138 MOLECULAR GROUPING 



temperature P the concentrations (7 [5 C u are equal, so that 



'RdT 



Here, however, the second term is a quantity independent 

 of the solvent, depending only on the heat of transition R, 

 and which, therefore, remains unaltered even if C l , C u refer 

 to vapour, so that their ratio is that of the saturation 

 pressures. 



This law, which might easily be proved for, say, calcite 

 and aragonite, or for the sulphur modifications, retains 

 a certain applicability even to isomers more widely 

 separated, which do not become identical on solution. 

 Carnelley and Thomas l found for isomeric benzene 

 derivatives (para- and meta-nitraniline) the following two 

 empirical laws : 



1. Of isomeric compounds, that with the lower melting 

 point has the greater solubility. 



2. The ratio of solubilities is independent of the solvent. 

 The latter rule corresponds literally with our conclusion 



as applied to solutions. The former equally corresponds 

 to the fact of higher vapour pressure shown above to 

 accompany lower melting point and greater solubility. 



As is to be expected, however, the rule thus found is not 

 universal, since it does not apply to polymorphic bodies, 

 and so Walker and Wood 2 found, e. g. for ortho-, meta-, and 

 para-oxybenzoic acid in water and benzene, the following 

 solubilities: 



Ortho Meta 



Ortho Meta Para 



Meta Para 



Water .... 0-264 1-337 0-765 0-197 1-75 



Benzene . . . 0-97 0-0121 0-0052 80-2 2-33 



2. ACTUAL MOLECULAK ARRANGEMENTS. 



The answer to the question as to molecular arrangement 

 involves two essentially distinct problems, one with regard 

 to the relative position of molecule and molecule, the other 



1 Journ. Chem. Soc. Trans. 1888, p. 782. 2 1. c. 1898, p. 618. 



