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SCIENCE PROGRESS 



which is the mirror-image of the other across the symmetry 

 plane of the system (OCGA in Fig. 2). 



It is a remarkable fact that the details of the crystalline 

 structure of tartaric acid bear out Pasteur's prophecy com- 

 pletely. There are two possible arrangements of the atoms 

 of the molecule, both of which are in accord with the crystal- 

 lographic and X-ray data, and either of which is the mirror- 

 image of the other. Fig. 6 {a and b) shows these two enantio- 

 morphous arrangements of the atoms {a' and b' are end views 

 of the molecules shown lengthwise in a and b). Moreover, 

 within the nucleus of each molecule is an irregular spiral 



7\ 



COG C B C 



Fig. 5. — Showing end view of unit cell of tartaric acid. 



formation of the four carbon atoms, to which we may ascribe 

 the characteristic optical rotation of the tartrates. In one 

 form the spiral is right-handed, and in the other left-handed. 

 But the model of the structure leads us much farther than 

 Pasteur's prophecy. There is yet another spiral formation 

 of the four hydroxyl groups, which is in an opposite sense to 

 that of the four carbon atoms of the nucleus. In the crystal- 

 line state, the existence of one spiral automatically brings about 

 the existence of the other. And this significant fact appears 

 to be at the root of a remarkable anomaly in the optical 

 behaviour of tartaric acid. 



As long ago as 1816, Biot observed that, whereas'in quartz 



