224 THE POPULAR SCIENCE MONTHLY 



mentation in a fermentable substance without any other direct rela- 

 tion. 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 fer- 

 mentable substances, were the cause of fermentation. He claimed that 

 "yeast produces fermentation in consequence of the progressive decom- 

 position which it suffers from the action of air and water." Fermenta- 

 tion and putrefaction were claimed to be processes of combustion or 

 oxidation. This theory was more fully elaborated, in 1848, by assign- 

 ing to the decomposing albuminoid fennents a peculiar molecular mo- 

 tion 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 fer- 

 mentation, 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 jDrocess. 



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 fermenta- 

 tion. About 1838 Cagniard-Latour and Schwann, by independent ob- 

 servations, rediscovered the yeast-granules of Leeuwenhoek, and, by 

 means of the better microscopes at their command, succeeded in prov- 

 ing 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 ad- 

 mission of oxygen, under conditions that excluded the germs, was with- 

 out 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 accu- 

 racy 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 



