550 LIQUID PARTS OF ANIMALS. 



CHAPTER XXV. 



OF THE AIR CONTAINED IN THE SWIMMING BLADDER 

 OF FISHES. 



MANY fish are furnished with a bladder filled with air, by means 

 of which they are supposed to rise or sink in the water. When 

 they wish to rise they are supposed to dilate their air-bladder ; 

 when they wish to sink they compress it. Whether this be the 

 use of the air-bladder of fishes is somewhat doubtful. Most fish 

 have a peculiar depth at which they almost always remain. Thus 

 the flat fish constantly affect the bottom of the sea, while there 

 are others that as constantly affect the surface. From the 

 observations of Biot it appears, that when a fish is suddenly 

 brought from a great depth towards the surface, the air-bladder 

 swells so much that the fish cannot again sink ; nay, it often 

 bursts ; arid the air making its way into the stomach, swells it 

 up, and forces it into the mouth or oesophagus. The air with 

 which these bladders is filled was first examined by Dr Priestley 

 in 1774. From his observations it appears that it varies in its 

 nature. The roach was the fish the air-bladder of which he ex- 

 amined. At first he found it filled with azote, but afterwards he 

 got a mixture of oxygen and azote. * 



Fourcroy long after examined the air in the air-bladder of 

 the carp, and found it almost pure azote : and similar results 

 were obtained by other chemists. But by far the most complete 

 analysis of this kind of air has been made by Biot, while in Yviza 

 and Formentera, two islands a little to the south of Majorca and 

 Minorca. He was employed by the French government to pro- 

 long the meridian of France to the Balearean islands, and em- 

 braced the opportunity which presented itself to examine the air 

 in the bladders of the different species offish caught in the neigh- 

 bourhood of these islands. Next season he returned to the same 

 islands with Mr Laroche, who repeated and confirmed his pre- 

 ceding experiments.! 



Biot found the air in the air-bladders a mixture of azotic and 

 oxygen gas in very variable proportions. No traces of hydrogen 



* Priestley on Air, ii. 462. 



f Biot's Memoirs are printed in the Mem. D'Arcueil, i. 252, and ii. 487. 



