CHAPTER II. 



BIOLOGY OF MICRO-ORGANISMS. 



THE distribution of micro-organisms is well-nigh universal. 

 They and their spores pervade the atmosphere we breathe, the 

 water we drink, the food we eat, and luxuriate in the soil 

 beneath our feet. 



They are not, however, ubiquitous, but correspond in dis- 

 tribution with that of the matter upon which they live 

 and the conditions they can endure. Tyndall* found the 

 atmosphere of high Alpine altitudes free from them, and 

 likewise that the glacier ice contained none; but wherever 

 man, animals, or even plants live, die, and decompose, they 

 are sure to be present. 



Their presence in the air generally depends upon their 

 previous existence in the soil, its pulverization, and dis- 

 tribution by currents of the atmosphere. Koch has shown 

 that the upper stratum of the soil is exceedingly rich in bac- 

 teria, but that their numbers decrease as the soil is pene- 

 trated, until below a depth of one meter there are very few. 

 Remembering that micro-organisms live chiefly upon organic 

 matter, this is readily understandable, as 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 consider- 

 able depth, micro-organisms may occur much farther below 

 the surface; yet they are rarely found at any great depth, 

 because the majority of them require free oxygen for suc- 

 cessful existence. 



The water of stagnant pools always teems with micro- 

 organisms; that of deep wells rarely contains many unless it 

 is polluted from the surface of the earth. 



It has been suggested by Soyka that currents of air 

 passing over the surface of liquids might take up organisms, 

 but, although he seemed to show it experimentally, it is 

 not generally believed. Where bacteria are growing in 

 colonies they seem to remain undisturbed by currents of 



* "Floating Matter in the Air." 



58 



