X. 



BACTERIA, AND BACTERIOLOGICAL 

 METHODS AND DIAGNOSIS. 



BACTERIA are minute cells, classified biolog- 

 ically far down the line in the vegetable king- 

 dom. "' They vary much in size, but all are so small as 

 ,to be beyond the range of unaided vision. An or- 

 dinary-sized bacterium would be one ten-thousandth 

 to one twenty-thousandth of an inch in diameter or 

 length. When seen singly, they are practically color- 

 less, but many bacteria, when growing in masses or 

 colonies, present distinct and even brilliant colors. 

 Such colonies often resemble, in gross appearance, the 

 ordinary moulds. 



While bacteria are so numerous and widespread as 

 to be commonly said to be ubiquitous, it is not true, 

 as many believe, that the air is swarming with them. 

 Like all bodies of greater specific gravity than air, 

 they tend to settle, so that the quiet atmosphere in 

 many places is comparatively free from microbes. 

 Exposed waters, the surface and the upper layers of 

 the soil, and the surfaces of solid bodies generally, 

 harbor bacteria in large numbers; but the deeper 



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