CHAPTER III. 

 INFLUENCE OF LIGHT AND OTHER RAYS. 



Microorganisms in their natural environment are temporarily but not 

 usually exposed to light. The organisms of decay, living in soil, in 

 foods, in the intestines of animals, will only occasionally come in contact 

 with the direct rays of the sun. Water bacteria and the organisms on 

 the surface of plants and animals are more commonly exposed to the sun. 



The influence of light varies with its intensity. Direct sunlight has a 



FIG. 52. These plates were heavily inoculated with B. coli and B. prodigiosus 

 respectively and then were exposed, bottom side up, to the direct rays of the January 

 sun, for four hours. On the instant of exposure, a figure O, cut from black paper was 

 pasted to the plate shading the bacteria underneath. After one, two and three hours, 

 the corresponding figures were pasted to the plates. The above picture was taken 24 

 hours after exposure, proving that three or four hours of direct sunlight weaken and 

 may even kill bacteria. B. prodigiosus proved more sensitive than B. coli. (Original.) 



very harmful effect upon microorganisms. Most bacteria are killed by 

 direct sunlight in a few hours; the time depends upon the organism 

 as well as upon the intensity of light; this again varies with the amount of 

 moisture and dust in the atmosphere, with the time of the day and with 

 the season; an absolute measure for the action of light cannot be fixed, 



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