The expression _spontaneous ferment_ may be applied to any ferment that appears in a fermentable liquid without having been purposely sown in it. In this respect the ferments mentioned in the preceding paragraph, those of all saccharine juices of fruit which ferment when left to themselves—the ferments of wine, for example—are spontaneous. The term, however, is not altogether appropriate, because, after all, the process is the same as if an actual sowing had been made, since, as we have shown, it is absolutely necessary for the juice to come into contact with the surfaces of the fruit, so that the ferment may be mixed with it, and so produce subsequent fermentation. Therefore, although we may apply the term _spontaneous ferment_ to the ferments of fruits, we intend that expression to apply in this paragraph solely to those ferments that are generated in a saccharine liquid, in which, by previous boiling, we have destroyed all ferment germs, and which, nevertheless, enters into fermentation after being exposed in free contact with air. In such a case it is entirely from the particles of dust floating in the air that the ferment germs that appear in the liquid are derived. Such are typical _spontaneous_ fermentations, and it is of the ferment so obtained that we are about to speak. In the course of the researches which we undertook in order to ascertain whether _mycoderma vini_, or vinous efflorescence, became transformed, in the case of beer, into actual alcoholic ferment—researches which were the more protracted and varied in consequence of their leading to the condemnation as erroneous, on the faith of new and more precise experiments, such as those given in Chap. IV. § 2, of that transformation, in which we had for long believed—we had occasion to observe several spontaneous fermentations of this kind in various saccharine liquids. We then proceeded to describe our method of conducting the experiments. Having brought about the development of a film of _mycoderma vini_ or _cerevisiæ_ on the surface of a liquid, fermented or not, we submerged that film in wort, which we afterwards put into long-necked flasks, in which alcoholic fermentation generally took place in the course of a few days. This fermentation in no way resulted from the transformation of the cells constituting the efflorescence into ferment. The mycodermic film merely acted as a receptacle of true ferment germs, wafted thither with the particles of dust floating in the air of the laboratory, which germs developed in the liquid into actual alcoholic ferments amongst the cells of the submerged mycoderma. By conducting experiments in this manner we brought about several spontaneous fermentations, the germs of which could have been introduced by nothing but the particles of dust in the air. These fermentations, which we were obliged to follow very carefully with the microscope from the time when they first manifested themselves, on