178 SCIENCE PROGRESS. 



this labile character of associating liquids which forms the 

 most important difference between liquids of this class and 

 those of the stable monomolecular order. 



Now hitherto it has been the general custom to com- 

 pare all liquids one with another in perfectly indiscriminate 

 fashion, in the hope of establishing general relationships 

 among liquids that might rank with those already estab- 

 lished for gases. This method has been attended with 

 a certain degree of success, and regularities of a striking 

 order have been observed among the molecular volumes, 

 the boiling-points, the molecular refractions, and other 

 properties of liquids. But exceptions of a very puzzling 

 order frequently arise along with the regularities observed, 

 exceptions for which in a large number of cases it has, up 

 to the present, been impossible to account satisfactorily. 

 It can now hardly be doubted that one great cause of such 

 exceptions is to be found in the fact that monomolecular 

 and associating liquids have been indiscriminately com- 

 pared one with another, when strictly speaking they are 

 not truly comparable. 



Investigation shows that such regularities as have 

 hitherto been observed appear in most marked manner 

 when monomolecular liquids alone are compared one with 

 another. The exceptions arise, as might be expected, when 

 associating liquids are included in the list, for associating 

 liquids are neither comparable with monomolecular liquids, 

 nor are they directly comparable with one another. 



In this connection it is important to call special atten- 

 tion to the case of water. Water occupies almost a unique 

 position among liquids on account of its high factor of 

 association, which is greater than that of nearly all other 

 associating liquids. In all comparisons of liquids one 

 with another, water should therefore strictly speaking 

 be given a place apart and be regarded as exceptional. 

 Unfortunately up to the present, for practical reasons, it 

 has been customary to select water as the typical liquid 

 compound, and to employ it always as the standard of 

 comparison. The choice is an unlucky one for the reasons 

 stated, and one that should be as far as possible abandoned. 



