A I R 



A I R 



T)."^<1 filh^s are found fwiinmiiifj on the furfiicc of the 

 water, hecaiit'e the miifclcs of tlie mcnibr.me ctafc to ac\ ; 

 and their belh'es nro uppermoll, as tlie hnck-hone cannot 

 yield, .id the dillcndcd fac is protruded into tlie abdomen, 

 and the back becomes c(inr«,<[uently heavicft at its upper part. 



In moll fillies there i;i a manifeft channel, kading from 

 the gullet or upper orifice of the ilomach to tiie air-blad- 

 der, which doubtlefs fer\-es for conveying air into it. In a 

 fturgeon Mr. Wil!u.;lil>y obfer\cd, that upon prefTing the 

 bladder the ftomach prefently fwellcd ; fo in that filh it 

 fcems the air pafles freely both ways. Poffibly the fifh 

 vliilc alive may have a power-to raife up this valve, and let 

 out air on occafion. 



In a variety of other fiflies there arc communications with 

 fomc parts of the alimentaiy canal, particularly the oe- 

 fophagus and (loniach. The falmon has an opening from 

 the fore-end of the air-bladder into the oefophagus, which 

 is furrounded by a kind of mufcular fibres. The herring 

 has a foil of funnel, connecting the bottom of the ftomach 

 with the air-bag ; by which the air probably comes out 

 from it. 



All the fiflies of the cartilaginous kind want air-bladders : 

 by what means it is they afcend and defcend in water is yet 

 unknown. The cetaceous kind, or fea-beafts, are alfo with- 

 out the air-bladder : indeed, as tliefe differ little from qua- 

 drupeds, but in the want of fett, the air which they re- 

 ceive into their lungs in infpiration, may ferve to render 

 their bodies equiponderant to water ; and the conflriclion 

 or dilatation of it, by the help of the diaphragm and muf- 

 cles of rcfpiration, may poffibly affill them to defcend, or 

 afcend, in the water, by a light impulfe thereof with their 

 fins. Moll of the eel-kind have bladders, yet they can 

 hardly raife themfclvcs in the water, by reafon of the length 

 and weight of their tails ; the air-bladder being near their 

 heads, may help them to lift up their heads and fore part. 

 Ray's Wifd. of God, &c. part i. p. 26. Phil. Ti-anf. 

 N" 1 14, Abridgment, vol. ii. p. 845. 



There is great diverfity in air-bladders, in refpecl of figure, 

 fubllancc, fituation, and connedlion, in different fifli. In 

 fonic, the air-bladder, is divided into two, e. gr. in airps ; 

 and in others, into three. Needham maintains, that all fifli 

 which have teeth in their jaws have only a iuigle air-bladder ; 

 vhereas thofe without teeth have a double one ; or, which 

 amounts to the fame, the air-bladder of thefe laft is divided 

 into two ceils. Sig. Redi refutes this diflinftion ; giving 

 indances of filti with teeth, whofe air-bladder is double ; and 

 of others without teeth, which have only a fingle air-bladder. 



Dr. PrielUey conjeftures that the air, enclofed in the air- 

 bladder of fiflies, ferves fonie farther purpofe in their economy 

 befides that of enabling them to rife or link in water. Some 

 iidi have no air-bladder, and yet rife or fink without diffi- 

 culty. That fifhes cannot live without air is a well-known 

 factjcflabliflied long ago by thecxperiments of Mr. Haukfbee. 

 The fiflies he employed were gudgeons, which are lively in 

 the water, and which fubfiil for a confiderablc time when 

 taken out of it. Having put three of thefe into a vefTel of 

 water, which had no communication with the external air, 

 and whith was defigned to refemble a frozen pond, and other 

 three into a veffel of water exhaufted of its air ; he obferved, 

 that in abont half an hour the latter manifefled figns of un~ 

 eafinefs by an unufual motion of their mouth and gills ; and 

 the former frequently afcended to the top of the veffel in 

 which they were confined and then funk down again, without 

 any fenfible alteration. After five hours the gudgeons in 

 the veffel exhaufled of its air, became lefs aftive ; and in 

 about three hours more thofe ia the confined air lay at the 



bottom of the veifvl with their bellies upwards, without 

 moving their fins or tail, but indicating life by a motion with 

 their mouths. On uncovcriag the veCel, they revived in two 

 or three hours, and were pcrfeclly veil next morning ; at 

 which lime thofe in the veffel purged of its air were alio re- 

 covered. When this lail veffel was put under the receiver 

 of an air-pump, and the air was exhaufled, they all died. 

 When the air was exhaufted, they remained at the top, but 

 on its readniilTion, they funk to the bottom. It is not eafy 

 to explain the manner in which fillies are fupplied with air, 

 nor the benefit they derive from it ; nor are the nature and 

 qualities of the air, contained in their aii-bladder, fatis- 

 faCforily afcertained. Dr. Prielllcy (Exp. and Obf. relating 

 to Nat. I'liilof. vol. ii. p. 138), confined miiiows, and other 

 fniaU llfiies in water without any accefs of common air, 

 till they died, and upon examining this water, he found 

 that it was fomcwhat worfe than air in which a candle jull 

 goes o.ut. Hence he infers, that air contained in water, in 

 an unclallic flate, is as neceffar)' to the life of fillies, as air 

 in an elallic ftate is to that of land-animals. Upon putting 

 fifhes into water impregnated with phlogiHicated air, he 

 found that it was not only injurious, but in procefs of time 

 fatal to them ; although he obferves, that fifhe.'., like infeCls 

 and fome other exfanguious animals, can live a confidevable 

 time without any thing equivalent to rcfpiration. In men- 

 tioning fome experiments on the flate of the air, which is 

 contained in the air-bladder of fifhes, he remarks, that when 

 thefe are taken out of the fifh, the air cannot be difcharged 

 from them by preffure through any exifling aperture, but he 

 was always obhgcd to cut or burfl them. The air itfelf, 

 obtained from many of them, was not affefted by nitrous 

 air ; but that of fome, particularly of roaches, exhibited 

 flight indications of the effeifl of this tcH. Upon the whole 

 it thus appears, that he feldom met with oxygen, and 

 with that only in a fmall quantity. Fourcroy made expe- 

 riments on the air contained in the air-veffel of the carp, 

 and found that for the moll part it was perfeflly pure azotic 

 gas, though it fometimes contained a fmall quantity of car- 

 bonic acid gas. From the nature of the fluid he infers, that 

 the air in the bladders of fifhes is produced in the flomach. 

 Ann. de Chem. i. 47. Dr. Monro, in his lectures, led his 

 auditors to conclude, that it was fixed air. But Dr. Brod- 

 belt of Jamaica, colledling about a quart of the air from the 

 bladder of a large fword-fifh, which, he fays, confiiled of 

 innumerable cells that had no communication with one 

 another, found to his furprife that it was oxygen. A flame 

 was brightened and an ignited flick was rekindled by it ; and 

 it was fo ftrong and pure, that the common experiment of 

 a piece of fleel-wire, heated and put into it, fucceeded well, 

 and threw out a moft vivid light when melting. This pure 

 air, he fuppofes, is adapted to ferve the purpofes of life, 

 when the fifh is far below the furface of the water. Dun- 

 can's Ann. of Med. for 1796, p. 393. 



The water-fnake, in lieu of a bladder, has a large mem- 

 branous air-bag on its back, which empties and fills with air 

 at pleafure, by an aperture, which it can fhut verj' clofe, 

 from without inwards, by means of a fort of valve, fo that 

 the leall globule of water cannot enter without its confent. 

 By this artifice it can enlarge or leffen the bulk of its body, 

 and inhabit all depths of the water ; though a conjefture 

 has been advanced by Mr. Ray, that it is by the help of 

 water which they take in and let out by two holes in the 

 lower part of their abdomen, near to the ventricle. They 

 fink in the water, by letting in fome of it at thefe holes ; 

 the orifices whereof are opened and fhut at pleafure, by 

 means of proper raufcles. The water being thus received 

 I into 



