56 THE STORY OF GERM LIFE. 



fermentation occurs in spite of all other bacteria. 

 Here also there is a possibility of an improve- 

 ment in the process by the use of pure cultures of 

 lactic organisms. Up to the present, however, 

 there has been no application of such methods. 

 The commercial aspects of the industry are not 

 upon a sufficiently large scale to call for much in 

 this direction. 



At the present time the only method we have 

 for the manufacture of lactic acid is dependent 

 upon bacteria. Chemical processes for its manu- 

 facture are known, but not employed commer- 

 cially. There are several different kinds of lac- 

 tic acid. They differ from each other in the 

 relations of the atoms within their molecule, and 

 in their relation to polarized light, some forms 

 rotating the plane of polarized light to the right, 

 others to the left, while others are inactive in this 

 respect. All the types are produced by fermenta- 

 tion processes, different species of bacteria hav- 

 ing powers of producing the different types. 



BUTYRIC ACID. 



Butyric acid is another acid for which we are 

 chiefly dependent upon bacteria. This acid is of 

 no very great importance, and its manufacture 

 can hardly be called an industry ; still it is to a 

 certain extent made, and is an article of commerce. 

 It is an acid that can be manufactured by chemical 

 means, but, as in the case of the last two acids, its 

 commercial manufacture is based upon bacterial 

 action. Quite a number of species of bacteria 

 can produce butyric acid, and they produce it from 

 a variety of different sources. Butyric acid is a 

 common ingredient in old milk and in butter, and 



