134 Scientific Proceedings, Royal Dublin Society. 



than those of isomeric substances in which the carbon chain is a normal one. 

 In the aromatic series the position of the subsfcituent group in the benzene 

 ring has an iniiuence on the distillation constant. Both in the case of the cresols 

 and the toluic acids the ortho compound has a greater distillation constant 

 than either the meta or the 2Mra derivatives. 



It is noteworthy that the distribution of o-, on-, and^j-toluic acids between 

 liquid and vapour phase in dilute solution bears the same relationship to each 

 other and to the constant for benzoic acid as the constants of o-, m-, and 

 ^-cresol and phenol. The distillation constants of n- and iso-butyric 

 acids show a similar relation to those of n- and iso-butyl alcohols. The 

 influence of constitution on the distillation constant is shown by o- and p- 

 hydroxy-benzoic acids, and by o- and^- nitro-phenols. The results in the case 

 of substances only sparingly soluble in water may be explained by a difference 

 in the vapour-pressure of the substances at the boiling-point of the solution. 



In the case of phenols and the lower fatty acids and alcohols the distilla- 

 tion constant appears to bear no relationship to the vapour-pressure. Pure 

 methyl alcohol at 100° exerts a. greater vapour-pressure than any of the other 

 homologous alcohols, yet in dilute aqueous solution there is a smaller dis- 

 tribution coefficient between liquid and vapour phases than is the case 

 with ethyl, butyl, or amyl alcohols. It seems to indicate other factors 

 which influence the distillation of aqueous solutions. The molecular weights 

 of the substance distilled have an influence in comparative distillations of 

 members of a series of compounds. In Tables II and III a comparison is 

 made between the Eamsay-Shields' association factor a;, the distillatiom 

 constant, and the boiling-point. It is seen that as the association factor 

 decreases the distillation constant increases. ' 



The association factor refers to the pure liquids ; consequently no direct 

 comparison can be made between the molecular stale of the pure liquid and its 

 aqueous solution. It shows, however, that for the lower members, as 

 the homologous series is ascended, a gradual modification occurs in the 

 attractions or forces exerted, between the molecules. The inter-molecular 

 forces are also modified by solution owing to the force exerted by 

 molecules of the solvent on those of the solute. For miscible liquids 

 or soluble substances, the greater the difference between the mean 

 attractions of the molecules of the solute for each other and the solute 

 for the solvent, the more abnormal will be the behaviour of the solution 

 on distillation.! "With the lower fatty acids and alcohols a gradual diminution 

 with increasing molecular weight occurs in the attraction between molecules of 



' Of. Berthelot, Compt. Rend., 1898, 126, 1703. 



