AEROBIC LIFE OF ANAEROBIC SPECIES 205 



them change, then the appearance of the ferment- 

 character is connected with the absence of oxygen, and 

 we are led to think that if the yeast and the analogous 

 plants can act thus on sugar during their anaerobic life, 

 it is because they have the ability to obtain from it the 

 oxygen which they need, the oxygen which serves, fur- 

 thermore, for their respiration, and which is given off 

 at once, more or less completely, in the form of carbonic 

 acid. 



According to this conception, everj' living cell having 

 need of oxygen, if deprived of this gas in a free state, and 

 if able to obtain it from substances which contain it in 

 a combined state, would be b. ferment for these substances. 

 Here is a theory of fermentation very directly related to 

 the facts, as we have come to see, and, furthermore, very 

 suggestive, for, if it notably enlarges the field of cellular 

 ferments, it at the same time restricts the field of fer- 

 mentable substances by showing that only those sub- 

 stances can be fermented which are capable of furnishing 

 to their ferment the oxygen which the latter uses by 

 burning a part of it. Every fermentable substance is 

 capable, consequently, of undergoing an internal com- 

 bustion, giving off heat thereby, for since there is a life 

 to maintain, it is necessary that there be somewhere a 

 source of energy. The living organism does not pro- 

 duce it; it consumes it in order to build up its tissues, 

 in order to make them live. So that a fermentable 

 substance, before producing heat by undergoing an 

 internal combustion, becomes, in a certain measure, 

 comparable to an explosive body, gun-cotton or nitro- 

 glycerin, which burns little by little in the labora- 

 tory of the yeast cell. Here is the very simple con- 

 ception which Pasteur called the physiological theory of 

 fermentation. 



We are forced to believe that it was not clear, since, 



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