134 BACTERIA AND THE CARBON DIOXIDE CYCLE 



The bleached solutions become blue again if shaken up with air, showing 

 that there has been no far-reaching decomposition of the molecule. All 

 that has happened is that nascent hydrogen has been taken up from the 

 molecules of the fermenting substance. Whether the hydrogen is set free 

 in consequence of the organism seizing oxygen from those molecules, or 

 whether part of it is due to some other disintegration going on in the living 

 protoplasm, is unknown to us. In any case, it is plain that Pasteur's dictum, 

 ' Fermentation is life without oxygen,' does not quite correspond with facts. 

 And the circumstance that fermentation (even alcoholic) can go on in the 

 presence of oxygen is a further argument against such a view, and against 

 the theory that fermentation is a form of intramolecular respiration. 



Another explanation, older even than Pasteur's, was that proposed by 

 Traube in 1858, the 'enzyme' theory. Traube imagined that ferment 

 organisms accomplished all their work by means of enzymes excreted by 

 their protoplasm. Such a view could of course be held only as long as it 

 was thought that fermentation was a simple splitting-up of complex 

 molecules into smaller ones : for instance, that alcoholic fermentation took 

 place according to the formula 



and that no by-products arose. Since we now know that a large number of 

 by-products are formed, and since it has not been possible to isolate the 

 supposed enzymes, Traube's theory had to be abandoned (111). In one case 

 only, that of the ammoniacal fermentation of urea, did it seem likely that the 

 change was effected by an enzyme, for the process is one of simple hydrolysis 



(NH 2 ) 2 CO + 2 H 2 O = (NH 4 ) 2 CO 3 . 



The enzyme in question, ' urase' (112), has now been isolated. It is a body 

 of great instability. 



The difference between the work of enzymes and organized ferments 

 must be here again referred to. Enzymes are doubtless active in all 

 fermentative processes, but the changes they bring about are only prepara- 

 tory ; for example, the inversion of sugars by yeasts, and the peptonizing of 

 albumens by putrefactive bacteria. The actual fermentative processes with 

 their numerous by-products are not to be explained by enzymic action, 

 they are effected by the living protoplasm of the cells *. 



Another explanation of the phenomena of fermentation, an explanation 

 of a totally different character, was proposed by Nageli in 1 879. According to 

 this, the process is entirely extra-cellular, being brought about by vibrations 

 set up by the molecules of the protoplasm and transmitted to the molecules 

 of the substances outside the cell, which are thereby split up into the various 

 bodies we call products of fermentation. This at first sight very plausible 

 hypothesis is rendered extremely improbable by the fact that the cell is 



* See note No. 111. 



