MIXED COMPOUNDS 73 



and the other left-handed. They are related to each other as 

 an object and its image, or as the right hand to the left. It 

 can be readily seen that one of these compounds might act 

 one way toward polarized light and the other would possibly 

 act in the reverse way. By one formula, A for example, we 

 can represent the lactic acid of flesh, the dextro-rotatory form, 

 and by the other the lactic acid obtained by bacterial action 

 on cane sugar, the levo-rotatory form. Thus we can explain 

 the two active forms. How about the inactive form? The 

 fact that sour milk lactic acid can be split into the two active 

 forms, and the same is true of inactive tartaric acid and many 

 other inactive compounds which contain an asymmetric car- 

 bon atom, gives reason for the view that the inactive compound 

 is composed of equivalent amounts of the active forms which 

 simply neutralize each other as to their effect upon polarized 

 light. 



To state it all briefly : van't Hoff and Le Bel's Theory of the 

 Asymmetric Carbon Atom, or, as we also call it, the Tetrahedral 

 Theory, explains the existence of three lactic acids by assuming 

 that one compound, the dextro-rotatory form, has the space 

 arrangement of one of the two formulas just given, say A. 

 The other active compound, the levo-rotatory form, has the 

 arrangement of the formula B. The third lactic acid, the 

 inactive sour milk acid, is composed of equivalent amounts of 

 the two active forms. 



Inactive Lactic Acid. — As has been stated before, the inac- 

 tive form of lactic acid which is in reality a mixture of the 

 dextro and levo forms is found in sour milk. It is produced 

 in milk by the bacterial fermentation of the milk sugar which is 

 present. 



C12H22O11 -f H2O -^ 4 C3H6O3 



Milk sugar Lactic acid 



This reaction consists both of a hydrolysis and of a splitting 

 of the molecule. It is brought about by a particular group 

 of bacteria, viz. lactic acid bacteria. The acid is also found 



