814 Mr. F. B. Young on the 



effects o£ traces of permanent gas upon the intensity of the 

 opalescence are mentioned in Section ¥ ; it seems probable 

 that these effects are indirect and consequent upon the 

 influence of the impurity upon the density of the ether. 



The variation of the intensity of the opalescence with tem- 

 perature was examined. It was observed that the intensity, 

 which was greatest at C , decreased as the temperature 

 steadily rose above or fell below that temperature but 

 decreased at a diminishing rate, so that the opalescence dis- 

 appeared insensibly. It was, therefore, difficult to determine 

 the limiting temperatures, particularly as the opalescence 

 was viewed through six layers of glass. The lower limit, 

 however, appeared to be about o, 5-0 G, 6 below C , whilst there 

 was a suspicion of opalescence at the highest temperature 

 reached, i. e. about 3° above 6 C . 



The Nature of Opalescence. 



Various explanations of the nature of the opalescence have 

 been suggested. 



Some investigators have considered it to consist merely of 

 an emulsion of one phase in the other (13, 14). Just below 

 the critical temperature the liquid and vapour differ so little 

 in density that either phase may exist dispersed throughout 

 the other in the form of fine particles. Donnan (15) supports 

 this view, but suggests certain conditions of surface tension 

 which might account for the stability of the opalescent 

 particles. 



Kiister (16) suggests that the opalescence is due to the 

 variation in the temperatures of the individual molecules 

 according to the kinetic theory. Let the mean temperature 

 of the substance be just above the critical temperature ; then, 

 according to the theory of probabilities, many of the mole- 

 cules will be moving with a velocity much lower (or higher) 

 than corresponds to the mean temperature. When a sufficient 

 number of slow-moving molecules occur together for an 

 instant they constitute a minute drop of liquid. Any 

 individual drop will only have a momentary existence 

 since by hypothesis the drop has a lower temperature than 

 its surroundings, but new drops will be created incessantly 

 throughout the substance. In this way the opalescence above 

 the critical temperature is accounted for ; the opalescence in 

 the liquid and vapour phases just below the critical tempera- 

 ture may readily be explained in a similar manner. 



The theories stated have some interest in connexion with 

 the present inquiry inasmuch as they implicitly assign 



