Stereoisomerism. 1 77 



opposite directions. Now it is characteristic of enantio- 

 morphs that they possess almost the same physical pro- 

 perties, such as melting points, solubilities, etc., although 

 they rotate polarized light (to an equal extent) in opposite 

 directions. The two salts formed with an optically active 

 base mentioned above, differ in their solubilities and other 

 physical properties. It is, therefore, possible to separate 

 them by fractional crystallization. By such means as this, 

 racemic forms of acids obtained synthetically have often 

 been separated into their components ; and on the same 

 principles optically active bases have been obtained from 

 synthetic racemic forms by combination with optically 

 active acids. The third method has, in fact, been very 

 largely applied for the preparation of optically active 

 substances. 



Other more recent methods for obtaining optically 

 active substances have been employed, but the most 

 general are the three classical methods of Pasteur given 

 above. 



SUMMARY. It has been shown tliat when substances 

 contain an asymmetric carbon atom that is, a carbon 

 atom united to four different elements or radicles sub- 

 stances are obtained which can exist in optically active 

 forms that is, forms which rotate the plane of polarized 

 light. Optical activity is thus conditioned by the presence 

 of the asymmetric carbon atom, and where one such atom 

 exists, the various elements or radicles can be arranged 

 round it in such ways that two forms are possible, one 

 of which is the mirror image of the other. These two 

 images are called "optical antipodes" or " enantio- 

 morphs," and rotate the plane of polarized light to an 

 equal extent, but in opposite directions. Substances related 

 to one another as such enantiomorphs are, are said to 

 be stereoisomeric. Substances containing an asymmetric 



(1,997) 12 



