III. MOLECULAR GROUPING 



(Polymorphism) 



WHILST polymerism is explained as difference in molecular 

 weight, and isomerism in molecular structures, there remains 

 a third difference in properties to account for, which is 

 characterized by the fact that it disappears on conversion 

 into the amorphous, i. e. liquid or gaseous state, conse- 

 quently on fusion, evaporation, or solution. As example 

 may be mentioned the rhombic and monosymmetric forms 

 of sulphur, with regard to which it has lately been shown * 

 that they form the same molecules S 8 on going into 

 solution; along with these are to be placed the various 

 crystallized modifications of ammonium nitrate, &c., studied 

 by Lehmann; and finally the numerous polymorphic 

 minerals, such as calcite and aragonite. 



It is natural to refer these differences to variation in 

 molecular grouping, since when the grouping ceases and 

 the substance passes into the amorphous state, the differ- 

 ences vanish. For that reason polymorphism is most 

 analogous to the physical change of state on melting or 

 freezing; it is well known that a molecular orientation 

 is then destroyed or created, whilst in a polymorphic 

 transition one orientation appears at the expense of another. 

 And so, if we are to distinguish between physical and 

 chemical isomerism, it is convenient to regard polymorphism 

 as physical isomerism, and polymerism and isomerism as 

 chemical, since they in common rest on a change within 



1 Aronstein and Meihuizen (see p. 60). 



