THE CHARACTERISTICS OF AN AQUARIUM. 13 



The old-fashioned fish globes were not Aquaria, in a 

 proper sense, because it was absolutely necessary to change 

 the water in them, pretty frequently, lest the fish should 

 die. The vitalization of the water without this change^ 

 comprehends the leading principle of the Aquarium. 



That principle, generally applied, may be reduced to 

 the common apprehension in the following simple, yet 

 sufficiently exact manner. 



Animal life absorbs oxygen, and throws off carbonic 

 acid gas. Vegetable life, on the contrary, absorbs carbonic 

 acid gas, and throws off oxygen. What one rejects the 

 other needs ; what would suffocate the one, if not removed, 

 the other would die of exhaustion if it could not obtain. 

 This is the universal compensating action of nature, and 

 applies, under certain circumstances, to man and a rose, 

 bud, as peculiarly as it does to an ox and an oak, a trout 

 and a water lily. 



An Aquarium exhibits a very accurate self-adjustment 

 of this delicate balance of vitalization and destruction. It 

 should contain precisely enough animal to sustain vege- 

 table life, and sufficient vegetable to meet the demands 

 of animal life. It is a very nice scale of physical equiva- 

 lents. The fish, insects and reptiles, must, to thrive, con- 

 sume the oxygen with which the plants impregnate the 

 water ; and they supply, in return, the carbonic acid gas, 

 all of which the plants must absorb for their own growth 

 and the water's purification. 



This reciprocal action, nicely maintained in equilihrioj 



