■FERMENTATION. 259 



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 natme. 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 in- 

 nocent 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 putre- 

 faction, is, according to Pasteur, absolutely deadly to 

 the vibrios which provoke the butyric acid fermentation. 

 This has been illustrated by the following beautiful 

 observation. 



A drop of the liquid containing those small organisms 

 is placed upon glass, and on the top 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. 

 Eound 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 the 

 film, round it the bacteria will pirouette and wabble 



