506 Dr. W. C. McC. Lewis on the Nature of the 



It is evident from the foregoing table that the average 

 surface densities are distinctly greater than the bulk 

 densities — in fact, approximately 2*5 times the bulk density 

 for the above-mentioned liquids. 



In view of the apparent generality of the phenomenon of 

 increased surface-density (over bulk) one is naturally led 

 to inquire how far this may be connected with, or exert an 

 influence upon, the molecular surface energy, which in the 

 hands of Eotvos * from the theoretical standpoint, and later 

 in the hands of Ramsay and Shields f from the experimental, 

 constitutes the basis of all our numerical conceptions of 

 molecular complexity, or state of aggregation in pure liquids. 

 Eotvos showed that the temperature coefficient of the 

 molecular surface energy would follow the same curve for 

 all liquids. Having carried out experiments with ether he 



found that - , — was constant over a wide range of tem- 

 dt to 



perature, and therefore of course this linear relationship 

 should hold for all liquids. Since the expression includes 

 the surface tension <y it is evident that in the first instance 

 at any rate any deductions as to molecular state must be 

 applied to the surface layer, and it is rather curious that the 

 mean numerical value for ' ; normal" liquids comes out 2*1, 



a number not differing much from the — ratio 2*5. 



It might therefore be thought at first sight that " molecular 

 complexity " was simply a different degree of surface density, 

 but that this is not so is amply demonstrated by the fact that 

 such characteristically " non -associated" (normal) liquids as 

 the esters and hydrocarbons (aliphatic and aromatic) have 



practically the same — ratio as bodies such as water, methyl 



alcohol, and acetic acid, which are characterized by marked 

 association. Further, Eotvos' original considerations, 

 depending as they do upon general principles of corre- 

 sponding states, would be practically independent of surface 

 density changes as long as the surface was in equilibrium 

 with the bulk, which of course is the case }. 



* Eotvos, Wied. Ann. xxvii. p. 448 (1885). 



f Ramsay and Shields, Zeitsch. fiir phys. Chemie, xii. p. 433 (1893). 

 % The form of Eotvos' expression does require a little modification, 

 but the conclusions as to complexity will probably not be altered. 



