CHAPTER II. 



THE LIVING ELEMENTS OF DUST ; WHAT THEY 

 ARE AND WHERE THEY COME FROM. 



ALL those forms of minute vegetable life 

 which swarm in myriads almost every- 

 where upon the earth's surface are called in 

 general micro-organisms or germs. Among 

 these there are three prominent forms which 

 are called bacteria, yeasts, and moulds (see 

 Plate I.). Among these the bacteria are by 

 far the most important. These tiny organisms 

 are for the most part so very small that many 

 thousands or millions of them clustered closely 

 together would not make a mass larger than 

 the head of a pin. Some of them are round 

 or ovoidal, some rod-like, some spiral (see 

 Plate I. Fig. 3). Most of them are harmless 

 to man, and serve a very important purpose 

 in the economy of nature in tearing asunder 



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