368 



H. M. Vernon 



In this experiment some of the coimtings after 48 hours in- 

 cubation were by an accident omitted. Those made after 24 hours 

 show a very rapid diminution in the number of germs. Thus on an 

 average about five sixths of them were killed off or rendered slowly 

 growing by a two hours exposure to the sun, and only A% survived 

 six hours' exposure. After five and a half days of almost coutinuous 

 sunlight however, the numbers had inereased again, so that on 48 hours' 

 incubatioD the colonies obtained averaged more than a third of the 

 original number. The next experiment, made under the sanie con- 

 ditìons, gave the following results: 



These figures show that an appreciable number of bacteria were 

 killed by even one hour's exposure to sunlight, whilst after three 

 hours the number of colonies obtained on 24 hours' incubation averaged 

 less than a quarter of the original number, and after 48 hours' in- 

 cubation, about a half. A further 3V2 hours' exposure seems to have 

 had no further action, whilst after Vji days, the number was very 

 much larger than that originally preseut. This increase was probably 

 owing to the sunlight during the intervening days having been only 

 very occasionai. For the next several days it vt^as continuously bright, 

 and hence we see that after four more days' exposure the number of 

 bacteria had diminished, on an average, to about 7^ of the original 

 number. After a further three days' exposure, the number of germs 

 was still further diminished. The vessel containiug the jar and flask 

 of water was now taken away from the direct sunlight, and kept in 



