The Relations between Marine Animai and Vegetable Life. 401 



and nitrate are kept below the above mentioned limìts. In Exp. 139 

 on the other hand, when .3730 qra. of nitrite was added, the marked 

 diminution of 8.9^ was etfected in the size of the larvai. 



These results therefore show that the nitrification of ammonium 

 salts by the action of bacteria has a very favourable effect upon the 

 physiological piopeities of the water, and also that nitrates are 

 distinctly less injurious to animai life than nitrites. But in that these 

 latter salts only exert such au action wheo preseut in considerable 

 quantities, it would seeni to be a matter of little moment whether 

 the nitrification proceed its complete course, or stop at the stage ol' 

 formation of nitrites. 



Part 3. Miscellaneous. 



The Effects of Aeration on the Purificatiou of Water. 



Aeration has commonly been regarded as one of the chief agents 

 in the purification of naturai waters such as river and well waters, 

 and it is also generally regarded as one of the most important aids 

 we possess for increasing the purity of aquarium water. Doubtless 

 it is very necessary to aerate the water of an aquarium well, in 

 Order to provide sufficieut oxygen for the respiration of the animals 

 contained in it, but the actual aeration can in itself bave but little 

 of that oxidising influeuce on organic impurities which is commonly 

 attributed to it. Thus in a former paper', where I bave given some 

 120 analyses of the gases in the Aquarium tank water, it may be 

 seen that in no case did the amouut of oxygen present in the water 

 sink below 3.28 ce. per kilogram of water, or below about 57^ of 

 the amount present in water fully saturated with air. As there is 

 always, therefore, a considerable proportion of oxygen present in the 

 water, the oxidising effect exerted upon organic impurities will be 

 little greater if the tension of the oxygen is at its maximum value, 

 or only half as much. The aeration also removes small quantities 

 of the excess of carbonio acid gas dissolved in the water, but how 

 small this quantity must be, and what a small influence is thereby 

 exerted, is proved by the following experiment, given in the paper 

 just referred to. Thus analysis showed that the Aquarium tank 

 water contained about 50^ more carbonic acid dissolved in it than 



1 Jouiu. Physiol. Cambridge Voi. 19 1895 p. 70. 



