THE YEASTS: A CHAPTEK IN MICROSCOPICAL 



SCIENCE ^ 



By A. Chaston Chapman, F. R. S., F. I. C, F. C. S., P. R. M. S. 



The subject on which I have the honor to address 3^ou this evening 

 is intimately connected with phenomena in which mankind has 

 taken a very deep interest from time immemorial. 



It is not my intention to deal with the antiquity of alcoholic 

 fermentation, except just to say that references to the use of fer- 

 mented beverages by ancient peoples, such as the Chinese and Egyp- 

 tians, chiefly in connection with religious and ceremonial observances, 

 appear to date back more than 4,000 years, and in the Vedic books — 

 the oldest literary monuments of the Indo-European races — there 

 are many references to intoxicating beverages. 



The word " fermentation," from fervere^ to boil or seethe, was 

 at first applied to all cases of chemical change whose cause was 

 unknown, and which were accompanied by the formation of large 

 quantities of gas, giving the liquid the appearance of boiling or 

 seething. In its widest sense the word is still occasionally applied 

 to a number of chemical processes in which micro-organisms are 

 the active agents, such, for example, as the souring of milk, the 

 conversion of alcohol into vinegar, the production of butyric acid, 

 and similar processes. In its restricted sense, hoAvever, and in the 

 sense in which I shall use it this evening, it is applied to the con- 

 version of sugar into (mainly) alcohol and carbon dioxide gas by 

 means of the organism known as yeast. 



The subject of fermentation is capable of rough division into 

 two parts — namely, the phenomenon, that is to say, the i^rocess, and 

 the origin or cause of that phenomenon — yeast. 



With fermentation regarded as a chemical process I do not 

 propose to deal, beyond referring very briefly to a few of the earlier 

 attempts made to explain an occurrence which was characterized 

 by so much that was mysterious. The word '• fermentation " is 

 very frequently met with in writings of the alchemists from the 

 thirteenth to the fiftenth centuries, but it is applied to a consider- 

 able number of chemical processes of all descriptions, and the ideas 



* Address of the President of the Royal Microscopical Society, read Jan. 21, 1925. 

 Reprinted by permission from the Journal of the Royal Microscopical Society, March, 1925. 



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