116 YEAST IV 



of the term " gas," calls " gas ventosum "), but to 

 that of a peculiar kind of air such as is occasionally 

 met with in caves, mines, and wells, and which 

 he calls " gas sylvestre." 



But a century elapsed before the nature of this 

 "gas sylvestre," or, as it was afterwards called, 

 "fixed air," was clearly determined, and it was 

 found to be identical with that deadly " choke- 

 damp " by which the lives of those who descend 

 into old wells, or mines, or brewers' vats, are 

 sometimes suddenly ended ; and with the poisonous 

 aeriform fluid which is produced by the combus- 

 tion of charcoal, and now goes by the name of 

 carbonic acid gas. 



During the same time it gradually became 

 evident that the presence of sugar was essential to 

 the production of alcohol and the evolution of 

 carbonic acid gas, which are the two great and 

 conspicuous products of fermentation. And finally, 

 in 1787, the Italian chemist, Fabroni, made the 

 capital discovery that the yeast ferment, the 

 presence of which is necessary to fermentation, 

 is what he termed a " vegeto-animal " substance ; 

 that is, a body which gives off ammoniacal salts 

 when it is burned, and is, in other ways, similar 

 to the gluten of plants and the albumen and 

 casein of animals. 



These discoveries prepared the way for the 

 illustrious Frenchman, Lavoisier, who first ap- 

 proached the problem of fermentation with a 



