ACTION OF LIGHT ON MICRO-ORGANISMS 391 



hydrogen or of some similar material, is essential for 

 its manifestation. 



4. A complicating factor in the study of the effect 

 of light on bacteria is the action exercised by the light 

 on the medium with which the bacteria are surrounded ; 

 but on the one hand it has been distinctly proved 

 that insolation is capable of destroying bacteria when 

 exposed in the absence of any medium (Downes and 

 Blunt, Marshall Ward), whilst on the other hand it has 

 been shown that insolation may so affect some, if not 

 all, culture media as to render them more or less unfit 

 for the cultivation of some micro-organisms. This 

 modification of the culture medium has only been found 

 to take place when the insolation was performed in the 

 presence of air (Eoux). It is true that numerous in- 

 vestigators have exposed culture media to insolation 

 without observing any such diminution in their nutri- 

 tive value, but negative results of this kind do not 

 disprove the positive ones, but only show that the con- 

 ditions were in some way or other different in the two 

 cases. 



5. As regards the precise duration of the exposure 

 to sunlight which is necessary to cause the destruction 

 of any particular bacterial form, the most divergent re- 

 sults have beefi obtained, not only by different workers^ 

 but even by one and the same investigator. That this 

 should be the case is not surprising, for not only is the 

 intensity of the sun's light subject to enormous varia- 

 tions, but also the inherent vitality of one and the same 

 organism differs greatly according to its source and 

 previous history. Thus, it has been shown by one of us 

 that anthrax spores obtained at 38° C. are much more 

 susceptible to the action of light than those which 

 have been obtained from the same original source at 

 18° to 20° C. 



