LIGHT 59 



of cases the effect of this decomposition is such that the food is 

 rendered unsuitable for the bacteria that happen to be present, 

 though it is not necessarily poisonous. The bacteria therefore die of 

 starvation and not from the effects of poison. 



The maximum effect of light is attained in large, shallow pools, 

 where the water is stationary or moving only very slowly. Buchner 

 found that the effect of light was bactericidal after penetrating 15-20 

 inches of water, though it must be stated that another investigator, 

 Arloing, holds that sunlight cannot be bactericidal to a depth of even 

 one inch. This contradictory evidence is possibly to be explained 

 by the fact that one may have worked with spore-material and the 

 other with vegetative cells, for the spores, of the soil-bacteria, for 

 example, may be exposed to the sun for weeks, possibly years, without 

 losing their vitality, whilst vegetative cells are much more susceptible. 

 Hence, in examining the effect of light on material contained in water, 

 we must first examine the nature of that material to ascertain whether 

 spores are present. A familiar experiment demonstrating the effect of 

 light, is to sow Bac. typhosus on nutrient gelatine in a Petri-dish, and 

 after covering up a portion of the surface with black paper, to expose 

 the plate to the light. Growth will take place only under the black 

 paper. The same can be performed in an agar-tube by covering the 

 lower portion of the tube with black paper, but leaving a small portion 

 uncovered. Growth will not take place where the light reaches the 

 the agar. 



Lately the 'effect of light on Bac. typhosus has been carefully 

 examined by the State Board of Health of Massachusetts. The table 

 shows the elimination of this organism in water on exposure to direct 

 sunlight. It shows that 15 minutes' exposure to direct sunlight has 

 a very powerful effect on this bacillus, as the number is reduced to 

 14 per cent, of the original number. 



No. of Bacteria No. of Bacteria 



EXPOSURE. (Average of (Average of another 



Experiments), set of Experiments). 



Start 716 562 



15 minutes - 35 9 



30 minutes - 9 4 



45 minutes - 1 2 



1 hour - 413 

 H hours - 1 4 



2 hours 4 

 4 hours 2 

 6 hours - 



