If the principles which we have laid down possess all the value that we attribute to them, if the cause of change in natural or artificial organic liquids does not exist in those liquids themselves, if change considered in itself depends upon the nature and number of the particles of dust in various places, if it is, besides, radically affected by the composition of the liquids, it must necessarily follow that wort or must, whilst, under certain circumstances of exposure to air, it remains absolutely free from life and its results, will, under other circumstances, give rise to a variety of organisms and their corresponding fermentations. This is, in fact, the lesson which direct proofs will teach us. Before entering upon these new observations in detail, we must call the reader’s attention to the difficulty, as experience has shown, of interpreting correctly the facts connected with the spontaneous impregnations of organic liquids. Gay-Lussac crushed some grapes under a bell-jar filled with mercury, after having washed them in hydrogen, to expel the air adhering to the grapes and the sides of the jar. Having waited for several weeks without detecting any signs of fermentation, he introduced some bubbles of oxygen, and fermentation showed itself the following day. Gay-Lussac concluded that the fermentation of must could not commence without the help of oxygen.[38] Under the conditions of his experiment nothing could be truer, and we must admire the diffidence with which this great natural philosopher interpreted the fact that he had observed. Another French natural philosopher, however, M. Cagniard-Latour, observed that the ferment of alcoholic fermentation was a little cellular plant. What was its origin in Gay-Lussac’s experiment? The advocates of the doctrine of spontaneous generation were ready with their explanation, and we have seen how MM. Trécul and Fremy, following many others, did not hesitate to maintain that the little plant with all its particles was produced by the action of oxygen on the albuminous substances contained in the juice of the grapes. The experiments, which we have given in the preceding paragraph, show us positively that germs of the ferment of must exist on the surface of the grape, and that, consequently, Gay-Lussac’s experiment has a more simple and natural explanation. The germs of the ferment existing on the surface of the grape become mixed with the juice of the grape when the latter is crushed; these germs remain inactive in the presence of hydrogen; they vegetate as soon as oxygen is introduced to them.