FIRST PRINCIPLES. 6 



oxygen gas. All water naturally contains this gas \ 

 it descends from the atmosphere in the form of 

 rain containing it, and in passing through the earth, 

 and bubbling up in springs, or rolling down to the 

 sea in rivers and forming the great ocean itself, it 

 never loses its oxygen gas but as it is withdrawn 

 from it by the aquatic animals that live in it. It 

 is thus that well- water and river-water even that 

 of the dirty Thames contains enough oxygen to 

 support the life of fishes for some days. But the 

 time comes when the fish will have consumed 

 nearly all the oxygen, and then they begin to die ; 

 they are then like the animals in the closed glass 

 jar ; for although there may be plenty of oxygen in 

 the air above the water, they cannot breathe this 

 by means of their gills, and they die for want of 

 oxygen. They also convert the oxygen into car- 

 bonic acid gas, which accumulating in the water 

 becomes a source of poisoning to them as to the 

 animals in the closed glass jar ; the fish then die of 

 suffocation, and are as much drowned in the water 

 as a man would be. 



It is on this account, then, that if we put fish 

 into water which has been boiled, they die imme- 

 diately, because the boiling expels the oxygen gas : 

 no amount, therefore, of cold boiled water will 

 keep fish alive. 



But now we find, that by putting growing plants 

 into the water, our fish will not only not die in the 

 course of a few hours, but that if we manage our 

 plants skilfully, they will live on for any length of 

 time without any change of the water at all. Let 

 us inquire how this is. If we take a water-plant 

 and place it in a jar, and expose it for a few hours 

 to the light of the sun, we frequently find stream- 

 ing up from its leaves a succession of little bubbles 

 of air : if we catch these bubbles, and this can 

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