AIR-BAG. 



in the same manner as the coats of the arteries in buds, 

 are understood to act upon the air in the air-cells of 

 these. 



In the respiration of animals, whether of the land 

 or the water, it is understood always to be the oxygen 

 of the air which is the efficient ingredient, not for the 

 purpose of adding any thing to the substance of the 

 animal, but for removing the superfluous carbon, or 

 superfluous carbon and humidity. Now the separa- 

 tion of oxygen from water, or, in other words, the de- 

 composition of that liquid, supposing it to take place 

 at a rate not more rapid than that at which the majo- 

 rity of fishes breathe, is rather a formidable operation ; 

 and if all the animals that are in the sea were to ob- 

 tain air in that way, they would boil themselves in a 

 very short time. These animals it is true receive the 

 current of water through their gills when free, or in 

 and out of their gill-openings when not free. But the 

 water is in all cases, at least of animals in which the 

 organisation is much developed, merely the vehicle 

 which conveys the air to the organs in respiration, 

 just as nitrogen appears to be the vehicle which con- 

 veys oxygen to the lungs of those animals which 

 breathe free air ; and that the water in time becomes 

 unfit for the respiration of gilled animals, in the same 

 manner as the residuum of breathed air becomes un- 

 fit for respiration by lungs, is a farther proof that the 

 oxygen is obtained from air contained in the water 

 and not from the water itself. If the water were de- 

 composed, the liberated hydrogen would escape and 

 the water would remain breathable by fishes as long as 

 a drop remained. But it is found by direct experiment, 

 not only that water which has been deprived of air 

 will not support the life of fishes, but that if they are 

 kept in a still vessel containing only a small portion 

 of water they languish and die, if the water is not 

 from time to time changed. 



These facts show that the agitation of the waters, 

 and the rolling and tumbling of their surfaces, which 

 are produced by wind and other causes, are not with- 

 out their uses to the aquatic tribes, any more than the 

 changes of air which are produced by causes nearly 

 similar, or at least something analogous, are to the 

 inhabitants of the land. Fishes which are kept in 

 small and sheltered ponds, even where they have 

 abundance of food, (as is proved by the greatness of 

 their increase and the rapidity of their growth,) are, 

 when they are allowed to increase beyond a certain 

 number, subject to general and fatal diseases, the 

 causes of which have never been very satisfactorily 

 explained ; but which are, no doubt, in part owing to 

 the over abundance of fishes exhausting the air in the 

 water faster than it can be replaced by absorption at 

 the surface of a sheltered and tranquil pond. It has 

 been found that a pond through which there is a con- 

 siderable run of water will support in health a larger 

 stock offish, even though the water cornes into it in 

 such a way as to preclude the possibility of its bring- 

 ing any supply of food. Indeed, in those cases of 

 mortality among pond fishes, it does not appear that 

 want of food is any part of the cause ; for they take 

 place under circumstances where there is every rea- 

 son to conclude that the supply of food is peculiarly 

 abundant. The fishes in the overstocked pond, bear 

 sonic resemblance to the prisoners in the black hole 

 at Calcutta ; and additional feeding, by increasing the 

 action of the system and quickening respiration, would 

 only increase the calamity. 



Of streams and rivulets, flowing through earth of 



NAT. HIST. VOL. I. 



nearly the same composition and containing equal 

 quantities of water, that which consists of a succession 

 of pools and rapids is always much more abundantly 

 supplied with the more delicate fishes than that which 

 flows sluggishly ; and the trout of the former, in 

 point of the quality of their flesh, bear nearly the 

 same relation to the trout of the latter, that beeves 

 and sheep fed on an open hill side, bear to those 

 which are fed in low, damp, and shaded pastures. 

 The cattle are the larger in size, but they are deficient 

 both in flavour and in nutriment. The pool under 

 the driving rapid or the dashing cascade, is always a 

 good troutirig pool ; and that not merely when it is 

 a difficulty or a nc j)lus ultra in ascending ; but appa- 

 rently from the abundance of pure air with which the 

 water is charged. 



The effects of prolonged calms at sea upon the 

 finny inhabitants of that element, have not been 

 observed with sufficient attention ; but so far as ob- 

 servation has gone, the fishes, especially die quick re- 

 spiring fishes which inhabit near the surface, such as 

 the herring and the mackerel, become languid and do 

 not feed so readily when the water has been long 

 still. The shoals of cod also "come up" in long- 

 calms, and whiten the water and even the sky with 

 their " blink," much more than when the surface is 

 agitated. 



In proportion as the respiration of fishes is slow, 

 they seem able the better to endure a diminished 

 supply of air in their watery element ; and their ca- 

 pability of bearing the diminution is always in pro- 

 portion to their tenacity of life when out of the water, 

 and also to their power of resisting impure substances 

 with which the water may be impregnated. As soon 

 as the water is foul, even though the foulness is only 

 a scum of oily matter upon the surface, the salmon 

 and the trout depart ; the flounder and the gudgeow 

 remain a little longer; and the eel continues to 

 wriggle for its existence even after the muddy stream 

 has become filthily impure. How much of the cause 

 may depend on the limited absorption of the air 

 which is in the water, so that the fishes are suffocated 

 for want of breath, or how much must be attributed to 

 the direct application of deleterious matters to the gills, 

 or other parts of the fishes, where they are absorbed 

 and act as poisons, has not been accurately deter- 

 mined, though it is a point of some importance in the 

 economy of the waters. 



The doctrine of the air, as an element or agent of 

 life, both animal and vegetable, is however so very 

 extensive, some parts of it are so very obscure, and 

 the uses of it to different species differ so much from 

 each other, that it cannot, though one of the most 

 important in the whole range of human knowledge, 

 be advantageously treated at length in a popular 

 article. In addition to what has been here said, 

 the reader will find some further explanation of 

 the leading principles in the article PHYSIOLOGY ; as 

 well us in the general article on plants, and the pri- 

 mary divisions of animals. 



AIR-BAG (called also the air-bladder, the sound, 

 the swim, and various other names), an organ with 

 which fishes are very generally provided, and re- 

 specting the use or mode of action of which there are 

 many theories and conjectures, but none that are 

 absolutely certain, either as to what is its use, or 

 whether that use, whatever that may be, be uniform 

 in all the species which have it. The common theory 

 is, that it assists the fishes which have it in adapting 



