CAGNIARD-LATOUR, SCHWANN, HELMHOLTZ 59 



it it was entirely a work of calculation. It had no 

 other foundation than the experiment of Lavoisier which 

 was evidently not exact, and had not been the object of 

 any verification. 



Any one who had wished, about 1850, to get an idea 

 of the degree of credence which the equation merited, the 

 equation of alcoholic fermentation accepted everywhere, 

 would have been justified in being entirely sceptical on 

 the subject, especially if he had asked himself why all 

 the chemists who were occupied with this question passed 

 by obstinately in silence this yeast which Lavoisier had 

 been obliged to add to make his sugar ferment, and with- 

 out which it was impossible to obtain any fermentation. 

 Why should this yeast, so necessary in the experiment, 

 disappear in its interpretation? 



Ill 

 CAGNIARD-LATOUR, SCHWANN, HELMHOLTZ 



This yeast was known in the vats of the brewery as a 

 kind of superficial scum, or as a precipitate on the bottom, 

 a scum or deposit in which resided an occult force. It 

 multiplied when introduced into a sweetened must and 

 caused it to ferment: apparently when it was not intro- 

 duced it formed there spontaneously, and Thenard had 

 shown, in 1803, that all sugared juices which ferment of 

 their own accord, give a precipitate having the external 

 appearance and the properties of the yeast of beer. 



This yeast seemed, therefore, necessary to the fer- 

 mentation. In an experiment which perplexed Pasteur 

 too much for us to pass over it, Gay-Lussac had shown 

 that something else was needed. That able physicist 

 caused to mount to the top of a test tube filled with 



