554 SCIENCE PROGRESS 



Phthalylphenylmethylhydrazide affords the best example of 

 dimorphism yet met with among compounds of this class, as 

 each form can be easily transformed into the other. It exists 

 in two modifications : the one stable at the higher temperature 

 crystallises in bright orange-coloured triclinic prisms, the other 

 stable at the lower temperature in monoclinic crystals of very 

 compact habit. There is no difficulty in obtaining either form. 

 When a boiling saturated alcoholic solution is allowed to cool, 

 the orange-coloured, triclinic modification separates, and when 

 collected and dried at ioo° is obtained pure. If this modification 

 is allowed to remain suspended in alcohol for some days, pale 

 yellow crystals of the monoclinic modification make their 

 appearance and grow at the expense of the orange crystals, 

 which become etched, dissolve away, and finally disappear. 



The solubilities of the two forms in alcohol were determined 

 by the method previously described, and are recorded, as before, 

 as the number of grams of the compound contained in ioo grams 

 of the saturated solution. 



Solubility of the Enantiotropic Forms of Phthalylphenylmethylhydrazide 



It was not practicable at higher temperatures to shake the 

 orange-coloured form with the alcohol sufficiently long to 

 ensure saturation without some transformation into the pale 

 yellow form occurring. The transition point could not there- 

 fore be fixed with exactitude by solubility determinations, but 

 by plotting the ratio of solubilities against the temperature and 

 extrapolating, it was seen that it must lie in the neighbourhood 



of 55°. 



Two other methods, however, which gave results correct 

 within very narrow limits were available in the case of this 

 compound. 



The two modifications are very obviously different in colour 

 and general appearance, and transformation in acetone is suffi- 



