36 SULPHUR BACTERIA 



It is not always possible to draw sharp lines of distinction 

 between these three classes of reactions. 



Fermentation.^ The history of the terms Ferment and 

 Fermentation will give some explanation of the slight confusion 

 that exists in the use of these words. The word ferment comes 

 from the Latin fervere, to boil, and referred originally to the 

 apparent " boihng " that took place when yeast in fermenta- 

 tion gave off copious bubbles of carbon dioxide. At f^rst the 

 liquid in which this bubbling took place was named the 

 ferment, and is so named by bakers to the present day. With 

 greater knowledge the term was restricted to the organism 

 that caused this bubbling. With a still further advance in 

 our knowledge the term fermentation was applied to other 

 processes in which a medium was chemically affected by 

 the activities of microorganisms even although there was no 

 obvious gas production ; such, for example, as the production 

 of lactic acid in the souring of milk. Then it was ascertained 

 that in many cases the actual chemical work was done, not 

 by the organism directly, but by a secretion formed from it, 

 which in some instances could operate after the organism itself 

 was dead. Thus arose the distinction between an organized 

 ferment and an unorganized ferment, the former referring to 

 the cases in which the protoplasm itself accomplished the 

 work, and the latter to those cases in which the secretion 

 was recognizable apart from the protoplasm producing it. 

 The special secretion then became known as the unorganized 

 ferment, or more simply as the ferment. 



The organized ferments fall into the first of the three classes 

 of katabolic processes cited above, and the unorganized fer- 

 ments into the third. 



Metabolism and Fermentation. 



The view is adopted in this work that all fermentative 

 processes are of a katabohc nature. Katabolic processes 

 as a whole all come under one of two categories :— 



I. Reactions which convert food substances to a form 

 more readily assimilable by the organism. 



