76 PASTEUR: THE HISTORY OF A MIND 



globules with the elements of their structure, and an 

 ammonia salt to provide them with nitrogen. If, in 

 this medium, completely robbed of that organic nitrog- 

 enous matter which Liebig declared to be necessary, 

 one obtains a fermentation, and if, at the same time, 

 the yeast multiplies and develops, deriving all the 

 complex elements for its tissues from the sugar and the 

 ammonia, it will certainly be impossible not to admit 

 a correlation between the fermentation and a phe- 

 nomenon of development and of life in this ferment 

 which Liebig considered a dead substance. 



With the same blow by which the theory of Pasteur 

 triumphed there fell into ruin not only the theory of 

 Liebig, but another theory then much less flourishing, 

 namely that of Berze'lius, according to which the ferment 

 exerted an action only by its presence, and provoked 

 the decomposition of the organic matter without deriving 

 anything from it or contributing anything to it, remain- 

 ing, furthermore, unchanged in quantity and quality 

 throughout the process. In our experiment, the ferment 

 must, on the contrary, increase in weight, taking all this 

 increase from the sugar. 



It is just because this experiment was so interesting 

 that it was difficult. It was necessary, in the first place, 

 for Pasteur to devote his attention to supplying the 

 yeast with a suitable mineral medium, and a rather com- 

 plex one too, including phosphates, salts of potassium, 

 magnesium, and ammonia. It matters not that the 

 cell of the yeast is small ; its needs are great and varied. 

 It was the first time that Pasteur had run up against its 

 exacting requirements, and the lesson which he drew 

 from this contact was not lost. Furthermore, even when 

 one gives to the yeast all that it needs in the way of 

 mineral substances, it has much more difficulty in living 

 in this medium where it must form all the substances 



