8 CONTENTS 



PAGK 



3. Double linkage and ring formation . . . . .121 



Carbon double linkage 121 



Ring formation . . . . . . . .124 



4. Stereochemistry of other elements . . . . .124 



Concluding remark on stereochemistry .... 125 



3. TAUTOMERISM 126 



III. MOLECULAR GROUPING (Polymorphism). 



I. LAWS REGULATING THE MUTUAL CONVERSION OF 



POLYMORPHIC SUBSTANCES 133 



A. The stable modification must have the smaller vapour 



pressure and the smaller solubility . . . .134 



B. The stable modification must have the higher melting 



point 134 



C. Possibility of a transition temperature . . . .135 



D. When there is a transition temperature, the modifica- 



tion stable at low temperatures is formed from the 

 other -with evolution of heat 136 



E. Polymorphic modifications have a constant ratio of 



solubilities, proportional to the differential coeffi- 

 cients of the saturation pressures, in solvents which 

 take up so little of the substance, that the laws of 

 dilute solutions are applicable 137 



2. ACTUAL MOLECULAR ARRANGEMENTS . . . .138 



A. Relative position of molecule centres in the crystalline 



figure 139 



1. The fundamental law of geometrical crystallography (F. C. 



Neumann) .......... 139 



2. Attempt at explanation of the geometrical law by arrange- 



ment of molecule centres (Frankenheim, Bravais) . -140 



3. Relations of symmetry in crystals . ..... 141 



4. Sohncke's point systems . . . . . . . 145 



5. The fundamental law of physical crystallography . . .146 



6. Molecular compounds . . . . . . . -147 



B. Orientation of the molecules in the crystal . . .148 



1. Relations between symmetry in the crystal and in the 



molecule 148 



2. Influence of changes in the molecule on the crystalline form 151 



