PROXIMATE CONSTITUENTS OF THE ANIMAL BODY 57 



In this tetrahedron, if we represent the four groups combining with the carbon 

 by B!, R 2 , R 3 , R 4 , they can be arranged either as in A or B. It is evident 

 that no amount of turning about will convert the tetrahedron A into tetra- 

 hedron B, but that, if we hold A before a mirror, its image in the mirror will 

 be represented by B. One arrangement is therefore the mirror image of the 

 other, and a compound containing one such carbon atom will be capable of 

 existing in two forms, namely, one form corresponding to A, the other form 

 corresponding to B. It is found that the unequal loading of the carbon 

 atom, which is present in such an asymmetric arrangement, causes the com- 

 pound containing the asymmetric carbon to have an action on polarised light. 

 One of the varieties will rotate polarised light to the right, while its mirror 

 image will rotate polarised light to the left. A mixture of equal parts of the 

 two compounds will rotate equally to left and right, i.e. will have no action 

 on polarised light. 



The variety rotating to the right is dextrorotatory, and the other laevo- 

 rotatory, while the mixture of the two is known as the racemic or inactive 

 variety. The three forms are said to be stereoisomeric, and are distinguished 

 as the d, I, and i forms respectively. If two asymmetric carbon atoms are 

 present in a compound, we may have four stereoisomers ; and generally if there 

 are n asymmetric atoms in a molecule, there will be 2 n possible stereoisomers. 

 These will not all be necessarily optically active, since the dextrorotation due 

 to one asymmetric carbon atom may be exactly neutralised by the laevorotation 

 due to another, so that ' internal compensation ' takes place and the substance 

 is optically inactive. Thus in tartaric acid four forms are known, namely, 

 d, Z, racemic or i, and mesotartaric, also inactive, in which internal compensa- 

 tion occurs. These four varieties may be represented as follows : 



inactive tartaric acid 



Several methods may be employed to separate the racemic form into its 

 two optically active components. One of these methods, first employed by 

 Pasteur, is to grow moulds in the solution. One of the optical isomers is 

 destroyed, leaving the other unchanged. Another method is the fractional 

 crystallisation of the salts with alkaloids, e.g. strychnine in the case of lactic 

 acid. 



