

18 THE PRINCIPLES OF 



is considerably heavier than common air, formed in the room, 

 and it did so very quickly from the continuous breathing 

 of this crowd of persons, it settled at the bottom, and filling 

 the room up as if it had been water, drowned the in- 

 mates as surely as if it had been that substance. Next 

 morning, when the door was opened, the persons confined 

 were found crowded together as near the window as they 

 could get, and dead and dying in large numbers. So, if we 

 place our fish in water and leave them there, they will, in 

 time, exhale enough carbonic-acid gas to impregnate the 

 water, and, at last, cause their own death. Bat, let us see 

 how nature has provided for this expediency — for in pond, 

 river, and ocean, the same action must be going on as in 

 our fish-globe. To provide oxygen for the aquatic animals, : 

 nature has given plants — those which grow in fresh water 

 resembling, very much, those living on the land, whilst 



those in the ocean are of a totally different character — a 



.3 

 difference which I will explain when we come to speak of 



the marine Aquarium. 



Plants inhale carbonic acid, and appropriating the carbon 

 to build up their own tissues, give off the oxygen for the i 

 use of animals, so that this gas oxygen becomes but a 

 carrier of carbon from the animal economy to that of the . 

 vegetable, and when the sun .shines on the plants in our 

 Aquarium, very often their leaves will be seen to be covered I 

 with an innumerable assemblage of minute globules of that I 

 gas, glistening like dew-drops upon grass. 



It is true that plants and fish, with nothing else, would 

 flourish for some time : but Mr. Warrington, who tried it, I 

 and to whom the most of our information concerning the 



