44 PA THOGENIC BA CTERIA . 



pendent upon their previous existence in the soil, its pul- 

 verization, and its distribution by currents of the atmo- 

 sphere. Koch has shown that the upper stratum of the 

 soil is exceedingly rich in bacteria, but that their num- 

 bers decrease as the soil is penetrated, until below a 

 depth of one meter there are very few. Remembering 

 that bacteria can live only upon organic matter, this is 

 readily understandable. Most of the organic matter is 

 upon the surface of the soil. Where, as in the case of 

 porous soil or the presence of cesspools and dung-heaps, 

 the decomposing materials are allowed to penetrate to a 

 considerable depth, the bacteria may occur much farther 

 from the surface, yet they are rarely found at any great 

 depth, because the majority of the known species require 

 oxygen. 



The water of stagnant pools always teems with bacte- 

 ria, but that of deep wells rarely contains many unless 

 it. is polluted from the surface of the earth. 



Being generally present in the soil, which the feet of 

 men and animals grind to powder, the bacteria, together 

 with the pulverized earth, are blown from place to place 

 into every nook and cranny, until it is impossible to es- 

 cape them. It has been suggested by Soyka that the 

 currents of air passing over the surface of liquids might 

 take up bacteria, but, although he seemed to show it ex- 

 perimentally, it is not generally believed. Where bac- 

 teria are growing in colonies they seem to remain un- 

 disturbed by currents of air unless the surface becomes 

 roughened or broken. 



Most of the bacteria which are carried about by the air 

 are what are called saprophytes, and are perfectly harm- 

 less to the human being ; but not all belong to this class, 

 nor will they do so while tuberculous patients are al- 

 lowed to expectorate upon the sidewalks, and typhoid 

 patients' wash to dry upon the clothes-line, and their 

 dejecta to be spread upon the ground. 



The growth of bacteria is profoundly influenced by 

 environment, so that a consideration of the conditions 



