ASSOCIATION AND DISSOCIATION. 177 



liquid) is, in the case of monomolecular liquids, about 75, 

 and has generally a higher value for associating liquids. 

 The latent heat of fusion affords another indication of 

 association in liquids. The latent heat of fusion of unit 

 volume divided by the temperature of fusion on the absolute 

 scale is approximately constant and equal to ot for mono- 

 molecular liquids. Associating liquids give a higher value 

 and one that is not far removed from o'\x. 



Liquids then are of two kinds, monomolecular and 

 associating. A full recognition of this fact is of the highest 

 importance from the physico-chemical point of view, as it 

 serves to throw light on many of the vexed questions of 

 physical chemistry. And in the first place it is necessary 

 to clearly understand the character of the difference between 

 the two classes of liquids. 



Monomolecular liquids are liquids the molecular weights 

 of which are identical with those of their vapours when 

 in the state of perfect gas. On the liquefaction of the 

 vapour no change takes place in molecular composition, and 

 the liquid on being heated or cooled also suffers no change 

 in molecular composition. Such a liquid is therefore from 

 this point of view a perfectly stable system, the composition 

 of which is not affected by outside influence. 



Associating liquids are liquids the molecular weights of 

 which are not identical with those of their vapours when in 

 the state of perfect gas. In this case on the liquefaction of 

 the vapour a change occurs in molecular composition, and 

 the molecules of the liquid produced are more complex than 

 those of the vapour. But the change is not complete on 

 liquefaction, for the researches of Ramsay and Shields have 

 clearly shown that the factor of association is not a constant 

 in the case of any one associating liquid, and that it alters 

 with the temperature. As the temperature falls the factor 

 of association increases and the liquid becomes more and 

 more complex in composition ; with a rise in temperature 

 this change is reversed. From this point of view then, an 

 associating liquid is not a stable but a labile system, the 

 composition of which is subject to alteration with change of 

 temperature and perhaps other outside conditions. It is 



