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THE POPULAR SCIENCE MONTHLY. 



wherever established as a pecuniary investment, it has never failed to 

 yield the most profitable results. Thus the aquarium at Hamburg 

 has proved an immense pecuniary success ; and that at Brighton, al- 

 though beginning its existence so recently as August, 1872, has never- 

 theless already made a gratifying return to its proprietors. 



Fig. 1. 



Main Tank, Brighton Aquarium (Half-Section). 



The aquarium further serves to illustrate an important biological 

 truth — one of the most subtile relations between the animal and vege- 

 table kingdoms. That truth is, that the two kingdoms exert comple- 

 mentary influences upon the atmosphere we breathe. Plants inhale 

 carbon and exhale oxygen ; animals do the reverse. Strike out all 

 the plants from existence, and we should, poisoned by our own breath, 

 die in heaps, with other animals ; while, if all the animals could, at one 

 blow, be swept away from the earth into space, the plants would be 

 destroyed by the want of carbon. And now the aquarium, which, 

 properly speaking, is an artificial sea, or lake, possessing all the con- 

 ditions necessary to the maintenance of aquatic life, both animal and 

 vegetable, beautifully illustrates this truth. Who has not observed 

 that fish, confined in water without plants, quickly die, unless the 

 water be repeatedly changed ? The fish die because, having breathed 

 out all the oxygen of the water, as there is nothing in it to produce 

 any more, they become poisoned with the suflbcatnig carbon. But, 

 when the plants also are put in, they take up the carbon from the fish 

 and go on producing oxygen all the while, so there is no longer any 

 necessity for changing the water. The fact that marine aquaria do 

 not require the introduction of plants has been supposed by some ob- 

 servers to furnish a contradiction of the truth just stated. But the 

 contradiction is apparent only, not real. Sea, or " salt " water, as it 

 is usually called, contains a great quantity of little germs or " spores," 



