282 



PRINCIPLES OF GENERAL PHYSIOLOGY 



OPTICAL ACTIVITY 



It is well known that compounds containing one or more carbon atoms 



D D 



C 



FIG. 74. DIAGRAM OF ASYMMETRIC CARBON ATOM. 



A, B, C, D represent four different groups, attached to the four angles of a tetrahedron, which 

 stands for a carbon atom. The two figures are mirror-images of one another, and cannot be made 



(van't Hoff, 1901, ii. p. 97.) 



to coincide. 



united to four different atoms or groups rotate the plane of polarised light, on 

 account of their asymmetry. 





FIG. 75. PHOTOGRAPH OF A MODEL OF AN ASYMMETRIC CARBON ATOM, TOGETHER WITH ITS 

 IMAGE IN A MIRROR. The four solids of different shape represent the four different 

 chemical groups. The reflected image can be detected by the double outline in places. 



The theory of the asymmetric carbon atom is due to Le Bel (1874) and van't 

 Hoff (1874). If we take lactic acid, 



CH 3 



H-C OH 



io 



OH 



