FERMENTATION 37 



form a most useful economic function in preparing food mate- 

 rial in the soil for use by the green plants. But the chief 

 biological interest of these forms is that they are able to build up 

 their own protein molecules directly from ielatively simple sub- 

 stances without the aid of chlorophyll, and to get energy from 

 such compounds in which it is locked up for all other kinds of 

 living things. Thus urea, thrown off by animals and plants as a 

 useless and to them harmful waste matter, is a source of food 

 and energy for some bacteria which convert it into free am- 

 monia, carbon dioxide and water. 



C. ENZYMES, HORMONES AND VITAMINES 



Alcoholic Fermentation.- -The control of alcohol production 

 in practical ways was well understood long before the explana- 

 tion of its production was worked out. The term fermentation 

 was early given to all processes involving the generation of gas, 

 probably because of the froth or foam which appears during 

 alcohol formation or when acids are allowed to act on carbon- 

 ates. In the i yth century, however, a distinction was drawn 

 between alcoholic fermentation and acid fermentation, and it 

 was recognized that alcohol is a new product of the fermenta- 

 tion process and quite distinct from the gas vlnorum (CO 2 ) 

 arising at the same time. In the iyth century also, Leeuwen- 

 hoek, the first microscopist, discovered that the scum or deposit 

 which is always present during fermentation is made up of 

 small spherical bodies which he did not attempt to identify as 

 animal or plant. With Lavoisier in the iSth century, chemistry 

 became a more exact science and, in connection with alcoholic 

 fermentation, it was found that the sugar in fermenting fluids 

 breaks down into alcohol and C0 2 gas, with traces of glycerine 

 and acetic acid. As with most other chemical processes, fer- 

 mentation was regarded by Lavoisier as a process of oxidation, 

 and the spherical bodies discovered by Leeuwenhoek were con- 

 sidered unimportant. Early in the igth century, however, a 

 reaction set in against many of Lavoisier's views, and its effects 

 are seen in the interpretation of alcoholic fermentation. On 



