A Theory of Ferments and Their Action. 
21 
causes the decomposition of sugar; and, second, that the force is sup- 
plied by living substances which the air carries — living ferments: 
Hoffman determined further, that yeast did not ferment grape sugar 
except when the sugar and yeast are in immediate contact. He ascer- 
tained this fact by interplacing in a vessel filled with grape juice a 
membranous or cotton filter, which he found quite sufficient to limit 
the fermentation which occurred when yeast was placed in the juice 
on one side, to that side of the vessel, notwithstanding that the juice, 
and all soluble matters it contained, could readily pass through the filter. 
The significance of these experiments seems not to have been appre- 
ciated at the time, as the chemical theories of Lavoisier and of Gay 
Lussac continued to fill the scientific mind. But many years later 
Lavoisier’s theory was overshadowed by the more brilliant and com- 
prehensive hypothesis of Liebig. The essential feature of Liebig’s “De- 
composition Theory” is, that the internal motions of decay, in the cells 
of an albuminous body undergoing decomposition, may be imparted to 
other albuminous bodies in contact and cause them also to undergo 
decomposition by exciting motions of decay in their cells. If the energy 
that causes substances to ferment is derived alone from the internal 
motions in cells undergoing decay and death, living cells as such, can 
have no place in Liebig’s theory of fermentation; in fact, the long and 
bitter controversy that took place between Liebig and Pasteur grew out 
of this question; Liebig holding that living cells did not in any man- 
ner influence alcoholic fermentation or its results, while Pasteur held 
that alcoholic fermentation is strictly a biologic process which living 
yeast cells alone are capable of inducing. 
Pasteur not only repeated and verified the experimental work of 
Schwann, Hoffman and of other observers, but he greatly added to this 
work until he succeeded in getting together a mass of facts in support 
of the claim, that the conversion of sugar into alcohol and carbon 
dioxide in alcoholic fermentation is due solely to the presence of living 
yeast cells, that no ground was left for the opposition to stand upon. 
But the struggle did not end here, the role that yeast cells play in 
causing the sugar molecules to dissociate had not yet been decided. 
Pasteur concluded from his observations, that the yeast cells accomplish 
this feat by taking oxygen, which the cells require, from the sugar 
molecules, and thereby causing their dissolution. He held that yeast 
cells when immersed in grape juice could not get from the outside air 
the oxygen they require for respiration, and must, therefore, get it from 
the sugar molecules; that “life without air” enabled the yeast to de- 
compose the sugar molecules, which recombine into alcohol and carbon 
dioxide. 
Liebig’s theory, notwithstanding its great popularity, was not uni- 
