136 BACTERIOLOGY. 



Dieudonne, in experiments upon the bacillus pro- 

 digiosus, found that direct sunlight in March, July, and 

 August killed these bacilli in one and a half hours; in 

 November in two and a half hours. Diffuse daylight 

 in March and July restrained development after three 

 and a half hours 7 exposure (in November four and a 

 half hours), and completely destroyed vitality in from 

 five to six hours. Electric arc-light inhibited growth 

 in five hours and destroyed vitality in eight hours. 

 Incandescent light inhibited growth in from seven to 

 eight hours and killed in eleven hours. Similar results 

 have been obtained with B. coli, B. typhosus, and B. 

 anthracis. According to Koch, the tubercle bacillus is 

 killed by the action of direct sunlight in a time vary- 

 ing from a few minutes to several hours, depending 

 upon the thickness of the layer exposed and the season 

 of the year. Diffuse daylight also had the same effect, 

 although a considerably longer time of exposure was 

 required when placed close to a window, from five to 

 seven days. 



Only the ultraviolet, violet, and blue rays of the 

 spectrum seem to possess bactericidal action; green 

 light is very much less so, red and yellow light not at 

 all. The action of light is apparently assisted by the 

 admission of air; anaerobic species, like the tetanus 

 bacillus, and facultative anaerobic species, such as the 

 colon bacillus, are able to withstand quite well the 

 action of sunlight in the absence of oxygen, the B. coli 

 intense direct sunlight for four hours. 



According to Kichardson and Dieudonne, the mech- 

 anism of the action of light may be at least partially 

 explained by the fact that in agar plates exposed to 

 light for a short time (even after ten minutes 7 exposure 



