20 THEORIES OF FERMENTATION. 



onslaught against this theory would be merely storming an un- 

 defended position. Moreover, as the subsequent editions of the 

 aforesaid work demonstrate, it was gradually modified by its 

 author, so that the form in which it was presented in his latest 

 exposition (II.) in 1870 differs in many particulars from the original. 

 The ocular demonstration, in individual instances, of the 

 untenable nature of the hypotheses supporting this theory, to 

 those whom the representations of Latour, Kiitzing, and Schwann 

 had not succeeded in convincing, was the congenial task under- 

 taken by Pasteur, and brought by him to a successful issue with 

 great experimental skill. 



16. Pasteur's Theory of Fermentation (III.). 



The victorious antagonist of the theory of spontaneous gene- 

 ration was not content with controverting the views of Liebig, he 

 also sought to erect a better theory in its stead. According to 

 this doctrine, it is the lack of free oxygen that leads to the fer- 

 mentation being set up by the organism as a means of supplying 

 itself with the energy it requires by seizing upon the oxygen 

 thereby obtainable. "Fermentation is life without air." A very 

 slight experience in this matter suffices for the recognition of the 

 fact that this theory takes no account of the several kinds of 

 fermentation in which the presence of oxygen is a necessary con- 

 dition, viz., the so-called oxidation fermentations the best example 

 of which is afforded by the acetic fermentation. In this respect 

 the theory cannot be further alluded to in the present chapter, 

 which is devoted to general considerations. It will be fully dealt 

 with in a subsequent chapter. The only remark to be made now 

 is that this theory also has proved untenable. 



The permanent value of the services rendered to Fermentation 

 Physiology by Pasteur are not diminished by the disproval of 

 his theory of fermentation, since they have their root in the 

 successful endeavour, by means of careful and extensive experi- 

 mental demonstration, to bring into universal recognition the 

 theory originated but only imperfectly formulated by Cagniard, 

 Kiitzing, and Schwann, of the causative connection between fer- 

 mentation and the vital activity of the microbe. 



17. C. Nag-eli's Physieo- Molecular Theory. 



Although it was by this time indubitably established that 

 without the vital activity of micro-organisms no fermentation 

 could occur, no clear account had been given as to hoio the 

 activity itself was exerted. Several explanations were possible. 

 According to one which was especially advocated by Kiitzing 

 and Pasteur, a decomposition was effected within the cells of 

 the organic ferments, which obtained their nourishment from the 



