iv.] YEAST. 77 



position of the other. From these considerations it became necessary 

 accurately to determine the constituent elements of the fermentable 

 substances ; and for this purpose [ did not make use of the compound 

 juices of fruits, the rigorous analysis of which is perhaps impossible, 

 but made choice of sugar, which is easily analysed, and the nature of 

 which I have already explained. This substance is a true vegetable 

 oxyd, with two bases, composed of hydrogen and carbon, brought to 

 the state of an oxyd by means of a certain proportion of oxygen ; and 

 these three elements are combined in such a way that a very slight 

 force is sufficient to destroy the equilibrium of their connection." 



After giving the details of his analysis of sugar and 

 of the products of fermentation, Lavoisier continues : 



" The effect of the vinous fermentation upon sugar is thus reduced 

 to the mere separation of its elements into two portions ; one part is 

 oxygenated at the expense of the other, so as to form carbonic acid ; 

 while the other part, being disoxygeuated in favour of the latter, is 

 converted into the combustible substance called alkohol ; therefore, if 

 it were possible to re-unite alkohol and carbonic acid together, we 

 ought to form sugar." 1 



Thus Lavoisier thought he had demonstrated that the 

 carbonic acid and the alcohol which are produced by 

 the process of fermentation, are equal in weight to the 

 sugar which disappears ; but the application of the more 

 refined methods of modern chemistry to the investigation 

 of the products of fermentation by Pasteur, in 1860, 

 proved that this is not exactly true, and that there is 

 a deficit of from 5 to 7 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 1| per cent, still remains 

 to be made good. According to Pasteur, it has been 

 appropriated by the yeast, but the fact that such appro- 

 priation takes place cannot be said to be actually proved. 



i " Elements of Chemistry." By M. Lavoisier. Translated by Robert 

 Kerr. Second Edition, 1793 (pp. 186196). 



