263 



nature of the plurivalent central-atom in the first place, 

 \vhirh are the preponderant conditions. In this way we get a 

 little HUM. 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 Bell's ingenious theory 1 ). 



29. The above remarks bring to the fore another gei, 

 question regarding optical activity and non-superposable hemihe- 

 drism. It is a well-known 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 only in the molten or dissolved state exhibiting 

 a rotatory power, which therefore must be an immediate manifesta- 

 tion 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, bo thin 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), 

 and where it is a characteristic property of their structure. The 

 rotatory power of the last mentioned group may be quite different, 

 and in opposite directions in the crystals and in the liquid state, 

 as far as the rotation of the crystalline substance is an inherent 

 property, and not caused by pseudo-symmetrical intercalation. 



Thus, according to Wyrouboff 8 ), while for instance the 

 rubidium-salt of dextrogyra.tory tartaric acid deviates the plane of 

 vibration of the polarised light to the right if dissolved in water, 

 the crystals of the substance are nevertheless /a^vogyrate. It is 

 difficult to give a rational explanation of such facts, as the 

 hypothesis of Liebisch 4 ), who attributes the phenomenon to 

 the circumstance that in aqueous solution the salt is evidently 



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

 (1915), loco cit; Chem. Weekblad 14. 726, 727. (1917). 



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

 8) G. Wyrouboff, Journ. de Phys. (3). 8. 451. (1894). 



4) Th. Liebisch, Grundziige der physik. Krystallographie, (1896), p. 428. 



