68 Lectures on Bacteria. [^ vii. 



when the smallest possible quantity of the organism is intro- 

 duced in the beginning, and its weight exactly determined. 

 The rest of the substratum is split up into the products of 

 fermentation in consequence of the processes of decomposition 

 which are connected with the vegetation, and which, as has 

 been already said, cannot be further considered here. The 

 best-known example of the kind is the alcoholic fermentation 

 of sugar by the Sprouting Fungus of beer-yeast, Saccharomyces 

 Cerevisiae, though it certainly does not strictly belong to the 

 subject-matter of these lectures. Pasteur states that in a suitable 

 solution about 1-25 per cent, of sugar was used for the formation 

 of yeast-substance, 4-5 for that of succinic acid and glycerine, 

 the remainder, 94-95 per cent., was broken up into alcohol and 

 carbonic acid. 



This example shows that the process of decomposition is 

 complex, and does not simply consist in the breaking up of all 

 the sugar into carbonic acid and alcohol. But these, in point 

 of quantity and from their importance to human requirements, 

 are the most prominent products of the fermentation in ques- 

 tion. Accordingly, we distinguish in this and all other cases 

 primary and secondary products of fermentations, and we name 

 the particular process of fermentation from some characteristic 

 primary product. 



It is known that the nature of the fermentations set up by 

 Bacteria is in general analogous with that of the case just men- 

 tioned. But in most of them the splitting-process is at present 

 less exactly understood, and in many only the primary products 

 are qualitatively known. Among these carbonic acid constantly 

 makes its appearance, as in Saccharomyces. Further remarks 

 will appear below along with special examples. At present we 

 will only briefly call attention to the colouring matters which 

 are observed not unfrequently in fermentations with Bacteria ; 

 they were noticed before on page 4, and have given rise to the 

 expression pigment-fermentations. 



Some, but not all, ferment-organisms give off into the fluid. 



