the wire coil. It was 

 observed that the opalescence gradually faded away whilst 

 the lines seen through the tube assumed the appearance of 

 fig. 5 B ; the transition zone was just beneath the lower limit 

 of the coil. After the heating current had been switched off, 

 the appearance of the tube remained unaltered for about three 

 minutes ; a faint opalescence then appeared in the transition 

 zone which soon spread and filled the lower part of the tube. 

 The upper part of the ether then grew opalescent, and finally 

 the appearance of the tube became the same as it had been 

 before the coil was heated. The phenomena scarcely differed 

 from those described in Section (4). 



The transition zone was reproduced as before. The heating 

 current having been switched off, the temperature was slowly 

 reduced. It was observed that the transition zone became 

 opalescent, the limits of the opalescence being sharply defined ; 

 the opalescence grew denser until at length a meniscus 

 appeared within it. The opalescence then faded away. The 

 phenomena of Section C (5) were thus reproduced. 



An attempt was then made to compare the relative per- 

 sistencies of the natural and the artificial transition zones. 

 The temperature was suddenly raised from about 193°'4 C. to 

 193°' 73 C. in order to produce the phenomena of Section C (4). 

 The time was then taken which elapsed before the zone of 

 transition had lost all definition and the opalescence had 

 spread to the top of the tube ; this proved to be about 

 10 minutes. After a short interval during which the tem- 

 perature remained at 193°' 73 C, the heating current was 

 switched on for two minutes, and the time was again taken 

 which had elapsed before the appearance of the tube was 

 sensibly the same as at the end of the former test : the period 

 was about 8 minutes. 



The tests with the heating coil show it to be highly pro- 

 bable that differences of density above 6 C which persist for as 

 much as 8 or 10 minutes may be ascribed to residual tem- 

 perature differences produced by vaporization. 



Mention may be made of two effects observed when the 

 heating coil was used below the critical temperature. 



It was possible, by passing a stronger current through the 



3H2 



