42 AGRICULTURAL BACTERIOLOGY. 



at low temperatures and also at high temperatures, and occur- 

 ring 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 (fenttmts) are all rendered inert or destroyed 

 by heat, a boiling temperature commonly destroying them 

 so completely that they are unable to renew their action even 

 after cooling. Their action is also checked by low tempera- 

 tures, but they are able to renew their activity if warmed again. 



5. Their action is commonly stopped by the accumulation 

 of the products of their activity. 



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 fer- 

 menting sugar solutions and the process is one of the most 

 widely known changes in nature. Until the present centuiy 

 its chemical nature has been wholly misunderstood. It was 

 formerly regarded as a purifying process, the yeasts which 

 were found in the fermented material being looked upon as 

 impurities which were separated from the rest. The chemical 

 nature was determined early in the iQth century, but its 



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

 Schwann demonstrated that fermentation would not occur ex- 

 cept 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 looked upon the process as a purely 

 chemical change, but this theory was eventually disproved by 

 Pasteur and others, who finally demonstrated that fermenta- 

 tion is a physiological process accompanying the growth of 

 yeast. 



At that time there was no conception of the numerous types 



