20 



THE BIOLOGY OF BACTERIA 



usual. After the agar had set, these plates were then exposed to the 

 sunlight. In one experiment with B. coli, the water culture was not 

 exposed to the sunlight in plates, but the exposure was made in a 

 clear, white glass bottle of the Blake pattern, holding 100 c.c., 

 samples being taken from this at the proper intervals, and plated 

 as usual. 



In all cases control cultures were made under exactly the same 

 conditions as were the cultures exposed, these, however, being pro- 

 tected from the sunlight by a heavy, opaque cloth, or some similar 

 material. The temperature of these cultures was, of course, consider- 

 ably lower than was the temperature in the sun. The numbers of 

 bacteria in the controls showed the usual variation to be expected 

 under the circumstances, usually a slight reduction in numbers being 

 noted during two or three hours' standing, although in one instance 

 the numbers increased quite materially. The data of these control 

 cultures are not shown in the accompanying tables. 



The brightness of the sun also varied considerably, and attempts 

 were made to measure the amount of light by photographic means, 

 but these measurements were unsatisfactory, and the data are not 

 included here. 



With typhoid, from 95 to 99 per cent, of all the germs were 

 destroyed by ten to fifteen minutes' exposure to direct sunlight. A 

 few germs may resist the sunlight for a somewhat longer time ; 

 usually, however, all the germs were destroyed by three or more 

 hours' exposure to bright sunlight. The results of the experiments 

 with typhoid are shown in the following tables : 



TABLE showing Elimination of Typhoid Germs in Water on 

 Exposure to Sunlight. 



