INTRODUCTION. 5 



on most of the surfaces of the body that open to the external 

 world. They are found in the water of rivers and lakes and in 

 the ocean. They appear in the soil down to a depth of several 

 feet. They float in the air, except at high altitudes and over 

 the ocean. Nansen found bacteria on the ice of the Polar sea. 

 Investigators have even reported finding them fossilized, indi- 

 cating, as we might expect, that they existed at remote periods 

 in the earth's history. But the vast majority of them are en- 

 tirely harmless so far as we are concerned, and many of them 

 are indispensable in maintaining the balance existing between 

 dead matter and living beings. 



Were it not for the putrefactive and nitrifying bacteria, the 

 dead bodies of plants and animals would lie practically un- 

 changed where they fall, and the fertilization of the soil neces- 

 sary for the life of most plants, by means of substances derived 

 from such dead material, would cease. 



Some kinds of bacteria are useful in taking nitrogen from the 

 atmosphere, and making it available as plant food in the soil, 

 and are thus employed in the place of chemical fertilizers. 

 Many of them have been made to subserve a useful purpose in 

 the ripening of cream and cheese, and in the manufacture of 

 vinegar from wine and cider. It has been suggested with 

 some plausibility that anaerobic bacteria played an im- 

 portant part in the formation of coal from vegetable substances. 



The ripening of ensilage in silos is a process of fermentation 

 caused by bacteria. 



In northern Siberia the bodies of the extinct species of ele- 

 phant called mammoths have been found imbedded in frozen 

 soil where they appear to have lain for thousands of years. In 

 this case the growth of putrefactive bacteria has been prevented 

 by cold, as in the modern refrigerator or cold-storage plant. 



The study of bacteria has led to the understanding of many 

 hitherto unexplained phenomena. The unaccountable devel- 

 opment of a moist, brilliant red deposit on bread and other 



