182 Prof. W. Ramsay and Miss E. Aston. [June 14, 



it is quite unnecessary to take measurements at temperatures very 

 close to the critical temperature, because, if that temperature is 

 known, there can be only one curve joining the points experiment- 

 ally found at somewhat lower temperatures and the critical tempera- 

 ture. The form of the curve is such that no doubt can exist as to its 

 course. Indeed, with chlorobenzene, measurements were not carried 

 out at all in the immediate neighbourhood of the critical point, but 

 only at much lower temperatures, and yet there could be no question 

 as to the course of the curve, when it was mapped. But these are 

 minor points ; and it is very gratifying to find that the material 

 provided by Dr. Shields and myself affords such a remarkable con- 

 firmation of the justice of Professor van der Waals' views. 18th May, 

 1894. 



III. " The Molecular Surface-Energy of Mixtures of non- 

 associating Liquids." By Professor WILLIAM RAMSAY, 

 Ph.D., F.R.S., and Miss EMILY ASTON, B.Sc. Received 

 April 26, 1894. 



It has been shown in the previous paper that it is possible to cal- 

 culate the degree of association of an associating liquid such as 

 alcohol, on the assumption that molecules of less complexity remain 

 uniformly distributed along with molecules of greater complexity 

 throughout the liquid, and that no one kind of molecule tends to 

 congregate on the surface to the exclusion of the other. It is neces- 

 sary, however, to justify this assumption ; and for this reason experi- 

 ments have been made on mixtures of liquids the molecules of which 

 do not unite to form complex groups ; such are most of the liquids 

 investigated by Ramsay and Shields (' Trans. Chem. Soc.,' vol. 63, 

 p. 1099, etseq.). 



The experiments of which an account is here given, show that 

 while the height to which a mixture of two liquids ascends in a capil- 

 lary tube is not the mean of the heights to which each singly would 

 ascend at the same temperature, while the surface-tensions and the 

 surface-energies are not necessarily the mean of those possessed by 

 the liquids unmixed with each other, regard being paid to their rela- 

 tive proportion in the mixture, yet the coefficient of decrease of mole- 

 cular surface-energy, and consequently the calculated molecular 

 weights, are true means of those of the two liquids. 



The substances used in these experiments were chosen in pairs, and 

 as it was necessary in closing the tubes to evaporate some of the con- 

 tained liquid in order to ensure the expulsion of air, mixtures of such 

 liquids were taken as possess approximately equal boiling points, so 



