794 Mr. F. B. Young on the 



o 



(ii.) Villard (2) points out that at the critical temperature 

 and just above it, the dilatation with rise or! tempe- 

 rature is extremely great. He attributes the differ- 

 ence of density to a slight but persistent difference in 

 temperature. In support o£ his explanation he has 

 reproduced the difference of density in a tube of 

 ethylene by gently heating the upper portion. Since 

 the mean specific volume was greater than the critical 

 volume, the effect must have been quite distinct from 

 that predicted by Gouy. 



(iii.) Onnes and VerschatTelt (3) ascribe the phenomenon 

 largely to the presence of traces of impurity in the 

 substance. If the substance contain a slight ad- 

 mixture of some other more volatile substance, the 

 latter might tend to collect in the vapour phase and, 

 by its partial pressure, would produce an effect similar 

 to that ascribed by Gouy to gravity. 



B. Liquidogenic Hypotheses. 



(iv.) Some investigators (amongst whom are De Heen 

 and Traube) * consider that the difference of density 

 corresponds to an actual difference of molecular 

 structure. The liquid molecules or liquidons are of 

 greater mass than the gaseous molecules or gasons ;. 

 the meniscus disappears when the two phases become 

 mutually soluble in all proportions, but homogeneity 

 of molecular structure occurs only at a temperature 

 (T c ), which is higher than that of the disappearance of 

 the meniscus (0 C ). It is urged that the differences of 

 density observed are both too great and too persistent 

 to be due to differences either of pressure or of tempe- 

 rature, whilst it is claimed that impurities are elimi- 

 nated by the methods of preparation. 



The investigation to be described was undertaken with the 

 purpose of examining the Cagniard-Latour phenomena in 

 the light of the various explanations offered; 



* The results obtained by De Heen (Mathias, Point Critique des 

 Corps Purs, p. 197, P. do Heen, Bull, de VAcad. roy. de Belgique [3] 

 t. xxxi. pp. 147 et 379, 1896), and by Teichner {Ann. d. Physik, Feb. 

 1904) have been critically examined in the laboratory of K. Onnes (loc. 

 cit.), and the results of the inquiries warrant a strong presumption that 

 the marked differences of density are to be attributed to the presence of 

 a small percentage of impurity in the substance assumed to be pure. 

 Since, however, Mathias regards the controversy between the Classical 

 and Liquidogenic Theories as still an open one, additional experimental 

 material relating to the critical phenomena of pure substances seemed 

 to be highly desirable. 



