USE OF BACTERIA IN THE ARTS. 55 



has been so successful in brewing namely, the 

 use, as a starter of the fermentation, of a pure cul- 

 ture of the proper species of bacteria. This has 

 been done experimentally and proves to be feas- 

 ible. In practice, however, vinegar makers find 

 that simpler methods of obtaining a starter by 

 means of which they procure a culture nearly 

 though not absolutely pure are perfectly satis- 

 factory. It is uncertain whether really pure cul- 

 tures will ever be used in this industry. 



LACTIC ACID. 



The manufacture of lactic acid is an industry 

 of less extent than that of acetic acid, and yet it 

 is one which has some considerable commercial 

 importance. Lactic acid is used in no large quan- 

 tity, although it is of some value as a medicine 

 and in the arts. For its production we are wholly 

 dependent upon bacteria. It is this acid which, 

 as we shall see, is produced in the ordinary 

 souring of milk, and a large number of species 

 of bacteria are capable of producing the acid 

 from milk sugar. Any sample of sour milk may 

 therefore always be depended upon to contain 

 plenty of lactic organisms. In its manufacture 

 for commercial purposes milk is sometimes used 

 as a source, but more commonly other substances. 

 Sometimes a mixture of cane sugar and tartaric 

 acid is used. To start the fermentation the mix- 

 ture is inoculated with a mass of sour milk or de- 

 caying cheese, or both, such a mixture always con- 

 taining lactic organisms. To be sure, it also 

 contains many other bacteria which have differ- 

 ent effects, but the acid producers are always so 

 abundant and grow so vigorously that the lactic 



