FERMENTATION 289 



These low organisms, wliich one miglit 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. Fur- 

 thermore, 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 fermenta- 

 tion. 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 drop is placed a circle 

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

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

 scope should come very close to the organisms. Eound 

 the edge of the circular plate of glass the liquid is in con- 

 tact 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 appropriating 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 



Science — VI— 13 



