INTRODUCTION. 1 5 



found bacteria on the ice of the Polar sea. Investigators 

 have even reported finding them fossilized, indicating, as 

 we might expect, that they existed at remote periods in the 

 earth's history. But the vast majority of them are entirely 

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

 are indispensable in maintaining the balance existing be- 

 tween the different kinds of living things. 



Were it not for the putrefactive and nitrifying bacteria 

 the dead bodies of plants and animals would lie practically 

 unchanged where they fell, and the fertilization of the soil 

 necessary for the life of most plants, by means of substances 

 derived from such dead material, would cease. 



In northern Siberia the bodies of the extinct species of 

 elephant 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. 



Some bacteria have been made to do work in industries, 

 like the bacilli whose growth in cream imparts an agree- 

 able flavor to the butter and cheese. 



Bacteria are also made use of in the manufacture of 

 vinegar. 



The study of bacteria has led to the understanding of 

 many hitherto unexplained facts. The unaccountable de- 

 velopment of a moist, brilliant red deposit on bread and 

 other articles of food, which was formerly believed by the 

 superstitious to be blood, deposited by some miraculous 

 agency, we know to be due to the growth of a common or- 

 ganism (bacillus prodigiosus). The emission of light by 

 decaying substances when seen in the dark may be caused 

 by bacteria as well as other organisms. 



It seems that in some cases in which death was attributed 

 to the suction of air into the veins, because air appeared to 



