ALCOHOLIC FERMENTATION OP SUGAR 133 



pre-existent in grape juice as had been imagined, e.g., by the 

 alchemist Basil Valentine. Becher considered that air was 

 necessary for the process ; according to the phlogistic theory, 

 an unknown substance, phlogiston, was set free on combustion. 

 As air was necessary, according to his theory, for fermentation, 

 he regarded it as a species of combustion in which likewise 

 phlogiston disappeared. The exact methods of Lavoisier 

 and Cavendish threw Ught upon this problem, as upon the 

 simpler problems of combustion and all chemical combinations 

 in general. Cavendish determined the amount of carbon 

 dioxide given off from a known weight of sugar. Lavoisier 

 weighed both the alcohol and the carbon dioxide. He 

 thought at first that they exactly made up the weight of the 

 sugar taken, and, his mind filled with the chemistry of oxygen 

 and the formation of oxides by combustion, he regarded sugar 

 also as an oxide splitting off into two simpler oxides, that is 

 alcohol and carbonic acid, by fermentation. He thought at 

 first that the yeast suffered no change, but found that this 

 was not the case, and recognised further that the breaking 

 up of the sugar was not so simple as he had at first imagined, 

 certain by-products in addition to the main substance formed 

 being always present. 



Gay Lussac in 1810 contributed some very interesting 

 experiments ; although these did not, in the light of subse- 

 quent investigations, confirm the conclusions which he drew 

 from them, yet they are highly instructive. He exposed 

 some grapes with unbroken skins to hydrogen gas so as to 

 ehminate all oxygen from their surface, he then expressed the 

 juice into a vessel over mercury in such a way that no air 

 could gain access. So long as air was not present, no fermen- 

 tation took place, but immediately oxygen was pumped into 

 the vessel, fermentation arose. He was also able to prevent 

 fermentation of grape juice by confining it in an atmosphere 

 of sulphur dioxide ; he naturally concluded that oxygen was 

 an essential factor in the fermentation process, and that in 



