76 BACTERIOLOaiCAL AND ENZYME CHEMISTRY 



possible to pick out crystals which difiered from each other 

 in structure only as the image in the mirror differs from its 

 real object, or as the right hand differs from the left; thus 

 certain small faces could be seen on one set of crystals on the 

 right hand, whereas the corresponding set of faces on the 

 other crystal were on the left (Fig. 16). ^ One of these crystal- 

 line forms turned the plane of polarisation to the right, the 

 other to the left, and the crystals were derivatives respectively 

 of dextro and Isevo tartaric acid. When these two forms were 

 crystallised together to form racemate, optical inactivity 

 resulted. 



The optical difference in these two modifications of tartaric 

 acid was here clearly referred to a difference in crystaUine 

 form. Pasteur at the same time suggested that the cause of 

 the difference lay deeper, viz., in the actual molecular structure 

 of the two acids, that is upon the arrangement of the atoms 

 in their respective molecules. This illuminating suggestion 

 of Pasteur found its full development in the theory of Van't 

 Hoff and Le Bel. 



These investigators found that every optically active 

 substance contained within its molecule a carbon atom to 

 which were attached four dissimilar groups ; such a carbon 

 atom they referred to as an asymmetric carbon atom. In order 

 to explain why such a grouping should give rise to actual 

 physical asymmetry they suggested that the arrangement of 

 the groups must be considered as occurring in three dimen- 

 sions. Now all investigation goes to show that the four com- 

 bining units, bonds, directions of affinity, or whatever term 

 may be used to express what is symbolised by the four lines 

 attached to the C in the formula of an organic compound, are 

 strictly equivalent. The only way to express this fact in 

 three dimensions is to consider the carbon element as being 

 at the centre C of a regular tetrahedron (Fig. 17), with its 



' Reproduced from Dr. C. A. Keane's Modern Organic Chemistry, by 

 permission of the publishers. 



