66 Lectures on Bacteria. [^ vii. 



ethyl-alcohol. Reductions are of rarer occurrence, as in the 

 splitting of sulphates by Beggiatoa, which will be described 

 presently. The last to be mentioned are the splittings which 

 end in other than simple products of oxidation and are included 

 under the general term of fermentations ; of these the best- 

 known example in every respect is the alcoholic fermentation, 

 which is the splitting of the different sugars into ethyl-alcohol 

 and carbonic acid. If these splittings are accompanied with a 

 development of offensive gas, especially in compounds containing 

 nitrogen, the term putrefaction is used, an expression rather 

 popular and expressive than strictly and scientifically defined. 



It is no part of our subject to enter further into the chemical 

 nature of these processes, the purely chemical and physical 

 sides of the theories of fermentation. With regard to the 

 general history of these theories also, we shall only observe 

 that it has been an established scientific truth since about the 

 year i860, that the entire series of phenomena of rotting 

 and fermentation above mentioned are the results of processes 

 of life and vegetation in certain lower organisms, especially 

 Fungi and Bacteria. To Pasteur belongs the entire credit of 

 having placed this vitalistic theory of fermentation on a firm 

 basis, in opposition to other views whi6h acknowledged no 

 causal relations at all between it and living organisms or causal 

 relations of a different kind, and of having extended it to all 

 phenomena of a similar kind. It is true that the same 

 vitalistic theory has been distinctly expressed in the case of 

 alcoholic fermentation since the time of Cagniard-Latour (1828) 

 and Schwann (1837), but it never obtained general accep- 

 tation. 



The vegetative process of living organisms is then the direct 

 cause of fermentations ; there is no fermentation if the organisms 

 are destroyed. Organisms of this kind are therefore termed 

 fermentation-exciters, ferment-organisms, or simply ferments 

 in the terminology of the school of Pasteur. In that of Nageli 

 they are known as yeast, and according as the ferment-organism 



