CHAPTER III. 



GENERAL CONDITIONS WHICH INFLUENCE 

 THE GROWTH OF BACTERIA. 



THE growth and multiplication of bacteria are checked 

 or increased by various external conditions, the chief 

 of which are the presence of light, heat, and oxygen, 

 and an adequate supply of water and food-materials ; 

 certain chemical substances mentioned later also have a 

 marked influence on the vital activity of these organisms, 

 i. The Action of Light. Although a small group of 

 bacteria appear to need exposure to light in order to 

 carry on their physiological functions, the majority of 

 them only thrive in very feeble light, or in darkness, and 

 their growth and development are retarded by exposure 

 to diffuse daylight. The action of direct sunlight, even 

 for a short time, results in the weakening of their vital 

 powers, and if continued destroys them altogether. The 

 blue, violet, and ultra-violet rays of light are most 

 destructive to bacteria. The deleterious effect of light 

 appears to be. partially dependent upon the presence of 

 atmospheric oxygen, since under anaerobic conditions 

 many bacteria resist sunlight for some time. In certain 

 cases the formation of hydrogen peroxide takes place in 

 the medium in which organisms are growing, and in 

 such instances the death of the bacteria may be attributed 



to the oxidizing action of this substance. 



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