78 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



These two arrangements cannot be symmetrically super- 

 posed ; they are what is known as enantiomorphous. Such a 

 difference can only occur when all four replacing groups are 

 different, that is to say, when there is an asymmetric carbon 

 atom in the molecule, as in the example just given, a, 6, c 

 and d, representing the replacing groups. 



If the substitution takes place by groups a, a, b, c, thus 

 (Fig. 20)- 



a CL a, & 



FIG. 20. 



it is easy to see that by turning the tetrahedron the two 

 forms are superposable, so that there is no essential differ- 

 ence between them. 



We are now in a position more fully to understand how 

 there exists more than one form of lactic acid and of tartaric 

 acid, even though the constitutional formulae as determined 

 by chemical reactions may be the same ; thus in the last 

 chapter it was shown that fermentation lactic acid or a-lactic 

 acid had the formula CH 3 C HOHC0 3 H, the centre carbon 

 atom is attached to four different groups and is therefore 

 asymmetrical. 



It is possible, therefore, to obtain a dextro, a Icevo, and an 

 inactive lactic acid. As already stated, these different forms 

 can be obtained by fractional crystallisation of the strychnine 

 salts, and also from the zinc salts. In writing what are known 

 as stereo-chemical formulae, i.e., formulae expressive of the space 

 arrangement of the atoms in the molecule, it is convenient, 



