378 H. M. Vemon 



water taken at from 4 km. to 15 km. from the sbore of Naples to 

 Vary between 6 and 78 per e.c, but the numbers obtained seemed 

 to bave little or no relation to the distance from the shore. Also 

 the number of bacteria seemed to vary bnt little with the depth of 

 the water, it being practieally the same for water at a depth of 

 825 metres as for surface waters. Agaiu, Fischer foimd the number 

 of germs at 200 and 400 metres depth to be as a rule considerably 

 greater than the number in surface water. At greater depths the 

 numbers rapidly diminished, so that below 1100 metres only 1 germ 

 per cubie centimetre was as a rule present. Fischer concluded, 

 however, that whatever the depth, the water was never absolutely 

 germ-free. This occurrence of bacteria in water at ali depths is of 

 interest with reference to the results obtaìned with water kept in 

 darkness. Thus it is obvious that as they are always present, they 

 will inevitably exert their purifying action on any nitrogenous con- 

 tamination communicated to the water, and so keep the water at its 

 highest possible limit of purity. 



It is of interest to inquire how far this favourable inÜuence of 

 keeping water in darkness enters into the purilication of the water 

 of Marine Aquaria. At the Naples Aquarium the water runs back 

 from the tanks into one of three reservoirs, which hold in aggregate 

 about 300 cubie metres. llere it remains from one to three days, 

 when it is again pumped into the various tanks. On making analyses 

 of this water pumped from a reservoir, and of that tlowing back 

 agaiu from the various tanks, it was always found that the latter 

 was much more impure than the former. After making numerous 

 analyses of water when it had just run into the reservoirs, and after 

 it had stood in tbem for a day or two, it was not however possible 

 to establish any Constant difference of composition. This, if it were 

 present, would in any case be slight, as never more than two thirds 

 of the water is pumped away from a reservoir before the water 

 from the tanks is allowed to run back again. How tlien was the 

 observed purification of the water brought about? It was eventually 

 found that this removal of ammonia from the water was efifected 

 during its passage along the lead pipes running from the subsidiary 

 reservoirs at the top of the buildings into the various rooms. Thus 

 the tap from which the water used in these experiments was drawn 

 was about 20 metres distant from the supplying tank. In one instance 

 the reservoir water contained .162 mgm. free, and .185 mgm. organic 

 ammonia, and the water drawn at the same time from the tap, at 



