ENZYMES OR FERMENTS 35 



trace of acetic acid and glycerine in the fluid. It was also 

 noted that a distinct deposit was always present in the form of a 

 scum which Leeuwenhoek, toward the end of the lyth century, 

 discovered was made up of small spherical bodies. He did not 

 attempt to determine their systematic position, but later 

 observers in the beginning, of the i8th century were divided as 

 to the question of their animal or plant nature. Their signifi- 

 cance as germs was not recognized however, for the entire range 

 of chemical processes was bound up in Lavoisier's theory of the 

 importance of oxygen, not only in the chemical, but also in all 

 vital activities. In 1818 Erxleben suggested that the Leeuwen- 

 hoek globules, which he regarded as vegetable in nature, might 

 be the cause of the fermentation a view confirmed and elabo- 

 rated by Cagniard de Latour in 1835, and also by Schwann who 

 found that oxygen had nothing to do with the process, and that 

 if air is heated fermentation does not occur in liquids properly 

 treated. He concluded that something in ordinary air is re- 

 moved by heat so that heated air has not the same effect in 

 producing fermentations as ordinary air. The older traditions, 

 however, prevailed until Pasteur's epoch making experiments 

 with heated or filtered air, and his proofs of the presence of 

 micro-organisms responsible for the fermentative and putre- 

 factive processes. 



Yeast, therefore, came to be looked upon as causing 

 fermentation through its own physiological activities, a 

 small initial quantity only being necessary because of its 

 rapid multiplication. The other chemical products of fer- 

 mentation were likewise explained; acetic acid for example 

 being the product, not of sugar breakdown but of the alcohol 

 breakdown through the agency of bacteria. The approximate 

 chemical reactions involved in the formation of alcohol and 

 acetic acid are shown as follows: 



C 6 Hi 2 O 6 (sugar) + yeast = 2C 2 H 6 O (alcohol) + 2CO 2 

 C 2 H 6 O (alcohol) + O 2 = C 2 H 4 2 (acetic acid) + H 2 O 



It has been found by analysis that only about 95% of the 

 sugar is converted into alcohol and gas, 4% is decomposed with 



