76 ANSWERS TO PRACTICAL QUESTIONS 



7. Must the air have then contained more 2>lctnt food ? 



Probably it did. 



8. What is the principle of the aquarium ? 



The inter-dependence of animals and plants, whereby each 

 supplies the wants of the other. The aquarium is a microcosm 

 a world in miniature.* 



* I have read somewhere a beautiful Persian fablo in which a nightin- 

 gale and a rose are represented as being confined in a cage together, and 

 being dependent upon each other for life. The fable is truth symbolized. 

 The idea has now become more practical, but not less beautiful. In the 

 modern aquarium, or drawing-room fish-pond, we see the world in minia- 

 ture. It is a self-regulating, self-subsisting establishment, and is con- 

 structed on the most perfect principles of chemical economy. 



" Before this truth of compensation between animals and plants was 

 discovered, many attempts were made to keep fish in small glass globes. 

 As they soon exhausted the oxygen, and impregnated the water with car- 

 bonic acid, it was necessary to change it daily. Finally, but a few years 

 since, it was discovered that plants evolve oxygen and consume carbonic 

 acid in the water as well as in the air. Starting out with this idea, about 

 the year 1850, a Mr. "Warrington, an Englishman, set about breeding fish 

 and mollusks in tanks by the aid of marine plants. He succeeded admi- 

 rably for a few days, but after a time a change came over his little world. 

 Without apparent reason, the water became suddenly impure, and the fish 

 died. Here was a new agency at work. "With the aid. of a microscope, Mr. 

 Warrington explored his tank for the poison that was evidently latent 

 there. He soon discovered that some of his plants had reached maturity, 

 and, in obedience to the law of nature, had died. The decaying matter 

 was the poison of which he was in search. How was this to be counter- 

 acted? In nature's tanks seas, rivers, and ponds reflected Mr. Warring- 

 ton, plants must die and decay, yet this does not destroy animal life. Wo 

 must see how nature remedies the evil. He hastened to a pond in the 

 vicinity, and examined its bottom with care. He found, as he had antici- 

 pated, an abundance of vegetable matter decayed. He likewise found 

 swarms of water-snails doing duty as scavengers, and devouring the putre- 

 fying substances before they had time to taint the water. Here was the 

 secret ; so beautiful a contrivance that it is said Mr. Warrington burst into 

 tears when it flashed upon him like a revelation. 



" He, however, quickly dried his eyes, gathered a quantity of snails, 

 and threw a handful into his little tank at home. In a single day the 

 water was clear and pure again. The fish throve and gamboled, grew and 

 multiplied ; the plants resumed their bright colors, and the snails not only 

 rollicked in an abundance of decaying branches, but laid a profusion of 

 eggs, on which the fish dined sumptuously every day." 



