LOUIS PASTEUR. 21 



gation which was undertaken by him. Every new problem in his 

 life was attacked by him from this standpoint. The great success 

 of Pasteur's work lay in the fact that his guiding principle was a 

 correct one, his great merit in his wisdom in early adopting it as a 

 law, and his genius in demonstrating it. If he had drawn an 

 inaccurate conclusion from these early experiments, he might in 

 time have corrected the error, but we must look upon the fact that 

 he had the wisdom to draw a correct inference from this first work 

 as the foundation of Pasteur's success in life. 



Pasteur now became interested in the subject of fermentation. 

 His home was in one of the important seats of fermentative 

 industries, and study of fermentation as a general phenomenon at 

 once received his attention, not only from its general interest, but 

 as especially appropriate to his life at Lille. He was thus led 

 away from the line of pure chemistry into biological work, but the 

 change was almost imperceptible. Up to the time when Pasteur 

 began his studies, fermentation had been regarded as a chemical 

 phenomenon, and it was natural that a chemist should study it. 

 In the few decades that preceded the work of Pasteur, fermenta- 

 tion had been carefully studied by a number of our chemists and 

 microscopists. While different theories had been advanced, the 

 theory of fermentation, which was almost universally held at the 

 time when Pasteur began his experiments, was that of the chemist 

 Liebig, and was a purely chemical theory. In accordance with 

 this theory of Liebig, fermentation is simply the chemical decom- 

 position of bodies produced by the unstable equilibrium of their 

 molecules. This theory held that the molecules of fermentable 

 materials were very unstable, and were easily broken to pieces into 

 simpler compounds. The ferment was held to be simply an 

 exciting cause which started this chemical decomposition. Fer- 

 mentation was thus a purely chemical subject at the time when 

 Pasteur began his studies, and the first work which he attempted 

 was to show that the chemical theory of the scientists of his day 

 should be replaced by the physiological or biological theory, 

 which he was convinced from his experiments was the correct one. 

 Upon this task he set himself at once, and by the study of the 

 lactic fermentation of milk, the butyric fermentation of milk, the 

 acetic acid fermentation in the manufacture of vinegar, and by the 



