Critical Phenomena of Ether. 815 



different values to the critical temperature. Whilst neither 

 of the theories makes any assumption either in favour of or 

 contrary to the liquidogenic hypothesis, the theories of 

 Altschul and Donnan imply that the Cagniard-Latour tem- 

 perature is below either the temperature of complete mutual 

 solubility of the liquidogenic hypothesis or that of uniformity 

 of state of the simple classical theory ; Krister's theory, how- 

 ever, would imply that the Cagniard-Latour temperature 

 coincides with one or the other of these. The fact that 

 ebullition may proceed concurrently with the existence of 

 opalescence of considerable intensity (v. Section C (15)) is 

 scarcely in accord with Altschul's simple explanation, since 

 it implies that an appreciable difference of density exists 

 between the phases. An observation of the condensation 

 phenomena also leads to the conclusion that there is a differ- 

 ence in kind between the stable opalescent cloud observed 

 above or below C and the cloud of condensation produced at 

 the moment of separation of the meniscus. If the tube, first 

 raised above C , is steadily and fairly slowly cooled down, the 

 opalescence increases in intensity as C is approached but 

 retains a " dry " nebulous appearance. At the moment of 

 appearance of the meniscus, however, the condensation cloud 

 appears to be superposed upon the opalescent cloud, some- 

 times flashing through the tube, sometimes spreading rapidly 

 from the bottom upwards. This is particularly evident when 

 the opalescence is localized, by the various means described, 

 to a shallow zone. The " dry " fog instantaneously changes 

 to a " wet ■" fog in a manner difficult to describe. When the 

 condensation cloud has settled, the opalescent cloud is still 

 visible enclosing the meniscus. The opalescent particles 

 probably supply the condensation nuclei, but seem themselves 

 to be something other than mere liquid drops or vapour 

 bubbles. This phenomenon may perhaps be explained in 

 accordance with Donnan's theory, if it is supposed that under 

 suitable conditions of cooling the increase in the bulk of the 

 opalescent material may tend to proceed rather by the increase 

 in the size of the existing particles than by the formation of 

 new aggregates, and that at length a certain proportion grow 

 beyond the limits of size for which the peculiar conditions of 

 surface tension which secure stability are true. 



The consideration of the intensity of opalescence as a 

 function of the temperature may, however, afford some 

 criterion in deciding between the theories stated : — 



Let a tube of capacity V contain 1 g. of the substance at 

 a temperature below 6 C ; if the volume of liquid is v whilst 

 u and Ui are the specific volumes of liquid and vapour 



