372 



H. M. Vernon 



light is gradually increased by the actiou of the algie. In the present 

 instance there would presumably be this action at work tending to 

 inerease the organic ammonia present, and a bacterial one tending 

 to decrease it. The net result of these two opposing actions has 

 been to leave the organic ammonia almost imchanged. 



It was remarked above that in the parallel observations made 

 with one jar kept in semi-darkness, and the other in absolute darkness, 

 the rate of purification seemed to be rather more rapid in the latter 

 case. That this is actually the case is confirmed by the following 

 two series of observations, in botli of which the jars of water were 

 first kept for 28 days in almost absolute darkness, and then for the 

 rest of the period kept in a cupboard, which had several cracks in 

 it, through which a fair amount of daylight filtered. In one of these 

 experiments ordiuary Aquarium water was used, and in the other, 

 some of the same water filtered through asbestos. 



Here we see that in both instances the organic ammonia in the 

 water begins to inerease slightly in amount after the transference of 

 the water from darkness to semi-darkness. This does not seem to 

 have been due to the formation of vegetable growth in the jars. At 

 least there was no visible trace of it after three weeks exposure to 

 semi-darkness. It may be concluded therefore, with a fair degree 

 of probability, that the bacteria exert their purifyiug action on the 

 organic ammonia, and possibly also upou the free ammonia, most 

 efficiently in absolute darkness. 



In connection with these experiments, the conclusions of other 

 observers on the purifying elTects of fresh water bacteria may be 

 recorded. It should be noted however, that most of these and other 

 observations not here mentioned ' differed from the above experiments 



1 TiEMANN und Gärtner, Untersuchung des Wassers 3. Aufl. pag. 497. 



