FISHES. 4(31 



composed of one, sometimes of two, and sometimes of three 

 divisions, situated towards the back of the fish, and opening inti? 

 the maw or gullet. Those who contend that this bag is designed 

 for raising or depressing the fish in the water, build upon the 

 following experiment. A carp being put into the air-pump, and 

 the air exhausted, the bladder is said to expand itself to such a 

 degree, that the fish swells in an extraordinary manner, till the 

 l)]adder bursts, and then the fish sinks, and ever after continues 

 to crawl at the bottom. On another occasion, the air-bladder 

 was pricked and wounded, which let out its air; upon which the 

 fish sunk to the bottom, and was not seen to rise after. From 

 thence it is inferred, that the use of the air-bladder must be by 

 swelling, at the will of the animal, thus to increase the surface 

 of the fish's body, and thence diminishing its specific gravity, to 

 enable it to rise to the top of the water, and keep there at plea- 

 sure. On the contrary, when the fish wants to descend, it is, 

 say they, but to exhaust this bladder of its air ; and the fisb be- 

 ing thus rendered slimmer and heavier, consequently sinks to 

 the bottom. 



Such is the account given of the use of the air-bladder ; no 

 part of which seems to me well siipported. In the first place, 

 though nothing is more certain than that a carp put into the air- 

 pump will swell, yet so will a mouse or a frog; and these we 

 know to have no air-bladders. A carp will rise to the surface ; 

 but so will all fish that want air, whether they have an air-blad- 

 der or not. The air-bladder is said to burst in the experiment ; 

 but that I deny. The air-blarider is indeed found empty, but it 

 has suffered no laceration, and may be distended by being blown 

 into like any oil'.er bladder that is sound. The fish after the 

 experiment, I grant, continues to creep at the bottom ; and so 

 will all fish that are sick and wounded, which must be the case 

 with this after such an operation. Thus these facts prove no- 

 thing, but that when the fish is killed in an air-pump the air-blad- 

 der is found exhausted, and that it will naturally and necessarily 

 be ; for the driiiii of air by which th:^ fish is supjjlied in the na- 

 tural way will necessarily oblige it to make use of all its hidden 

 stores ; and, as there is a communication between the gullet and 

 the air bladder, the air which the latter contains will thus be oli- 

 viously drawn away. But still farther, how comes the air-i)lad- 

 der, according to their hypothesis, to swell under the ex|)criment 



