MIXED COMPOUNDS 69 



sideration of one of the most striking phenomena of organic 

 chemistry. 



Stereo-isomerism or Space-isomerism 



Not only are there known the two structurally isomeric 

 hydroxy-propionic acids which are explained as just described, 

 but there are also known two other acids which are likewise 

 hydroxy-propionic acids and furthermore which are structurally 

 identical with lactic acid, i.e. a-hydroxy-propionic acid. 



When milk sours, the sugar contained in the milk is converted 

 by means of bacterial organisms into lactic acid or, as it is further 

 distinguished, lactic acid of fermentation or sour milk lactic acid. 

 This acid is inactive toward polarized light. In the flesh of 

 animals another lactic acid has been found which, as just stated, 

 is also a-hydroxy-propionic acid or a true lactic acid. This 

 acid, however, possesses the characteristic property. of activity 

 toward polarized light and turns the plane of polarization to 

 the rights or as it is termed, it is dextro-rotatory. Also when the 

 lactic acid of sour milk which is inactive toward polarized light 

 is treated in a definite way (conversion of it into its strychnine 

 salt) it has been split into two different lactic acids, one of which 

 is active and dextro-rotatory identical with lactic acid from flesh, 

 and the other is oppositely active, i.e. is left-handed or levo- 

 rotatory. This levo-rotatory lactic acid is also produced 

 directly when cane sugar is acted upon by a particular form of 

 bacterium. We thus have three isomeric lactic acids all of 

 which are the same structurally, i.e. are a-hydroxy-propionic 

 acids, viz. inactive lactic acid found in sour milk, dextro lactic 

 acid found in animal muscle, levo lactic acid produced by bac- 

 terial fermentation of cane sugar. 



How now can we harmonize the existence of three lactic 

 acids of identical structure, viz. a-hydroxy-propionic acids, 

 CH3 — CH(OH) — COOH, with our previous statements in re- 

 gard to the structure of carbon compounds and with these 

 definite properties in connection with polarized light? 



Optical Activity. — Without explaining the physical action 



