THEORY OF FERMENTATION 311 



The conclusions to be drawn from the whole of the pre- 

 ceding facts can scarcely admit of doubt. As for our- 

 selves, we have no hesitation in finding them the foundation 

 of the true theory of fermentation. In the experiments 

 which we have described, fermentation by yeast, that is 

 to say, by the type of ferments properly so called, is pre- 

 sented to us, in a word, as the direct consequence of the 

 processes of nutrition, assimilation and life, when these 

 are carried on without the agency of free oxygen. The 

 heat required in the accomplishment of that work must 

 necessarily have been borrowed from the decomposition 

 of the fermentable matter, that is from the saccharine sub- 

 stance which, like other unstable substances, liberates heat 

 in undergoing decomposition. Fermentation by means of 

 yeast appears, therefore, to be essentially connected with 

 the property possessed by this minute cellular plant of 

 performing its respiratory functions, somehow or other, with 

 oxygen existing combined in sugar. Its fermentative 

 power which power must not be confounded with the fer- 

 mentative activity or the intensity of decomposition in a 

 given time varies considerably between two limits, ^xed 

 by the greatest and least possible access to free c xygen 

 which the plant has in the process of nutrition. If we 

 supply it with a sufficient quantity of free oxygen for the 

 necessities of its life, nutrition, and respiratory combus- 

 tions, in other words, if we cause it to live after the manner 

 of a mould, properly so called, it ceases to be a ferment, 

 that is, the ratio between the weight of the plant developed 

 and that of the sugar decomposed, which forms its princi- 

 pal food, is similar in amount to that in the case of fungi. 18 

 On the other hand, if we deprive the yeast of air entirely, 

 or cause it to develop in a saccharine medium deprived of 

 free oxygen, it will multiply just as if air were present, 



afterwards, long after the death of the plant, we examined this liquid. It 

 contained 0.3 gramme (4.6 grains) of alcohol, and 0.053 gramme (0.8 grain) 

 of Tegetablc matter, dried at 100 C. (212 F.). We ascertained that the 

 pores of the fungus were dead at the moment when the flask was opened. 

 Whrn sown, they did did not develop in the least degree. 



M We find in M. Raulin's note that " the minimum ratio between the 

 weight of sugar and the weight of organized matter, that is, the weight of 

 fungoid growth which it helps to form, may be expressed as 4.^=3.1." 

 JULES RAULIN, Etude* chimtquts svr la vtpttation. Reckerckes stir It 

 dHftlopfemtnt d'unt muctdintt dans wn mi/iVtt ortificifl, p. 192, Paris. 

 1870. Wt Iwvt teen in the case of yeast that this ratio may be as low as |. 



