178 Biological Chemistry. 



carbon atom, which are obtained synthetically, are optically 

 inactive. The " racemic " form is always obtained that 

 is, a mixture in equimolecular amounts of the two enantio- 

 morphs. An account of the methods for " resolving " 

 racemic forms into the optically active enantiomorphs 

 has been given. Where more than two asymmetric carbon 

 atoms exist in the molecule, the number of possible stereo- 

 isomerides increases. In such cases, in addition to the 

 ordinary racemic form, a meso-form can exist, which is 

 optically inactive, but which cannot be resolved into active 

 enantiomorphs. In this case, the molecule is inactive 

 owing to " internal compensation,' 9 whereas in the racemic 

 forms this inactivity is due to " external compensation" 

 The modern conceptions of stereoisomerism are due chiefly 

 to Le Bel and Van 't Hoff, and are founded mainly on 

 the work of Pasteur on the tartaric acids, and Wislicenus 

 on lactic acids, of which a short historical account has 

 been given. The various stereoisomerides, although under- 

 going the same chemical reactions, behave very differently 

 towards living objects. For this reason the study of 

 stereoisomerism is one of considerable importance to 

 biologists. 



