THE VITAL CONDITIONS OF BACTERIA. 103 



The ultra violet, violet and blue light have a power- 

 ful injurious effect, green light has a feeble effect, 

 and red and yellow have none at all. 



The action of light seems to be dependent in part 

 on the oxygen of the air. Strict anaerobic (tetanus) 

 and facultative anaerobic varieties (bacterium coli) 

 tolerate sunlight very well if there is complete exclu- 

 sion of oxygen. 



Richardson and recently Dieudonne have discov- 

 ered a fact which possesses great importance in re- 

 gard to the mechanism of the action of light, al- 

 though it does not explain everything. They found 

 that hydrogen hyperoxide (H 2 O 2 ) develops in a short 

 time (in ten minutes in direct sunlight) upon illumi- 

 nated agar plates, but only in blue to ultra violet 

 light. * An agar plate, half covered with black paper, 

 is exposed to the light, then a paste containing a 

 small amount of potassium iodide is poured over it 

 and this followed by a weak solution of sulphate of 

 ferric oxide, the illuminated side turns a bluish-black. 

 In gases which contain no oxygen H^O, does not form 

 and light does not give rise to any injury. This 

 also explains the fact that slight " attenuation" of the 

 bacilli is also observed frequently when agar plates 

 which have been standing in the sun \ are inocu- 

 lated. Bacteria which have been previously ex- 

 posed to the light develop with special difficulty on 

 an illuminated nutrient medium. 



* Hours elapse before H 2 O 2 can be demonstrated upon gelatin. 



f Other decompositions of the nutrient media by sunlight may 

 interfere occasionally with the subsequent growth of bacteria, 

 for example, the development of formic acid from tartaric acid 

 (Duclaux) . 



