1870—1872 221 



ing the typical ferment, the yeast of beer, an article inserted in 

 the reports of the Academic des Sciences for 1861, and entitled, 

 The Influence of Oxygen on the Development of Yeast and on 

 Alcoholic Fermentation. In this article Pasteur, a propos of 

 the chemical action connected with vegetable life, explained in 

 the most interesting manner the two modes of life of the yeast 

 of beer. 



1. The yeast, placed in some sweet liquid in contact with air, 

 assimilates oxygen gas and develops abundantly; under those 

 conditions, it practically works for itself only, the production 

 of alcohol is insignificant, and the proportion between the 

 weight of sugar absorbed and that of the yeast is infinitesimal. 

 2. But, in its second mode of life, if yeast is made to act upon 

 sugar without the action of atmospheric air, it can no longer 

 freely assimilate oxygen gas, and is reduced to abstracting 

 oxy^gen from the fermentescible matter. 



*'It seems therefore natural," wrote Pasteur, **to admit that 

 when yeast is a ferment, acting out of the reach of atmospheric 

 air, it takes oxygen from sugar, that being the origin of its 

 fermentative character." It is possible to put the fermenta- 

 tive power of yeast through divers degrees of intensity by intro- 

 ducing free oxygen in variable quantities. 



After comparing the yeast of beer to an ordinary plant, 

 Pasteur added that **the analogy would be complete if ordinary 

 plants had an affinity for oxygen so strong as to breathe, by 

 withdrawing that element from unstable components, in which 

 case they would act as ferments on those substances." He sug- 

 gested that it might be possible to meet with conditions which 

 would allow certain inferior plants to live away from atmo- 

 spheric air in the presence of sugar, and to provoke fermenta- 

 tion of that substance after the manner of beer yeast. 



He was already at that time scattering germs of ideas, with 

 the intention of taking them up later on and experimenting on 

 them, or, if time should fail him, willingly offering them to any 

 attentive scientist. These studies on beer had brought him 

 back to his former studies, to his great delight. 



*^What a sacrifice I made for you," he could not help saying 

 to Dumas, with a mixture of affection and deference, and 

 some modesty, for he apparently forgot the immense service 

 rendered to sericiculture, "when I gave up my studies on 

 ferments for five whole years in order to study silkworms!!!" 



No doubt a great deal of time was also wasted by the endless 



