277 



posed by Pasteur, may be hidden in many cases to such an extent, 

 that it may be doubtful as to whether it be present in such cases 

 at all. Surely in most cases a parallelism between both phenomena 

 is present: where the dissymmetry of the molecule is betrayed 

 by the rotatory power of the molecules, there is exhibited in 

 most cases also a non-superposable hemihedrism of the crystalline 

 forms. But this need not always be the case. The greater the 

 chemical contrast between the substitutes round the asymmetric 

 atom present in the moleciile, the greater chance will there evi- 

 dently be, that crystallographical hemihedrism will appear to accom- 

 pany the rotatory power of the substance. For the magnitude of 

 the rotatory power of the molecule, this chemical contrast between 

 the substitutes is only of secondary significance: here it is the non- 

 superposable stereometrical arrangement and the special nature 

 of the plurivalent central-atom, which are the preponderant condi- 

 tions. In this way we get a little more insight into the mechanism 

 of optical activity and its dependence on molecular configuration, 

 as the beginning of a fuller understanding of the relation between 

 Pasteur's law and Van 't Hoff-Le Bel's ingenious theory 1 ). 



29. The above remarks bring to the fore another general 

 question regarding optical activity and non-superposable hemihe- 

 drism. It is a wellknown fact, that there are a number of sub- 

 stances which crystallise in undeniably enantiomorphous crystal- 

 forms, but which behave quite differently with respect to 

 polarised light. 



According to Pope 2 ) three classes of substances may be 

 distinguished, as: 



a. Compounds exhibiting a rotatory power only in the molten 

 or dissolved state, which, therefore, must be an immediate mani- 

 festation of the dissymmetry of the molecules. 



b. Compounds whose crystals show a rotatory power and 

 hemihedrism, but whose solutions are optically inactive. 



c. Substances which exhibit a rotatory power, both in solution 

 and in the crystalline state. Among classes b and c a further 

 distinction can be made between such crystals, the rotatory power 

 of which is caused by pseudo-symmetrical intergrowth (Chapter VII), 



1 ) F. M. Jaeger, Proceed. Kon. Akad. v. Wet. Amsterdam, Vol. 17 and 18, 

 (1915), loco cit; Chem. Weekblad, 14, 726, 727, (1917); Receuil d. Trav. d. Chim. 

 d. Pays-Bas, 38, 171, (1919). 



2) W. J. Pope, Zeits. f. Kryst. u. Miner., 27, 406, (1896). 



