ASSOCIATION AND DISSOCIATION. 175 



of state takes place without any change in molecular 

 complexity, which is, in many cases at any rate, highly 

 improbable. In fact if the vapour density of a given 

 substance is known, it would be necessary to ascertain 

 whether and to what extent molecular changes accompany 

 liquefaction and solidification, before the molecular weight 

 of the substance as liquid or as solid would be known. 



A large number of attempts have been made from 

 different sides to gain an insight into the molecular con- 

 ditions of liquids and solids, and in the case of liquids the 

 problem has now been brought within a measurable distance 

 of a successful solution. For methods have been devised 

 which serve to show whether the molecules of a liquid are 

 more complex than those of its vapour, and which enable 

 the degree of complexity to be determined approximately 

 in certain cases. 



The most successful of these methods is undoubtedly 

 that due to Ramsay and Shields. This method is based on 

 the fact that the molecular surface energies of different 

 liquids are found to be the same at comparable tempera- 

 tures. As the molecular surface energy is the product of 

 the surface tension, and of the surface which contains unit 

 number of molecules, in practice the method is reduced to 

 the determination of the surface tension of the liquid 

 at different temperatures, for which only simple apparatus 

 is necessary and no special skill on the part of the experi- 

 menter is required. 



The investigation of the molecular surface energy of 

 different liquids has clearly shown that liquids must be 

 divided into two classes. A^ large number of liquids turn 

 out to have the same molecular weights as their vapours, 

 and these liquids have therefore been termed normal or 

 monomolecular. On the other hand certain liquids are found 

 to have molecular weights which are greater than those of 

 their vapours, and such liquids have been termed associa- 

 ting. With liquids of this last class, if M is the molecular 

 weight deduced from the vapour density, the molecular 

 weight of the liquid will be xM, where x is called the 

 factor of association. As far as has been at present ascer- 



