I 34 MOLECULAR GROUPING 



Let us now put together the chief results with regard 

 to mutual conversion ; these refer firstly to cases of poly- 

 morphism in which a conversion takes place slowly in one 

 direction or the other, due to contact with the other modi- 

 fication. In these cases we may clearly describe one 

 modification as stable, the other as meta-stable. 



A. The Stable Modification must have the Smaller 

 Vapour Pressure and the Smaller Solubility, 



The necessity of this rule follows because only in this 

 case would a change from the meta-stable to the stable 

 phase necessarily occur by means of the vapour, or 011 

 contact with a solvent by means of it. If the reverse 

 condition of solubility held, the transition would take 

 place in the reverse direction on suitable application of 

 a solvent; the direct conversion would then allow of a 

 possible cyclic process that would be equivalent to a per- 

 petual motion. A direct confirmation of the greater solu- 

 bility of the meta-stable variety has been given in the 

 case of magnesium-chloride octohydrate *, of which the 

 saturated solutions possess the following composition : 



MgCl 2 . 8H 2 O a (stable) MgCl 2 . 11-43 H 2 O (-16 8) 



MgCl 2 . 8 Ho Op (meta-stable) MgCl 2 . 11-04 H 2 O (-16-8) 



B. The Stable Modification must have the Higher 



Melting Point. 



In Fig. 25 the saturation pressure P of the two modi- 

 fications I and II is represented as a function of the 

 temperature, and I possessing the higher pressure is con- 

 sequently the meta-stable phase. If now we draw a third 

 curve, III, representing the vapour pressure of the liquid 

 substance, the points of intersection, A and B, with I and II 

 obviously represent the melting points of the two forms, 

 and, as may be seen, A, the melting point of the meta- 



1 Van 't Hoff, Meyerh offer, Zeitschr.f. Phijs. Chem. 27. 87. 



