QUANTITATIVE BACTERIOLOGICAL ANALYSIS. 15 



prevented the growth of bacteria in Pasteur's solution ; direct 

 sunlight had the greatest influence, but diffused daylight also 

 appeared to have a restraining effect. Experiments were also 

 made by them which showed that the blue and violet rays of the 

 spectrum entirely prevented the growth of bacteria, while the 

 red and orange-red rays merely delayed development. 



Downes and Blunt also found that spores were uninjured by 

 exposure to light in a vacuum, and came to the conclusion that 

 the action of light was due to a gradual process of oxidation set 

 up by the sun's rays in the presence of oxygen. 



Duclaux experimented with Tyrothrix scaber and Micrococci, 

 and demonstrated that the power of resistance to light varied 

 with the species, the kind of nutrient medium and the intensity 

 of the light. 



Arloing showed that anthrax spores were killed more quickly 

 than anthrax bacilli. Roux confirmed Arloing's results, and 

 pointed out that the action of light might be due to the pro- 

 duction of oxygen. 



Engelmann showed that all bacteria are not unfavourably 

 influenced by light, the Bacterium photo-metricum being 

 motionless in the dark but mobile when exposed to light. 



Janowski studied the influence of light on the B. typhosus, 

 and found that diffused daylight delayed its growth both on 

 gelatine and in broth. The same cultures were killed in four 

 to ten hours by exposure to sunlight, although the temperature 

 of the media had not reached 40 C. Further investigations 

 showed that the chemical rays were especially harmful. 



Buchner introduced B. typhosus, B. coli and B. pyocyaneus 

 into water containing a little Liebig's extract and exposed the 

 cultures to direct sunlight and diffused daylight. His results 

 are shown in the following table : 



B. TYPIIOSUS. TS. COLT. 



Exposed Kept in Exposed Kept in 

 to light, the dark. to light, the dark. Conditions. 

 Number of bacteria at 



. 3,000 4,600 4.600 , 



the commencement . ) Direct 



Number of bacteria |. 7 600 12.600 I 8unli ht - 



after two days . . J 

 Number at the com- ^ ooo ^ m ^QQ \ jy. 



mencement ... V 

 After three hours . J 5,000 9,800 J sunll S ht - 



