SUGAR IN THE NUTRIENT MEDIUM. 151 



A strong acid reaction is now secured with phos- 

 phoric acid and the volatile acids are distilled off 

 with the aid of a current of steam. The distillation 

 must be prolonged because the complete removal of 

 the volatile acids is difficult. The lactic acid is left 

 in the distillate, is obtained by shaking repeatedly 

 with pure ether, and the ether is then distilled off. 



The lactic acid obtained is always ethylidenlactic 

 CH 3 



acid CHOH, which occurs in two stereoisomeric forms : 

 COOH 



(1), dextro-rotatory with laevo-rotatory zinc salt, and 

 (2) Isevo-rotatory with dextro-rotatory zinc salt. If, 

 as happens very frequently, exactly the same number 

 of molecules of left and right lactic acids are present, 

 then the combination is optically inactive and forms 

 the so-called "fermentation lactic acid." I assume 

 that both lactic acids often develop from sugar, but 

 that some varieties of bacteria thrive mainly on one, 

 some on the other form, so that sometimes there is a 

 uniform combination, sometimes one form predom- 

 inates or alone remains. 



Since Schardinger (Mitt. f. Chem., XL, 545) dis- 

 covered that the previously unknown left lactic acid 

 was the product of a short rod bacillus in water, the 

 pupils of Nencki and Eubner have made numerous 

 investigations on the lactic acids formed by the dif- 

 ferent varieties, in the hope of utilizing the results 

 for purposes of differential diagnosis. 



For the method of determining the form of lactic 

 acid, vide Nencki (C. B., IX., 305) and Gosio (A. H., 

 XXL, 115). 



