220 THE VITAL FUNCTIONS. 



fresh portion, which can only take place whpn the water 

 freely communicates with the atmosphere: and if this re- 

 newal be by any means prevented, the water is no longer 

 capable of sustaining life. Fishes are killed in a very [ew 

 hours, if confined in a limited portion of water, which has no 

 access to fresh air. When many fishes arc enclosed in a 

 narrow vessel, they all struggle for the uppermost place, 

 (where the atmospheric air is first absorbed,) like the unfor- 

 tunate men imprisoned in the black-hole at Calcutta. AVhen 

 a small fisli pond is frozen over, the fishes soon perish, un- 

 less holes be broken in the ice, in order to admit air: they 

 may be seen flocking towards these holes, in order to re- 

 ceive the benefit of the fresh air as it is absorbed by the 

 water; and so great is their eagerness on these occasions, 

 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 respiration. They found that river- 

 water generally contains about one 3Gth of its bulk of air, 

 of which quantity one-third consists of oxygen, being about 

 one per cent., of the whole volume. A tench is able to 

 brQ^the when the quantity of oxygen is reduced to the 

 5000th part of the bulk of the water, but soon becomes ex- 

 ceedingly feeble by the privation of this necessary element. 

 The fact, however, shows the admirable perfection of the or- 

 gans of this fish, which can extract so minute a quantity of 

 air from water to which that air adheres with great tenacity.* 



• The swimming bladder of fishes is regarded by many of the German 

 naturalists as having some relations with the respiratoiy function, and as be- 

 ing the rudiment of the pulmonary cavity of land animals; the passage of 

 communication wit'i the oesophagus being conceived to represent the trachea. 

 The air contained in the swimming bladder of fishes has been examined 

 by many chemists, but although it is generally found to be a mixture of 

 oxygen and nitrogen, the proportion in which these gases exist is observed 

 to vary considerably, liiot concluded from his experiments, that in the air- 



