STEREOISOMERISM AND OPTICAL ACTIVITY 229 



all these ideas were brought into chemistry, is against them. 

 Crystallographists recognise 230 possible point-systems, grouped 

 in 32 classes, of which 1 1 classes give enantiomorphous crystal- 

 forms. So, all optically active crystals, like quartz or sodium 

 chlorate, belong to one of these 1 1 classes ; but the converse ot 

 this is not true, and there are cases known where the crystals 

 are enantiomorphous but optically inactive, e.g. barium nitrate. 1 

 This clearly shows that enantiomorphism is not always accom- 

 panied by, and cannot therefore be the cause of, optical 

 activity. 



This must hold good even in stereo-chemistry ; and thus we 

 may get cases where the configuration of the molecule is enan- 

 tiomorphous and still the substance is inactive. 



Secondly, even if enantiomorphism were always accompanied 

 by optical activity, it can hardly be regarded as the efficient 

 cause of it. The nature of the phenomenon rather suggests 

 something analogous to a twisted or screw-spiral structure in 

 the substance. Not only the rotation produced by a naturally 

 active substance can be removed by retraversing it, but also an 

 optically active body can be, and has been, artificially prepared 

 by piling together a number of mica plates in such a manner 

 that the optical axis of each is turned through a definite angle 

 with respect to that of the preceding plate. This makes it very 

 probable that the cause of optical activity is screw-spiral 

 structure of some sort, enantiomorphism being another simul- 

 taneous effect of the same cause. 



This fact seems to have been well recognised in crystal- 

 lography. It is by resorting to this that Sohncke 2 has tried to 

 explain why barium nitrate crystals are optically inactive, while 

 sodium chlorate crystals, belonging to the same crystal class, are 

 active. According to him, barium nitrate possesses a point- 

 system, in which there is no screw-spiral structure, while such a 

 structure is present in the point-system belonging to sodium 

 chlorate. 



In stereo-chemistry, however, this fact has been entirely 

 ignored, and we still find enantiomorphism described as the 

 cause of optical activity. Logically speaking, if screw-spiral 

 structure is the cause of optical activity, it must be assumed to 

 be present in the configurations of optically active compounds. 



1 Tutton's Crystallography and Practical Crystal Measurement, p. 139. 



2 Tutton's Crystals, p. 151. 



