Critical Phenomena of Ether. 819 



only the layers of ether at the junction of the two masses is 

 of the density most favourable to the production of opal- 

 escence. The rate at which the opalescence spreads is 

 dependent upon the rate of diffusion of the gaseous impurity, 

 which is very slow. Even the slight impurity contained in 

 tube XIII. seemed sufficient to delay visibly the establish- 

 ment of equilibrium. If tubes XIII. and XIV. were raised 

 simultaneously slightly above 6 C both tubes quickly showed 

 opalescence in one portion (in tube XIII. this was above the 

 transition zone), but the opalescent column in XIII. remained 

 well defined for a few minutes after that in XIV. had become 

 diffuse. It appears then that the readiness with which the 

 opalescence becomes uniform throughout the substance, when 

 the tube is heated slightly above C , may be taken as a 

 criterion of the freedom of the substance from gaseous 

 impurity. 



For the purpose of exaggerating the effects of impurity 

 a tube XVII. was filled with ether contaminated with 5 per 

 cent, of alcohol. The air was expelled from the tube, pre- 

 paratory to sealing, by boiling off the excess of ether over 

 mercury. The ether was heated, however, near the surface, 

 so that the lower layers were unaffected and retained the 

 dissolved air. The critical temperature 6 C for this tube was 

 approximately 19 6°'2 C. It was maintained at the highest 

 temperature for which the thermostat was constructed (about 

 196°'7 C.) for 30 mts. At the end of that time a very well- 

 marked transition zone was still made evident by the squared 

 paper placed behind ; it extended through a height of about 

 10 mm. only and was slightly opalescent. The temperature 

 was then very slowly reduced, the reduction of o, 5 occupying 

 about 30 mts. ; at the same time the transition zone was 

 watched carefully through a telescope containing a scale eye- 

 piece. It was observed that the transition zone contracted 

 slightly in height, whilst the relative displacement of the 

 upper and lower parts of the vertical lines became more 

 marked. The opalescence in the zone became gradually 

 denser and more sharply defined. At the same time it be- 

 came shallower : at 196°*4 C. it was a dense cylinder about 

 2*5 mm. in height, and at 196°*2 C. its height was scarcely 

 1 mm. The opalescence appeared to have contracted into 

 the upper part of the transition zone, for the curvature of the 

 vertical lines was still visible underneath. At 196°*2 C. the 

 meniscus appeared in the opalescent zone. The transition 

 zone had persisted for an hour, and at the end of that time 

 was still clearly marked. The concentration of the opal- 

 escent material was probably only apparent, and the zone of 



