1894.] Molecular Surface-energy of Mixtures of Liquids. 191 



We see, from these experiments, that a mixture of two liquids 

 may either behave as a mean, or the liquids may influence each 

 other. With the first two pairs, toluene and piperidine, and 

 benzene and carbon tetrachloride, the liquids belong to very different 

 chemical types. With the second pair, the heights and densities 

 differ very greatly from each other, and although the values of 

 7 and of 7(Mv) il approach more nearly, there is still a marked differ- 

 ence. Yet the values of p, the density, 7, the surface-tension, 

 and (y(Mw)*, the molecular surface-energy of the mixtures, are iden- 

 tical with those calculated, within limits of experimental error. 

 Perhaps this statement should, in strictness, not apply to the den- 

 sities, yet there is no great divergence from the mean. 



It is therefore legitimate to state that in certain cases the molecular 

 surface energy of a mixture is the mean of those of its constituents 

 determined at the same temperature. 



The third pair of liquids, chlorobeiizene and ethylene dibromide, 

 give results belonging to a different category. Here the calculated 

 density is greater than that found. This implies expansion on 

 mixing. The values of 7 are also greater, and together with these 

 the values of 7(Mv) ! . But the rate of alteration of 7(Mu) 5 with 

 temperature is practically normal, and the mean molecular weight 

 can therefore be calculated with fair approach to accuracy. The 

 fourth pair of liquids give still more abnormal results, owing probably 

 to the fact that one of them has some power of association. 



It would be premature to discuss these results without much more 

 extended experimental evidence. Experiments have already been 

 made with mixtures of alcohol and other liquids, and an investigation 

 of mixtures of acetic acid is still in progress. The problem is a- 

 complex one ; we have to deal with the extent to which the associa- 

 tion of an associated liquid is altered by dilution ; and it will form 

 the subject of a further communication. We have, however, thought 

 it advisable to bring forward some results in this paper to avoid the 

 possible generalisation from the behaviour of the first two pairs of 

 liquids that the molecular surface-energy of all liquids is the mean of 

 those which they possess when unmixed. 



One question remains to be considered ; it is this : Is it justifiable 

 to assume that the molecular surface-energy of a mixture of the 

 associated and dissociated molecules of a substance whose molecular 

 complexity alters with temperature is the mean of each taken singly ? 

 For, on that assumption, the mean molecular weights of associating 

 liquids have been calculated. In our opinion it is ; but, as direct ex- 

 perimental evidence is not as yet attainable, it may be well to bear in 

 mind that, although a fair working hypothesis, it cannot be taken as- 

 proved fact. 



