7C INTRODUCTION. 



salt water, that a lighter gas, such as nitrogen, is 

 requisite to such fishes as inhabit the former, while, 

 to those which live in the latter, a heavier gas, such 

 as carbonic acid, is adequate for the purpose. They 

 are thus enabled to rise or to sink in the water 

 without much muscular exertion; all that is re- 

 quired being, in the former case, to distend the 

 organ in question, — and, in the latter, to contract 

 it : but in what manner they effect this change in 

 its volume is not very well understood. The com- 

 mon impression is, that the air-bladder, in its 

 ordinary state, is subjected to a certain uniform 

 pressure by the contraction of the contiguous, 

 muscles ; and that it is by relieving it from a part 

 of this pressure, by relaxing these muscles, and 

 thus allowing of a rarefication of the air which it 

 contains, that fishes rise in the water; whereas, 

 when they desire to sink, they contract these 

 muscles to a still greater degree than usual, by 

 which means this air i«, in a corresponding degree, 

 condensed. Upon these principles, the actual quan- 

 tity of air contained in the air-bladder may be pre- 

 sumed to be at all times the same, and this may 

 possibly be the case in those fishes in which the air- 

 bladder does not communicate with the neighbouring 

 passages ; but in those in which such a communica- 

 tion exists, it is obvious that any compression of 

 the bladder will not merely condense the air, but 

 expel a portion of it through the mouth or over the 

 gills ; and there must consequently be some means 

 by which such air is renewed, independently of any 



