308 THE VITAL FUNCTIONS. 



air is constantly vitiated by the respiration of 

 these animals, and requires to be renewed by 

 the absorption of a fresh portion, which can 

 only take place when the water freely commu- 

 nicates with the atmosphere ; and if this renewal 

 be by any means prevented, the water is no 

 longer capable of sustaining life. Fishes are 

 killed in a very few hours, if confined in a 

 limited portion of water, which has no access 

 to fresh air. When many fishes are enclosed in 

 a narrow vessel, they all struggle for the upper- 

 most place, (where the atmospheric air is first 

 absorbed,) like the unfortunate men imprisoned 

 in the black-hole at Calcutta. When a small 

 fish-pond is frozen over, the fishes soon perish, 

 unless holes be broken in the ice, in order to 

 admit air : they may be seen flocking towards 

 these holes, in order to receive the benefit of 

 the fresh air as it is absorbed by the water ; 

 and so great is their eagerness on these occa- 

 sions, that they often allow themselves to be 

 caught by the hand. Water, from which all 

 air has been extracted, either by the air-pump, 

 or by boiling, is to fishes what a vacuum is to 

 a breathing terrestrial animal. Humboldt and 

 Provencal made a series of experiments on the 

 quantities of air which fishes require for their res- 

 piration. They found that river-water generally 

 contains about one 36th of its bulk of air ; of 

 which quantity, one- third consists of oxygen, 



