90 PHYSIOLOGICAL ACTIVITIES 



acid — stops the reaction. If the acid is neutrahzed by the 

 addition of an alkah — calcium or magnesium carbonate is 

 best — then the sugar may all be split up. Where such fer- 

 mentation occurs under natural conditions, the products 

 are further split up, partly by molds and partly by acid- 

 destroying bacteria into simpler acids and eventually to 

 carbon dioxide and water, so that the end-products of the 

 complete fermentation of carbohydrate material in nature 

 are carbon dioxide, hydrogen, marsh gas, and water. 



In all of these fermentations the bacteria are utilizing the 

 carbon both as building material and for oxidation and the 

 fermentations are incidental to this use. As a rule, the acid- 

 forming bacteria can withstand a higher concentration of 

 acid than the other bacteria that would utilize the same 

 material, and in a short time crowd out their competitors 

 or inhibit their growth, and thus have better conditions for 

 their own existence, though finally their growth is also 

 checked by the acid. 



SPLITTING OF FATS, 



The splitting of fats into glycerin and the particular acid 

 or acids involved may be brought about by bacteria. An 

 illustration is the development of rancidity in butter at 

 times and the "strong" odor of animal fats on long keeping 

 and of many kinds of cheese — "Limburger" in this country. 

 Generally speaking, however, fats are not vigorously attacked, 

 as is illustrated by the difficulties due to accimiulation of 

 fats in certain types of sewage-disposal works. The chemi- 

 cal change is represented by the equation: 



Fat. Glycerin. Fatty acid. 



C.,H6 (C„H2„.i02)3 + 3 H2O ^ CsHs (OH)i + 3 (C„H2„ O2). 



