FERMENTATION. 275 



strong impregnation with hop juice of all beer intended 

 for exportation. 



These low organisms, which one might be disposed 

 to regard as the beginnings of life, were we not warned 

 that the microscope, precious and perfect as it is, has no 

 power to show us the real beginnings of life, are by no 

 means purely useless or purely mischievous in the 

 economy of nature. They are only noxious when out 

 of their proper place. They exercise a useful and 

 valuable function as the burners and consumers of dead 

 matter, animal and vegetable, reducing such matter, 

 with a rapidity otherwise unattainable, to innocent 

 carbonic acid and water. Furthermore, they are not all 

 alike, and it is only restricted classes of them that are 

 really dangerous to man. One difference in their habits 

 is worthy of special reference here. Air, or rather the 

 oxygen of the air, which is absolutely necessary to the 

 support of the bacteria of putrefaction, is, according to 

 Pasteur, absolutely deadly to the vibrios which provoke 

 the butyric acid fermentation. This has been illus- 

 trated by the following beautiful observation. 



A drop of the liquid containing those small organisms 

 is placed upon glass, and on the drop is placed a circle 

 of exceedingly thin glass ; for, to magnify them suffi- 

 ciently, it is necessary that the object-glass of the 

 microscope should come very close to the organisms. 

 Round the edge of the circular plate of glass the liquid 

 is in contact with the air, and incessantly absorbs it, 

 including the oxygen. Here, if the drop be charged 

 with bacteria, we have a zone of very lively ones. But 

 through this living zone, greedy of oxygen and appro- 

 priating it, the vivifying gas cannot penetrate to the 

 centre of the film. In the middle, therefore, the 

 bacteria die, while their peripheral colleagues continue 

 active. If a bubble of air chance to be enclosed in tbe 



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