224 



PHYSICAL INFLUENCES 



thrive better in light than without it. Direct sunlight, does not kill 

 them, it rather attracts them and they move toward the hght. This is 

 called phototaxis or heliotaxis. The pigment, bacteriopiirpurin, does 

 not take the place of chlorophyl, however, since the bacteria do not pro- 

 duce oxygen in light and always need organic food. 



The effect of hght upon microorganisms is mainly brought about by 

 a chemical change in the protoplasm, and also, to some extent, by a 

 chemical change in the medium, namely the formation of a peroxide or a 

 similar oxidizing agent. 



The germicidal action of hght is of importance in the purification of 

 rivers. It is appUed also in curing diseases of the skin, as lupus and 



Fig. 107. — Two cultures of an Aspergillus, one grown in the dark the other in 

 diffused light, showing rings, (flriginal.) 



leprosy, by exposing the diseased parts to a very concentrated Ught of 

 the electric arc. This hght contains plenty of blue and violet rays and 

 is preferable to suidight because it is always ready for use and its com- 

 position and intensity can be controlled easily. Ultra-violet hght is 

 used in the steriUzation of water and of mUk. 



Diffuse hght is not nearly as harmful to microorganisms as direct 

 sunhght. Long exposures to diffuse hght will kill most bacteria, while 

 molds are not at all sensitive. They rather like a very dim Hght, and 

 many molds grown in a dark room with hght only from one side will 

 grow toward the light. This property, which is characteristic for all 

 green plants, is called heUotropism or phototropism (Fig. 107). It has 



