The same reference to the swelling up of the fermenting substance is seen in the Gallo-Latin terms “levure” and “leaven.”

It is highly creditable to the ingenuity of our ancestors that the peculiar property of fermented liquids, in virtue of which they “make glad the heart of man,” seems to have been known in the remotest periods of which we have any record.  All savages take to alcoholic fluids as if they were to the manner born.  Our Vedic forefathers intoxicated themselves with the juice of the “soma”; Noah, by a not unnatural reaction against a superfluity of water, appears to have taken the earliest practicable opportunity of qualifying that which he was obliged to drink; and the ghosts of the ancient Egyptians were solaced by pictures of banquets in which the wine-cup passes round, graven on the walls of their tombs.  A knowledge of the process of fermentation, therefore, was in all probability possessed by the prehistoric populations of the globe; and it must have become a matter of great interest even to primaeval wine-bibbers to study the methods by which fermented liquids could be surely manufactured.  No doubt it was soon discovered that the most certain, as well as the most expeditious, way of making a sweet juice ferment was to add to it a little of the scum, or lees, of another fermenting juice.  And it can hardly be questioned that this singular excitation of fermentation in one fluid, by a sort of infection, or inoculation, of a little ferment taken from some other fluid, together with the strange swelling, foaming, and hissing of the fermented substance, must have always attracted attention from the more thoughtful.  Nevertheless, the commencement of the scientific analysis of the phenomena dates from a period not earlier than the first half of the seventeenth century.

At this time, Van Helmont made a first step, by pointing out that the peculiar hissing and bubbling of a fermented liquid is due, not to the evolution of common air (which he, as the inventor of the term “gas,” calls “gas ventosum"), but to that of a peculiar kind of air such as is occasionally met with in caves, mines, and wells, and which he calls “gas sylvestre.”

But a century elapsed before the nature of this “gas sylvestre,” or, as it was afterwards called, “fixed air,” was clearly determined, and it was found to be identical with that deadly “choke-damp” by which the lives of those who descend into old wells, or mines, or brewers’ vats, are sometimes suddenly ended; and with the poisonous aeriform fluid which is produced by the combustion of charcoal, and now goes by the name of carbonic acid gas.

During the same time it gradually became evident that the presence of sugar was essential to the production of alcohol and the evolution of carbonic acid gas, which are the two great and conspicuous products of fermentation.  And finally, in 1787, the Italian chemist, Fabroni, made the capital discovery that the yeast ferment, the presence of which is necessary to fermentation, is what he termed a “vegeto-animal” substance; that is, a body which gives of ammoniacal salts when it is burned, and is, in other ways, similar to the gluten of plants and the albumen and casein of animals.