FERMENTATION AND ANAEROBIOSIS 135 



surrounded by a rigid membrane which would at least very much weaken 

 the vibrations if it did not quench them. It is true that the explanation is 

 difficult to disprove, but that is always the case with views based on purely 

 hypothetical molecular movements. 



The view which perhaps more than any other agrees with the physio- 

 logical or ' biochemical ' nature of the phenomena is that which looks upon 

 them as being the result of chemical processes going on in the living sub- 

 stance of the cells. The fermentative and putrefactive organisms would 

 seem to have properties not possessed by other organisms, properties which 

 enable them to live in places and under circumstances where a complete 

 combustion of the food-stuffs to C0 2 and H 2 O is not possible *. The 

 energy that all other plants and animals gain by respiration is obtained 

 by these organisms from a less complete disintegration of the molecules 

 of the respirable substance, the relative smallness of the amount of energy 

 set free being compensated by the large quantity of substance decomposed. 

 In other words, the ordinary respiration of plants and animals is a complete 

 combustion of a few molecules, that of the ferment organisms an incom- 

 plete combustion of many molecules. The products of fermentation there- 

 fore are, unlike the products of respiration, bodies whose heat of combustion 

 is still high ; alcohol, for instance, with 3,246 calories, butyric acid with 

 3,679 calories. 



The degree of adaptation (if the expression be allowed) to anaerobic 

 habitats is very different in different organisms. The butyric ferments and 

 the methane bacteria, for instance, are absolutely (obligatory) anaerobic, and 

 have lost the power of ordinary respiration. The alcohol yeasts, lactic 

 bacteria, and most other ferment organisms are only facultatively anaerobic. 

 They are not yet fully weaned from oxygen, which, while not indispensable, 

 is far from being a poison to them. They can, even in the presence of 

 oxygen, exercise their remarkable faculty of partly disintegrating the com- 

 plex molecules of organic compounds, and gaining a small amount of energy 

 thereby. At the same time these facultative anaerobes can actually respire, 

 that is to say, can burn up completely some of the food-stuffs given them. 

 This is proved by the fact that when alcoholic fermentation goes on in the 

 presence of air a larger amount of C0 2 is evolved than corresponds to the 

 alcohol formed. Perhaps it is for this reason that the yeasts grow so quickly 

 when air is available ; the extra energy derived from the combustion of the 

 sugar to carbon dioxide and water being used to build up new cell-sub- 

 stance. If oxygen is absent, respiration is not possible ; the other source 

 of energy alone is available, and growth and multiplication are necessarily 

 slower. 



* E. g. the mud of ponds and ditches, the interior of decomposing carcases, the contents of the 

 intestine, and below the surface of dung- hills — all places in short where fermentation and putrefaction 

 are anaerobic. 



