290 LOUIS PASTEUR 



laws of chemistry. We can readily see that fermentatl 

 occupy a special place in the series of chemical and bio- 

 logical phenomena. What gives to fermentations certain 

 exceptional characters of which we are only now begin- 

 ning to suspect the causes, is the mode of life in the 

 minute plants designated under the generic name of fer- 

 ments, a mode of life which is essentially different from 

 that in other vegetables, and from which result phenomena 

 equally exceptional throughout the whole range of the 

 chemistry of living beings. 



The least reflection will suffice to convince us that the 

 alcoholic ferments must possess the faculty of vegetating 

 and performing their functions out of contact with air. 

 Let us consider, for instance, the method of vintage prac- 

 tised in the Jura. The bunches are laid at the foot of 

 the vine in a large tub, and the grapes there stripped 

 from them. When the grapes, some of which are uninjured, 

 others bruised, and all moistened by the juice issuing from 

 the latter, fill the tub where they form what is called the 

 vintage they are conveyed in barrels to large vessels fixed 

 in cellars of a considerable depth. These vessels are not 

 filled to more than three-quarters of their capacity. Fer- 

 mentation soon takes place in them, and the carbonic acid 

 gas finds escape through the bunghole, the diameter of 

 which, in the case of the largest vessels, is not more than 

 ten or twelve centimetres (about four inches). The wine 

 is not drawn off before the end of two or three months. 

 In this way it seems highly probable that the yeast which 

 produces the wine under such conditions must have de- 

 veloped, to a great extent at least, out of contact with 

 oxygen. No doubt oxygen is not entirely absent from the 

 first; nay, its limited presence is even a necessity to the 

 manifestation of the phenomena which follow. The grapes 

 are stripped from the bunch in contact with air, and the 

 must which drops from the wounded fruit takes a little 

 of this gas into solution. This small quantity of air so 

 introduced into the must, at the commencement of opera- 

 tions, plays a most indispensable part, it being from the 

 presence of this that the spores of ferments which are 

 spread over the surface of the grapes and the woody part 





