548 SCIENCE PROGRESS 



Of the two crystalline modifications the more stable of the 

 two at any temperature must have the lower vapour pressure, 

 for if this were not so, on placing the stable and labile forms at 

 the two ends of a sealed tube, the stable, since it possessed the 

 higher vapour pressure, would distil over and the unstable 

 variety would grow at its expense, which is contradicted by 

 all experience. 



In the figure the continuous lines a, a and b, b represent the 

 vapour pressure curves of the polymorphic solid forms and the 

 dotted lines i, i and 2, 2 the vapour pressure curves of the liquid 

 in the two cases. 



The statement that in monotropic pairs of compounds the 

 melting-point is below the temperature of transformation is 

 misleading, since such a transformation could only be referred 

 to change within individual molecules or to a breaking down of 

 complex aggregates, and such changes according to current 

 views would be classed as isomeric or polymeric rather than 

 polymorphic. What is really intended is, of course, to express 

 the opinion that in all cases of monotropy the ratio between 

 the vapour pressure of the unstable variety and that of the 

 stable variety progressively diminishes as the temperature 

 approaches the melting-point. Ostwald in this explanation 

 apparently overlooks the possibility of cases where the transition 

 points lie much below the ordinary temperature and thus cannot 

 be realised or experimentally determined except approximately 

 by extrapolation on account of the slow rate of change at the 

 low temperature. It is probable, therefore, that mai.; r sub- 

 stances are classified as monotropic though potentially enan- 

 tiotropic. 



In a course of lectures delivered in 1897-8, van't Hoff 

 put forward certain theoretical deductions relating to the 

 reversible transformation of polymorphic forms. He bases his 

 conclusions on the assumption that polymorphic differences are 

 due to variations in molecular grouping, which disappear when 

 the substances pass into the gaseous or liquid state or into 

 solution. He shows that the stable modification must have 

 the smaller vapour pressure, the smaller solubility, and the 

 higher melting-point ; that a reversible polymorphic transforma- 

 tion must proceed to completion at one temperature, the direc- 

 tion of the change depending on the direction in which the 

 temperature is altered, and that the modification stable at low 



