Stereoisomerism. 1 69 



compounded of two active acids, which rotated polarized 

 light equally in opposite directions, and produced thereby 

 an optically inactive mixture. They further suggested to 

 Pasteur the conception of an asymmetric arrangement of 

 atoms in the molecules in the two sets of crystals, either 

 along the spirals of left-handed or right-handed screws, or 

 at the corners of irregular tetrahedra, or in some other 

 asymmetric forms which could yield non-superposable 

 images. According to this conception, the racemic acid 

 would result from a mixture of two such images. 



The Theories of Le Bel and Van't Ho/. In 1874 

 theories to explain " optical isomerism " * were promulgated 

 almost simultaneously by the French chemist Le Bel and 

 the Dutchman Van't Hoff, which were nearly identical. 

 The Van 't Hoff method of explanation is the one now 

 generally adopted, and will be retained in this place. If 

 the quadrivalency of carbon is accepted, compounds are 

 possible in which this atom is united to four different 

 elements or radicles. Suppose, now, that the carbon atom 

 is represented in the centre of a tetrahedron, and the atoms 

 or radicles to which it is united are represented at the 

 four corners of this figure. It will then be obvious that 

 the atoms or groups to which the carbon is united can be 

 grouped at the corners in such a way that the positions 

 relatively to one another are different. If, for example, 

 a compound of the type ABCD is considered, where 

 is the carbon atom, and A, B, C, D other elements or 

 radicles, two different figures can be obtained, namely 



* The meaning of this expression should be clear from the foregoing re- 

 marks on the tartaric acids. 



