This experiment no doubt resembles those made with the alcoholic and lactic ferments. But it is distinguished from them by one circumstance eminently worthy of attention. The butyric ferment, by its motions and by its mode of generation, furnishes the irrefutable proof of its organisation and of its life. This ferment, moreover, presented to Pasteur a new and unexpected peculiarity. The vibrios live and multiply without the smallest supply of air or of free oxygen. Not only, indeed, do they live without air, but the air destroys them and arrests the fermentation which they initiate. If a current of pure carbonic acid is made to pass into the liquid where they are multiplying, their life and reproduction do not appear to be at all affected by it. If, on the contrary, instead of the current of carbonic acid we employ one of atmospheric air for only one or two hours, the vibrios fall without movement to the bottom of the vessel, and the butyric fermentation which was dependent on their existence is immediately arrested. Pasteur designated this new class of organisms by the name of _anaérobies_; that is to say, beings which can live without air. He reserves the designation _aérobies_ for all the other microscopic beings which, like the larger animals, cannot live without free oxygen. 'It matters little,' added Pasteur, 'whether the progress of science makes of this vibrio a plant or an animal; it is a living organism, endowed with motion, which is a ferment and which lives without air.' * * * * * In meditating upon these facts, and upon the general character of fermentation, Pasteur soon found himself in a position to approach more nearly to the essential nature of these mysterious phenomena. In what way do microscopic organisms provoke the phenomena of fermentation? The organism eats, if one may say so, one part of the fermentable matter. But how does this phenomenon of nutrition differ so much from that of higher beings? In general, for a given weight of nutritive matter which the animal takes in, it assimilates a quantity of the same order. In fermentation, on the contrary, the ferment, while nourishing itself with fermentable matter, decomposes a quantity great in comparison to its own individual weight. Again, the butyric ferment lives without free oxygen. Is there not, said Pasteur, a hidden relation between the property of being a ferment and the faculty of living without free oxygen? Are not vibrios which imperatively require for their nutrition and multiplication the presence of oxygen gas those which will never have the properties of ferments? Pasteur then contrived a series of experiments with the view of placing in parallelism these two curious physiological facts: life without air and the characteristics of ferments.