PUTRID FERMENTATION. 163 



besides these, though the alcoholic and the acetous 

 are by far the most important. It is evident, how- 

 ever, from the great facility with which starch, gum, 

 sugar, &c., are converted into each other, that all of 

 these forms of organic matter may be readily made 

 to undergo any of these kinds of fermentation, and 

 likewise that the nature of the change thus brought 

 about will depend entirely on circumstances. 



380. Organic matter, placed under proper circum- 

 stances, may be made to pass through several suc- 

 cessive stages of fermentation ; the last of these is 

 usually the putrefactive fermentation, the results of 

 which are chiefly carbonic acid, ammonia, and water. 

 When it is desired to excite any particular kind of 

 fermentation, care and attention are always requisite 

 to prevent it from proceeding too rapidly, or too far, 

 as, when this is the case, it very commonly changes 

 into the putrefactive fermentation, and, when this has 

 once commenced, it is hardly possible to stop its 

 progress. 



381. A number of important arts are wholly based 

 upon these facts ; and many of the arts of life are 

 more or less dependent in principle upon the effects 

 just described. The manufactures of wine, spirit, 

 beer, vinegar, bread, cheese, starch, leather, &c., may 

 be mentioned as examples. 



382. Grapes contain sugar, and a small quantity 

 of azotized matter, capable of acting on the sugar as 

 a ferment; but no fermentation can take place, so 

 long as air is altogether excluded. The azotized 



