Critical Phenomena of Ether. 813 



which, therefore, presumably possessed the critical density, 

 it seems that opalescence can occur in ether only if the 

 density approximates to the critical density. Again, since 

 conversely in every case in which the intensity of the opales- 

 cence was not uniform the meniscus appeared in the part 

 in which the opalescence was most marked, it may be inferred 

 that the critical density is the density most favourable to the 

 production of opalescence. 



Travers and Usher (7) observed that if the mean density 

 of filling differs from the critical density, so that as 6 C is 

 approached the meniscus is either rising or falling in the tube, 

 then the disappearing phase becomes markedly more opales- 

 cent than the increasing phase. They suggest an explanation 

 based upon Donnan's theory concerning the surface-tension 

 of small drops, and connect the localization of the opalescence 

 with the motion of the meniscus. This phenomenon was 

 strikingly apparent in tubes XIII. and XIV. when they were 

 raised simultaneously to the critical temperature ; the liquid 

 phase in XIV. became intensely opalescent before the vapour 

 showed more than faint indications, whilst in tube XIII. 

 the conditions were reversed. The phenomenon, however, 

 receives a simple explanation from the dependence of the 

 intensity of the opalescence upon the density of the substance. 

 Unless the mean specific volume is equal to the critical 

 volume, the meniscus can only disappear within the limits of 

 the tube if, at the time of disappearance, the two phases 

 differ in density, and hence at 6 C only one of the phases can 

 approximate to the critical volume. If the mean specific 

 volume is less than V c , this phase will be the phase of lesser 

 density, i. e. the vapour phase, which is also the diminishing 

 phase ; it is this phase whose density will be more favourable 

 to the existence of opalescence. The reverse is true if the 

 mean specific volume is greater than the critical volume. 

 This explanation is in effect identical with the conclusion of 

 ►Sidney Young (12) that the position of maximum opalescence 

 depends upon the mean specific volume, since of course the 

 actual specific volume at any given height in the tube and 

 the mean specific volume are interdependent. 



That the presence of the meniscus is unnecessary for the 

 production of the phenomenon is shown by the effects 

 described in Section C (4) where the local opalescence 

 appeared after the fading of the meniscus ; under the con- 

 ditions of Section C (4) the same striking difference was 

 apparent between tubes XIII. and XIV. The same indeed 

 was true when the transition zone produced above 6 C by 

 means of the heating-coil was allowed to disappear. The 



