TYPES OF FERMENTATION AND DECAY. 27 



1. They are all closely associated with life processes; i.e., are 

 brought about directly or indirectly by living agents. 



2. They are all closely dependent upon temperature, ceasing 

 at low temperatures and also at high temperatures, and occurring 

 with vigor within limits of temperature not far apart. Most of 

 them occur most vigorously at temperatures between 80 and 100 F. 



3. They are all produced by the stimulating action of some 

 special body, present in the fermenting material in a quantity 

 which is very small, considering the great changes produced. 



4. These bodies (ferments) are all rendered inert or destroyed 

 by heat; a boiling temperature commonly destroys them so completely 

 that they are unable to renew their action even after cooling. Low 

 temperatures simply check their activity, which they are able to 

 renew if warmed again. 



5. Their action is completely stopped by an accumulation of 

 the products of their own activity. 



If we ask what is the body producing the action (the ferment), 

 we find that the first and last of the types described differ from the 

 second in one radical point. Whereas the alcoholic fermentation 

 and putrefaction are directly produced by living germs, either yeasts 

 or bacteria, the amyolytic fermentation is not produced by a living 

 organism, but by some non-living substance secreted from a living 

 being. To explain this a brief account is required of the develop- 

 ment of our knowledge of fermentations in general. 



Fermentations have been known for centuries. Even in ancient 

 Egypt the production of alcohol was familiar. Every savage tribe has 

 its own method of obtaining alcohol by the fermenting of fruit juices, 

 and the process is one of the most widely-known changes in nature. 

 For a time it was regarded as a putrefying process, the yeasts found 

 in the fermented material being looked upon as an impurity which 

 was separated from the rest. The chemical nature of alcoholic fer- 

 mentation was determined early in the nineteenth century, but its 

 relation to the yeasts was not determined until 1837, when Schwann 

 demonstrated that fermentation would not occur except under the 

 influence of yeasts. The conclusion that it was the result of the 

 growth of yeasts was vigorously combated for years by Liebig, who 



