66 PASTEUR : THE HISTORY OF A MIND 



white wines, Liebig, who had much imagination, had 

 an explanation all ready. All the fermentable liquids 

 contain what he called gluten, what we would call to-day 

 albuminoid substances. In contact with air this 

 gluten oxidized and was precipitated in the form of 

 yeast: this is the explanation of the experiment of Gay- 

 Lussac. Consequently, in proportion as one part of 

 the yeast destroys itself by acting on the fermentable 

 substance, another forms: if more is formed than is 

 destroyed we have the case of the brewery vats; if 

 more is destroyed than is formed, we have the case of 

 Thenard's experiments concerning which we have just 

 spoken. 



For the fundamental explanation of the phenomena, 

 Liebig had only to take the ideas of Willis and of Stahl 

 on the internal movement of a mass in fermentation, 

 attributing the motive power to the ferment. ^'The 

 yeast of beer, and in general all animal and plant sub- 

 stances undergoing putrefaction, impart to other sub- 

 stances the state of decomposition in which they find 

 themselves. The movement which is imparted to 

 their own elements, as the result of the disturbance of 

 the equilibrium, is communicated equally to the elements 

 of the substances which are found in contact with them." 

 For example, sugar is a stable compound with respect 

 to a great number of external influences, air, light, even 

 heat. On the contrary, it is an unstable structure with 

 respect to the molecular movement of organic sub- 

 stances in decomposition: under their action it breaks 

 up easily into alcohol and carbonic acid. 



Thus the theory of Liebig, without denying or accept- 

 ing formally the organization of the yeast globule, 

 confined itself to denying its vital role in fermentation, 

 and collected all these phenomena into one single 

 formula. From all sides, it presented a good face, 



