128 



LACTIC ACIDS. 



The free acid is dextro-rotatory, but the true value of (<x) D is unknown 

 owing to uncertainty as to the purity of the acid. The salts on the 

 other hand are all leevo-rotatory. For the zinc salt, when one part is 

 dissolved in 18 of water (a) D = 7*6. 



FIG. 2. ZINC SAKCOLACTATE. 

 (After Kiihne.) 



FIG. 3. CALCIUM SABCOLACTATE. 

 (After Kiihne.) 



Both this acid and the preceding one yield an intense yellow colour- 

 ation when added to an extremely dilute (almost colourless) solution of 

 ferric chloride. This reaction is sometimes useful 1 . 



When the formula of ethylidene-lactic acid is examined it is found to contain 

 what is known as an asymmetric carbon atom : that is to say an atom of carbon 



H 



whose affinities are saturated by four dissimilar radicles. Thus H 2 C C COOH. 



OH 



According to the hypothesis of Van't Hoff and Le Bel such a substance must be 

 possessed of optically active properties since all substances which do rotate the 

 plane of polarised light contain an asymmetric carbon atom. It is known however 

 in certain cases, as for instance racemic acid, that although the substance contains 

 one (or more) asymmetric carbon atoms it may still be optically inactive since it is 

 composed of a mixture of isomeric bodies possessing equal and opposite rotatory 

 powers. From this point of view it is probable that ethylidene-lactic acid may be 

 such a mixture and that at present only one of the optically active isomers of which 

 it is composed has been obtained, viz. sarcolactic acid. 



In support of this view it is interesting to notice that a dextro-rotatory lactic 

 acid can be obtained from the optically inactive ethylidene-lactic acid, by applying 

 to its ammonium salt Pasteur's method for the separation of a mixture of 

 isomeric substances whose rotatory powers are equal and opposite. This consists 

 in growing the organism Penicillium glaucum in a dilute solution of the mixture ; 

 one of the isomers is found to be more readily destroyed by the plant than is the 



1 Uffelmann, Arch.f. klin. Med. Bd. xxvi. (1880), S. 431. 



