Polytechnic Association. 797 



of the products of fermentation by Pasteur, in 1860, proved that this 

 is not exactly true, and that there is a deficit of from five to seven 

 per cent of the sugar which is not covered by the alcohol and carbonic 

 acid evolved. The greater part of this deficit is accounted for by the 

 discovery of two substances, glycerine and succinic acid, of the 

 existence of which Lavoisier was unaware, in the fermented liquid. 

 But about one and one-half per cent still remains to be made good. 

 According to Pasteur, it has been appropriated by the yeast, but the 

 fact that such appropriation takes place cannot be said to be actually 

 proved. 



However this may be, there can be no doubt that the constituent 

 elements of fully ninety-eight per cent of the sugar which has vanished 

 during fermentation have simply undergone rearrangement; like the 

 soldiers of a brigade, who at the word of command divide themselves 

 into the independent regiments to which they belong. The brigade 

 is sugar, the regiments are carbonic acid, succinic acid, alcohol and 

 glycerine. 



From the time of Fabroni, onward, it has been admitted that the 

 agent by which this surprising rearrangement of the particles of the 

 sugar is effected is the yeast. But the first thoroughly conclusive 

 evidence of the necessity of yeast for the fermentation of sugar was 

 furnished by Appert, whose method of preserving perishable articles 

 of food excited so much attention in France at the beginning of this 

 century. Gay-Lussac, in his Memoir sur la Fermentation,* alludes 

 to Appert' s method of preserving beer-wort unfermented for an indefi- 

 nite time, by simply boiling the wort and closing the vessel in which 

 the boiling fluid is contained in such a way as thoroughly to exclude air ; 

 and he shows that if a little yeast be introduced into such wort, after 

 it has cooled, the w r ort at once begins to ferment, even though every 

 precaution be taken to exclude air. And this statement has since 

 received full confirmation from Pasteur. 



On the other hand, Schwann, Schroeder and Dusch, and Pasteur, 

 have amply proved that air may be allowed to have free access to 

 beer-wort, without exciting fermentation, if only sufficient precautions 

 are taken to prevent the entry of particles of yeast along with the air. 



Thus, the truth that the fermentation of a simple solution of sugar 

 in water depends upon the presence of yeast, rests upon an unassailable 

 foundation ; and the inquiry into the exact nature of the substance 

 which possesses such a wonderful chemical influence becomes pro- 

 foundly interesting. 



* Annates de Chimle, 1810. 



