The Relations between Marine Animai and Vegetable Life. 417 



conditions of aeration were purposely varied. The niethod employed 

 might be either direct gas analysis, or the simpler method of 



SCHÜTZENBERGER and RiSLER 1. 



The free and organic aramonia eould of course be easily as- 

 certained from time to time, and a very fair criterion of the amount 

 of fouling of the water be thus obtained. 



If the water in an Aquarium is but seldom renewed, it would 

 be advisable from time to time to make careful determinations of 

 the specific gravity, and if necessary, diminish the salinity by the 

 addition of fresh water. This could of course be easily done in a 

 large aquarium by allowing fresh water to flow continuously at the 

 required rate into oue of the large tanks or reservoirs. A sudden 

 addition of the water necessary to reduce the salinity to the normal 

 might bave disastrous conseqiiences. Thus delicate pelagic animals 

 are especially aflfected by changes of salinity, and probably one of 

 the chief causes of their ver}' short life in aquaria is due to the 

 shock of sudden transference from open sea-water to the denser 

 aquarium water. Tu the few comparisons made, the salinity of the 

 Naples Aquarium water was fouud to be on an average about a 

 hundredth j)art more than that of the open sea-water: an apparently 

 very small amount, but nevertheless enough to cause a difference of 

 about 180 mm. of mercury in osmotic pressure. To reduce this specific 

 gravity to the normal, it would bave been necessary to add about 

 5000 litres of fresh water to the cireulating water of the Aquarium, 

 for the total volume of this amounted to about 500 cubie metres. 



General Conclusions. 



In addition to their practical hearing, the observations and ex- 

 periments described in this paper may be held to throw some light 

 on the processes of water puritìcation taking place on a vast scale 

 in nature. They may, I think, be taken to show that as far as sea- 

 water is concerned, by far the largest amount of puritìcation is 

 effected through bacterial ageucy. In that nine tenths and more of 

 the water of the ocean is plunged in darkness too deep for the growth 

 of any Chlorophyll containing organism , one must conclude that 

 bacteria and bacteria alone are there the source of purificatioo. Next 

 to bacteria probably come the Diatomaceae and the minute algae. 



1 Untersuchung des Wassers pag. 288. 



