32 FUNDAMENTALS OF SUBMIC ROSCOPI C MORPHOLOGY 



TABLE IV 



COHESIVE FORCES BETWEEN ORGANIC GROUPS, ACCORDING TO 

 MEYER AND MARK I93O 



weight or the molecular refraction. Accordingly, the contribution of 

 the characteristic groups to the cohesion has been denoted as mo/ar 

 cohesion (Meyer and Mark, 1930). For example, the heat of vaporiza- 

 tion of ethyl alcohol, which amounts to 10 kcal per mole, is additively 

 composed of the molecular cohesions of CH3, CHg, and OH. The 

 values concerned can be found in Table IV. 



This table shows that, in neighbouring molecules, methyl and 

 methylene groups and also oxygen bridges attract each other only 

 slightly. The attraction between amino and ketone groups is twice as 

 large and, in the polar hydroxyl and carboxyl groups, the cohesion 

 assumes quite considerable values. None the less, all the values for 

 molar cohesion are 10 to 100 times smaller than the energy equivalents 

 of the main valency bonds, and accordingly the secondary valency 

 bonds are at least 10 times weaker. Consequently, whenever secondary 

 valencies play a decisive role in the crystal lattice, the distances are 

 much greater than those between atoms bound by primary valencies. 

 In organic crystals, therefore, in which both bond types occur : primary 

 valencies inside the molecule (intramolecular) and secondary valencies 

 between the molecules (intermolecular), the lattice distances are essen- 

 tially of two different orders of magnitude. 



