55 6 SCIENCE PROGRESS 



two forms and has widely differing solubilities in different 

 solvents. 



The following table gives the solubilities of the two forms of 

 this compound in five different solvents at 25 , the temperature 

 furthest removed from the transition point at which a complete 

 series of determinations was made : 



The agreement between the ratios is remarkably close. 



Phthalylphenylmethylhydrazide is not so suitable for the 

 purpose, on account of its greater solubility. A similar con- 

 firmation was found, however, on comparing the ratios of the 

 solubilities of the two forms in ethyl and in methyl alcohol at 

 30°C 



Solubility of 

 Solvent. pale yellow 



Solubility of Solubility of 



pale yellow orange 



modification. modification. 



Ethyl alcohol . . 2*239 2-474 1*105 



Methyl alcohol . . 4*547 5'°33 1-107 



It ought, perhaps, to be pointed out that the solubilities 

 given are not concentrations, but the densities of the correspond- 

 ing pairs of solutions do not differ sufficiently to affect materially 

 the ratios. 



From the first recognition of polymorphism attempts have 

 been made to gain an insight into its nature and to relate it 

 with the phenomena of isomerism. Although no strict line can 

 be drawn between isomeric, polymeric, and polymorphic changes, 

 the first two are almost universally regarded as being due to 

 changes within the chemical molecule, the last are regarded as 

 due to change of molecular grouping within the crystal. 



That some change in the relationship of the atoms within 

 the molecule occurs in both isomeric change and polymorphic 

 change is certain; the difference appears to be as to whether this 

 is sufficiently great to cause the molecules to react differently 

 or not. In the former case the change is designated isomeric, 

 in the latter polymorphic. 



