FISHES. 325 



The depth at whicli the fishes had been caught, increases 

 from the beginning to the end of the table, and the propor- 

 tion of oxygen observes the same rule. This last circum- 

 stance induced Biot, and his friend De Laroche, to en- 

 deavour to ascertain the proportion of oxygen contained in 

 sea- water at different depths. They were unable to per- 

 ceive any difference. M. Configliachi has more lately 

 repeated these experiments, and found that the proportion 

 of oxygen in sea-water, bore no relation to the depth from 

 which the water had been obtained *. 



These experiments lead to the conclusion, that the air 

 contained in this sac is a secretion of the organ ; that in 

 fishes which live near the surface ' azote is separated ; but 

 in fishes which live at great depths, the quantity of oxygen 

 is proportionally increased. The purposes accomplished 

 by this arrangement have never been explained in a satis- 

 factory manner. The red organ which we have already 

 taken notice of, as existing in some fishes, is now generally 

 considered as the part which separates this air from the 

 blood. But, as this organ is not always present when there 

 is an air-bag, we are still left in doubt on the subject. 



To the systematic ichthyologist, the characters furnished 

 by the air-bag are of considerable importance, although 

 seldom sufficiently attended to. They are easily traced, 

 and they are not subject to variation. 



To the economist, the air-bag or sound is considered as 

 an article of value. This organ in the cod or ling, when 

 salted, forms a nourishing and palatable article of food, 



" Annnls of rhi,!. vol. v. p. 40, 



