INTRODUCTION. 7^ 



supply of it from the atmosphere, since, otherwise, 

 a fish which had once sunk below the water by 

 expelling a portion of air from its air-bladder, could 

 never have risen again by the help of this organ. 

 Further, the character of the contained gas, at 

 least in the salt-water fishes, is such, as to be in- 

 compatible with the idea that it is derived from the 

 atmosphere, which, abounding as it does in nitrogen, 

 contains a very insufficient quantity of carbonic acid 

 gas ; nor can it be derived from the water, in either 

 the fresh-water or the salt-water fishes, since water 

 is destitute alike of nitrogen and of carbonic acid, 

 at least in any thing like what may be supposed to 

 be a sufficient proportion for this purpose. It is 

 manifest, therefore, that at least such fishes as 

 expel at intervals a portion of the air from their 

 air-bladders, must have the power of renewing it 

 by a process going on within themselves in other 

 words, that they form this air from their blood, by 

 a process called secretion, in the same manner as 

 they form their gall, or any other of their natural 

 fluids ; nor will the suddenness with which such air 

 must be presumed to be frequently formed, occur as 

 an objection to this doctrine, to any one who reflects 

 on the almost instantaneous effect of certain emo- 

 tions of the mind exciting in man a copious flow 

 of tears, or bathing the whole body in perspi- 

 ration, effects which are confessedly the results 

 of secretion. This was the opinion of Dr. Monro 

 and Mr. Hunter, respecting the source of the air 

 within the air-bladder of fishes; and the former 



