FERMENTATION BY BACTERIA. 597 



definition given both in tins work and also by various 

 other authors. 



As a matter of fact, development of carbonic acid is 

 always observed in the ordinary lactic fermentation ; and 

 Bontroux, as well as Hueppe, has observed the develop- 

 ment of carbonic acid in fermentation set up by pure 

 cultivations of lactic acid bacteria. Former observers incomplete- 

 have also found, besides carbonic acid, other fermentative previous 6 

 products (alcohol, butyric acid, mannite, gum). It is formula, 

 probable that these substances often owe their origin 

 only to the simultaneous presence of adventitious 

 organisms which stand in no relation to the lactic fer- 

 mentation ; nevertheless, it is not by any means a priori 

 impossible that the various bacteria which are capable of 

 setting up lactic fermentation can occasion decomposi- 

 tions which are dissimilar, and combined with a larger 

 number of bye-products. In any case, however, the 

 above formula is incorrect for the lactic fermentation by 

 the bacillus acidi lactici, because no account is taken in 

 it of the carbonic acid which is constantly observed. 



The course of the fermentation and its dependence on 

 various external conditions has been already discussed 

 on page 365. 



Butyric Acid Fermentation. 



The materials for this fermentation are furnished by Butyric acid 

 starch, dextrine, inuline, cane-sugar, and dextrose ; and fermentation. 

 also by milk-sugar, but only after hydration. The 

 butyric acid fermentation occurs spontaneously, and 

 is very widely distributed ; thus it is found in milk 

 which has stood for a long time, in " sauer-kraut," in 

 cheese, in the ripening of which it probably plays a 

 part, &c. In order to obtain it artificially it is well, 

 according to Fitz, to mix together 100 grammes of 

 potato starch (or dextrine), one gramme of sal ammoniac 

 and the ordinary nutrient salts, with two litres of water, 

 and to add 50 grammes of carbonate of lime, in order to 

 neutralise the butyric acid as it is formed. According 



