mentation in a fermentable substance without any other direct relation. In 1840 Liebig presented a theory of fermentation which was generally adopted by chemists. He recognized fermentation and putrefaction as essentially similar processes. Albuminoid substances, from the complex arrangement of their molecules, were assumed to be in a state of unstable equilibrium tending to decomposition, and their putrefactive transformations, which were communicated to fermentable substances, were the cause of fermentation. He claimed that "yeast produces fermentation in consequence of the progressive decomposition which it suffers from the action of air and water." Fermentation and putrefaction were claimed to be processes of combustion or oxidation. This theory was more fully elaborated, in 1848, by assigning to the decomposing albuminoid ferments a peculiar molecular motion which communicated to fermentable substances a similar vibration of their particles, and a consequent decomposition. This was in fact but a revival of the theory of Willis and Stahl more than two hundred years before. Notwithstanding its general acceptance by chemists, Liebig's theory failed to recognize one of the essential factors of fermentation, and we must now turn our attention to a brief outline of some of the discoveries which disproved it, and furnished on the other hand a complete and satisfactory explanation of the process.
Leeuwenhoek, in 1680, made the discovery that yeast was composed of minute granules, but, with the imperfect lenses of that time, he failed to determine their real character. Fabroni, in 1787, described the yeast-granules as a vegeto-animal substance; and Astier, as early as 1813, claimed that this ferment was endowed with life, and derived its nourishment from the fermenting materials, thus causing fermentation. About 1838 Cagniard-Latour and Schwann, by independent observations, rediscovered the yeast-granules of Leeuwenhoek, and, by means of the better microscopes at their command, succeeded in proving that they were vegetable cells which were reproduced by budding. Schwann, by a series of ingenious experiments, proved that the germs of the living ferments were conveyed to fermentable substances by the air, and that they were the cause of fermentation, while the free admission of oxygen, under conditions that excluded the germs, was without effect. The experiments of Schwann were, in themselves, sufficient to establish the truth of the physiological theory of fermentation, but they were entirely ignored by Liebig and the advocates of his chemical theory. A complete demonstration of the true theory of fermentation was finally made by Pasteur (1857-'79) in a series of experiments which, from the skill displayed in their conception, and the remarkable accuracy secured in conducting them in accordance with strictly inductive methods, may safely be classed among the most brilliant records in the history of science. He repeated the experiments of his predecessors, invented new methods of investigation by which he was enabled to eliminate all possible sources of error, and answered his opponents by
