MOLECULAR GROUPING 133 



the molecule. The word allotropy may then be retained 

 for the phenomena of isomerism in the elements, whether 

 they are to be characterized as polymerism, like ozone (O 3 ) 

 and oxygen (O 2 ), or polymorphism, like rhombic and mono- 

 symmetric sulphur. 



The general remark may be added that, as was explained 

 earlier (p. 82). the phenomena of polymerism and isomerism 

 are rare in the inorganic province, and the differences 

 accompanying similarity of composition there met with 

 are almost entirely to be referred to polymerism. It is 

 remarkable that the tenacity with which organic com- 

 pounds retain a molecular structure which is, nevertheless, 

 not that of equilibrium, a property mostly wanting in 

 inorganic compounds, is replaced by tenacious retention 

 of molecular grouping. Two points of view are advan- 

 tageous for complete treatment. First, we will bring 

 forward the laws of transition in polymorphic substances. 

 Secondly, we may discuss the details of molecular grouping. 



I. LAWS REGULATING THE MUTUAL CONVERSION OF 

 POLYMORPHIC SUBSTANCES. 



These laws may be deduced from the two facts that the 

 polymorphic substances are solids of similar composition, 

 and that on passing over into vapour, solution, or the 

 fused state, the difference between the two modifications 

 disappears. 



We have, however, in the following discussion, to bear 

 in mind the conditions of equilibrium, that mutual con- 

 version of the two modifications, especially in the case of 

 polymorphic minerals, such as calcite and aragonite, does 

 not occur ; we have, therefore, to deal, in one of the two 

 cases, with a state of apparent equilibrium (Part I, p. 236) 

 which is not destroyed even by contact with the other 

 modification, and is not to be referred to an extremely 

 slow transition. It is natural to associate this rigidity 

 with the relatively great hardness of the minerals. 



