SEPARATION OF OPTICAL ISOMERS 115 



succinic, and other acids. Lactic acid, itself a product of many carbo- 

 hydrate fermentations, is converted by butyric bacteria into butyric acid, 

 carbon dioxide, and free hydrogen. The biochemistry of all these processes 

 is as yet but little understood (Chap. XIII). 



Very remarkable decompositions of the optically 'inactive' varieties 

 of organic acids are effected by some bacteria*. These consist, as is 

 known, of equal parts of the dextro-rotatory and laevo-rotatory acids, 

 which twist the plane of polarized light to right and left respectively. In 

 a solution of the ammonium salt of racemic acid, for instance, only the 

 dextro-rotatory vaiiety is used by the bacteria, the laevo-rotatory modi- 

 fication being thereby as it were set free. The inactive varieties of lactic 

 and mandelic acids are biochemically separable into their components in 

 the same way (85). 



The profound chemical disintegration that these changes seem to show 

 is only apparent. As a matter of fact they are only selective changes, only 

 one of the two active compounds of the salt being used up. One of the 

 mucigenous bacteria, for instance, uses up fumaric acid, but leaves the 

 stereo-isomeric compound, malei'c acid, untouched. A similar phenomenon, 

 which may be termed the converse of those just described, is exhibited by 

 a variety of J5. coli communis which ferments grape sugar into lactic acids 

 differing in rotatory power according to the source of nitrogen employed. 

 If it be supplied with phosphate of ammonia, laevo-rotatory acid is formed, 

 but if the nitrogen be given to it in the shape of peptone the dextro- 

 rotatory variety appears ; whereas the lactic acid produced by other 

 fermentations is generally inactive. 



We must for the present be content to record these facts an explana- 

 tion is not possible. The stereochemical hypotheses, with which modern 

 chemistry seeks to unravel the molecular structure of these isomeric com- 

 pounds, throw no light whatever at present on the biochemical phenomena 

 associated with them. 



* Some of the mould -fungi act in a similar way. 



i a 



