74 PASTEUR: THE HISTORY OF A MIND 



always borrows something from the sugar, and makes 

 a part of its own tissues out of this food. Learn also 

 that it is only on the condition of keeping a little of the 

 sugar for itself, that it consents to give you the rest in 

 the form of alcohol. 



Ah! Do you believe that you can write an equation 

 for the alcoholic fermentation as you write the equation 

 for the preparation of oxygen? Very well! Simply 

 to account for the production of the glycerin and the 

 succinic acid, the equation must be very complicated, 

 and if you wish to take into consideration the things 

 borrowed by the yeast from the sugar, it will become so 

 complex that it would be better not to write it at all. 

 Would you dream of writing in the form of an equation 

 the series of transformations which are undergone by 

 the sugar in the tea or coffee you drink? The yeast is a 

 living thing just as you are. 



This is a resume of the attack directed by Pasteur 

 against the old ideas, and when he had demolished them, 

 he set about their reconstruction. Omitting some of 

 the less important details, and taking up the exposition 

 proper, the edifice, it must be admitted, is simple: it 

 amounted to this, to produce regular fermentation under 

 conditions in which none of the prevailing theories could 

 explain the phenomenon. 



It is curious to see how the idea of this pertinent ex- 

 periment came to Pasteur little by little. 



Thenard, we have seen, had determined that there was 

 a diminution in the weight of the yeast during fermen- 

 tation, which is true for the conditions of his experiment. 

 He had found, furthermore, that this yeast, when ex- 

 hausted in the presence of an excess of sugar, no longer 

 contained nitrogen. This last was an error, due to the 

 imperfection of the methods in use for the detection of 

 this substance. 



