INTRODUCTION. 5 



ing, 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 between the 

 different kinds of living things. 



Were it not for the putrefactive and nitrifying bacteria the 

 dead bodies of plants and animals would He 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. 



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 modem refrigerator or cold-storage plant. 



Some bacteria have been made to do work in industries, like 

 the bacilli whose growth in cream imparts an agreeable flavor 

 to the butter and cheese. 



Other bacteria are also made use of in the manufacture of 

 ^•inegar. 



Some kinds of bacteria of the soil are employed to take 

 nitrogen from the atmosphere, adding nitrogen compounds to 

 the soil, which take the place of chemical fertilizers. 



The study of bacteria has led to the understanding of many 

 hitherto unexplained facts. The unaccountable development 

 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 organism (bacillus prodigiosus). 

 The emission of light by decaying substances when seen in the 

 dark is caused by bacteria as well as other organisms. 



It seems that in some cases in which death has been attributed 

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

 be present inside the heart, the air was in reality a gas, formed 



