288 THE CIRCULATION OF NITROGEN [PART III 



that it could be utilised by the plants. As the nitrogen in solution 

 in the water became used up more would be dissolved from the 

 atmosphere. Therefore the denitrifying and nitrogen-fixing 

 bacteria would balance each other. 



One can demonstrate this on a small scale by setting up a sea 

 water aquarium. A capacious but rather shallow tank or jar may 

 be filled with sea water and stocked by some marine animals. 

 Sea anemones, fishes like the butter fish {Centronotiis), limpets, 

 small shore crabs, small plaice or flounders (but not dabs), shrimps, 

 and other marine animals live well in such small aquaria. Some 

 kind of alga should be introduced, but usually the water contains 

 the zoospores of such and it may be found that algae will spring 

 up in the tank even if not intentionally introduced. An air 

 circulation should be set up, not so much with the object of 

 supplying oxygen as for keeping the water in movement. If such 

 an aquarium be successfully established a balance between the 

 animal and plant life will be struck. If too many animals be 

 introduced at first some of these may die, and, if unnoticed, may 

 poison the water by forming objectionable putrefactive products. 

 But in a successful experiment the waste products of the one 

 category of organisms will provide the food-stuffs for the other. 

 The carbon dioxide exhaled by the animals will be assimilated 

 by the plants, and the oxygen given off by the latter will be inspired 

 by the animals. The nitrogenous excretions of the animals will be 

 nitrified by the bacteria present, and the products of these re- 

 actions will afford the food-materials utilised by the plants. A 

 condition of equilibrium, varying from season to season will be 

 established, and the tank will become a self-supporting sea-area 

 on a small scale. 



Transfer of nitrogen from sea to land. Thus inorganic 

 compounds of nitrogen are formed on the land as the result of the 

 activity of micro-organisms, which break down the excretory 

 products of organisms, or the dead bodies of the latter. These 

 inorganic nitrogen compounds are washed down into the sea as 

 sewage matters and land drainage. There a certain quantity is 

 utilised as food by plants, which again are utilised as food by 

 animals. A certain quantity of the latter are caught by fishermen 



