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 
