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GENERAL PHYSIOLOGY 



direct poison. The germicidal power of light was first clearly 

 defined and proved experimentally by Downs and Blunt in 

 1878. This germicidal action of direct sunlight seems to be 

 particularly well marked in the case of those bacteria which 

 produce disease and is therefore a matter of a great deal of 



FIG. 40. Appearance of a gelatine culture of Bacterium anthracis, exposed to 

 the light over only the area E, and then incubated for 48 hours. In the area 

 E no colonies have developed. (From Ward, '93.) 



practical importance. Not only are bacteria readily killed 

 in direct sunlight when they are in a vegetative stage, but 

 Marshall Ward has shown that some of the spores are more 

 quickly killed than are the vegetative cells. 



In order to demonstrate the action of direct sunlight on 

 bacteria it is only necessary to seed an agar plate heavily 

 with the organism to be tested. When the agar is thoroughly 



