POLYMORPHISM 549 



temperature must be formed from the other with evolution of 

 heat ; and, finally, that polymorphic modifications must have 

 a constant ratio of solubilities in solvents which take up so 

 little of the substances that the laws of dilute solutions are 

 applicable. 



It has only recently been possible to put several of these 

 deductions to the test of experiment on account of the lack of 

 suitable material. Such material has, however, been found in 

 various anilides and hydrazides. Para-bromoacetanilide and 

 2 :4-dibromoacetanilide, both easily obtained by the bromination 

 of acetanilide, are perhaps the most easily accessible examples 

 of monotropic polymorphs and show excellently the different 

 solubilities of polymorphic forms. 



When a hot saturated alcoholic solution of />-bromoaceta- 

 nilide or of 2 : 4-dibromoacetanilide is allowed to cool, needle- 

 shaped crystals of the unstable form, generally of considerable 

 length, separate. These long, slender crystals, which form an 

 interlaced mass, if left in the solvent at the ordinary tempera- 

 ture slowly redissolve, whilst crystals of a second form much 

 more compact in habit make their appearance. Crystals of 

 this compact form generally appear spontaneously, but if none 

 appear after some hours, a few may be added. The appearance 

 (Plates I. and II.) during the transformation is very characteris- 

 tic. In the mass of interlaced needles, which if a concentrated 

 solution is cooled completely fills the liquid, cavities make their 

 appearance, each containing a crystal or group of crystals of 

 the compact form. These rest on the needles below and gradu- 

 ally sink, producing irregular, worm-like tubes. The whole mass 

 becomes filled with such holes and tubes and crumbles away, 

 whilst the compact crystals subside into a comparatively thin 

 layer at the bottom. 



The photographs reproduced in Plate I. illustrate the gradual 

 transformation of the unstable into the stable modification of 

 />-bromoacetanilide. Fig. 1 shows a beaker, about half its 

 actual size, filled with crystals of the unstable form immediately 

 after crystallisation has taken place. Fig. 4 shows the same 

 beaker, similarly reduced in size, after complete transformation 

 of the unstable into the stable form. Figs. 2 and 3 show a part 

 of the same beaker, about the actual size, on successive days 

 during the progress of the transformation. 



A similar series of reproductions on the same scale of photo- 



