128 LACTIC ACIDS. 



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 whose affinities are saturated by four dissimilar 

 H 



I 

 radicles. Thus H 2 C C COOH. 



OH 



According to the hypothesis of Van't Hoff and Le Bel such a sub- 

 stance must be possessed of optically active properties, since all sub- 

 stances which do rotate the plane of polarised light contain an 

 asymmetric carbon atom. It is known however in certain cases, as for 

 instance raceinic 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 other, so that at a certain stage only one is left in solution. 1 

 When treated in this way ethylidene-lactic acid yields a dextro- 

 rotatory solution. 2 When a current of dry air is passed through sar- 

 colactic (or ethylidene-lactic) acid heated to 150, two molecules of the 

 acid lose two molecules of water and yield a solid crystalline substance 

 known as lactide (C 3 H 4 2 ) 2 . When boiled with water this is recon 

 verted into optically inactive lactic acid, thus effecting the reconver 

 sion of the optically active into the inactive form of the acid. 



The Van't Hoff-Le Bel hypothesis of what was originally called 

 ' physical ' isomerism is based upon considerations of the spaciaC 

 relationships of the constituents of an organic substance ; hence the 

 more recent use of the expression ' stereochemical ' instead of 

 ' physical.' 3 



The acid reaction of dead muscle is undoubtedly due to the 

 presence of sarcolactic acid, as was first clearly shown by Liebig 

 in 1847. 4 In certain cases the reaction of muscle which is still 

 irritable may become acid, and this has usually been regarded as 

 due to the development of this acid during its activity. In recent 



1 Compt. Rend. T. LI. (1860), p. 153. 



2 Lewkowitsch, Ber. d. d. chem. Gesell. Jahrg. 1883, S. 2720. 



3 See Miller's Elements of Chem. (Armstrong and Groves), Part III. Sec l 

 (1880), p. 983, for details of the Van't Hoff-Le Bel hypothesis. 



4 That living (irritable) muscle in a state of rest is reallv alkaline was first 

 demonstrated by Du Bois Reymond in 1859. Monatsber. d. Berl. Akad. 1859, S. 

 288. See his Gesammel. Abhdl. Bd. 11. 1877, S. 3. 



