I C H T H 1 
pofed to exifl in perfefHon while the fifn are in the ele¬ 
ment lo analogous to their nature, and where, enjoying 
all their faculties, thefe colours are kept up and renewed 
by continual fprings from within; for there, the oily v.ar- 
nifh which is prepared by their organs is rendered more 
beautiful and tranfparent by that liquid through which 
we regard them. When taken out of the water, their 
powers are diminifhed, their ftrength declines, their mo¬ 
tions grow languid, their colours fade, the vifcous juice 
dries away ; the fcales, no longer foftened by that oily 
matter, nor refrefhed by the water, are prefently changed ; 
the veflels deftined for their reparation are obflrucTed 
from the fame caufes ; the briglueft colours of the fcales 
or of the body change, and often difappear altogether, 
Without.leaving any traces of what once they were. 
Fifties, having a compact folid body, are heavier than the 
element in which they live: confequently they would al¬ 
ways remain at the bottom, had not Providence given them 
feveral organs peculiarly conftrufted to enable them to 
perform rapid motions in every direction in that element. 
The principal of thefe organs is the Jamming-bladder, 
Which is found in molt fiffi except thole which have an 
extremely fiat body, as the rays and foies; the inferior 
organs, but no lefs effential, are the fins. The fwimming- 
bladder is membranous, and varies in lhape according- to 
the fpecies. It is always of a lengthened form; but fome- 
times the extremities are fharp, fometimes rounded ; fome¬ 
times the front part divides into two prolongations; fome¬ 
times is parted tranfverfely into two hollow lobes commu¬ 
nicating with each other ; fometimes the two lobes are lon¬ 
gitudinal, fide by fide; and in feme fpecies it has three, and 
even four, cavities. This bladder communicates with the 
front part of the ftomacl\, fometimes, though rarely, with 
the hinder part, by a little canal called the air-tube, which 
goes out from the middle of the bladder, or that extremity 
-which is neared: the head, when the bladder is fingle, but 
from the hinder lobe if there is one behind another. 
This conduit brings to the fwimming-bladder (called alfo 
the air-bladder) a certain gas, which dwells it, extends it, 
and renders it much lighter than water, thus giving the 
animal the faculty of riling amidlt that liquid. When, 
on the contrary, the fifh wants to fink, it compreffes this 
bladder by means of the mufcles which furround it; the 
gas efcapes through the air-tube into the ftomach ; gets 
out of the body by the throat, the gills, or the anus; and 
lhe weight of the folid and foft parts of the body carry 
the fiffi with greater or lefs rapidity to the bottom of the 
■water. 
This effe&of the air-bladder in railing or finking fillies 
is little doubted; but it may be eafily proved by any per- 
fon who will pierce the air-bladder of a live fifh, which 
afterwards will never be able to rife to the furface of the 
water, unlefs in dome fpecies whole mufcles are very 
ftrong and their fins very broad'. In dome countries, where 
certain fifh are to be preferved with great care in reser¬ 
voirs, they mutilate them in this manner to keep them 
from overleaping their boundaries. 
Some controverfy has been maintained as to the nature 
of the gas contained in the air-bladder. Fourcroy found 
azotin the air-bladder of a carp; Dr. Prieftley found oxy¬ 
gen mixed with a gas whofe nature he does not deferibe;' 
Dr. Broadbelt, of Jamaica, found nothing but pure oxy¬ 
gen in a fword-filh which he examined. The air-bladder 
of leveral tench, examined by La Cepede, contained hy¬ 
drogen gas, which he very reafonably fuppofes to be the 
material that organ is moft commonly filled with. He 
conceives that water is decompofea in the gills, (it is well 
known that water is a compound of hydrogen gas and 
oxygen;) this furnilhes to' the blood the oxygen neceffary 
for that fluid ; and then, if the animal does not feek to 
fwell out the air-bladder, the hydrogen fet at liberty is 
either diilipated at the branchial apertures or through the 
mouth, or combines with the various parts of their bo¬ 
dies, which by analyfis are known to furnilh much of 
this gas ; but when, oil the contrary, the nib would dif- 
Vol. X. No. 703, 
O L O G y. 741 
tend that organ, then the hydrogen gas, in (lead of, difti- , 
paring or combining, rufhes through the air-tube info the 
fwimming-bladder. Without this, continual decompoli- 
tion of water, how can a filh, whp expands and contrails 
this bladder feveral times in a minute, find on the ihftant 
the quantity of gas it infpires and rejects ? and how can 
it have at command, in the unfathomable depths of the 
ocean, the quantity of oxygen needful for fefpiration ? 
This decompolition, which is Well- known to modem 
chemifiry as operated in vegetables, camlet be fuppofed 
more difficult in living animals,.though cold-blooded ani¬ 
mals. Hence Cepede concludes, that, when it is necef¬ 
fary to break the ice of a pond to preferve the fifh from 
death, it is rather to clear the water from the miafmata 
produced by emanations from their own bodies, or from 
putrid animal and vegetable fubftances, than to procure 
them atmofpherical air, of which they have no need. .This 
theory was hinted at by our countryman John Mayovv as 
long ago as tl;e year 1698; who indeed foretold feveral of 
the linking difeoveries of modern chemifiry. 
Moreover, feveral fpecies of filh, as the baliftes and 
tetrodons, have another remarkable property, by which 
they can rife or fink in the water with extreme facility. 
They can quickly fwell up the lower part of their belly 
by introducing a gas lighter than water, and thus, by 
greatly increafing their lize, leiffen th&ir fpecific weight. 
See the article Tetrodon. 
But it is not fufficient that fiffies can rife and fink in 
the water; they nip it be able to move forward and,in 
every direction. This is effected by means of the fibs, 
but principally by the tail. This part moves on the hinder 
portion of the body as on a pivot, and according as it is 
longer or fliorter, more or lefs elaftic, or fortified with 
mufcles more or lefs ftrong, impels the fifh forward with 
greater or lefs velocity. A tail much flattened on the 
iides afts on a larger furface of water, and impels the filh 
withincreafed force. And the fin which terminates the 
tail, being placed Vertically, if it be very broad, fo much 
the more increafes the ftrength of a lever which it length¬ 
ens, &c. The action of the tail is affifted by the pedtoral 
fins; thefe, being moveable in different dire< 5 lions, and 
more or lefs expanded, enable the animal to move to right 
or left, and even backwards. The play and eft’eft of the 
peftoral -fins is very fimilar fo the palmated feet of geefe, 
ducks, and other water-fowl. The fame may be faid of 
the lower fins, but in a lefs degree, as they do not prefent 
in general fo broad a furface. The principal ufe of the 
anal fins is to lower the centre of gravity, and keep the 
fifh in a certain pofition. When they reach to the caudal 
fin, they increafe the furface of the tail, and confequently 
accelerate the motion ; they alfo, by alternate contraction 
and dilatation, operate in changing the direction,of the 
fifh’s courfe. The dorfal fins, when placed over the tail, 
have the fame effect in directing the courfe and increafing 
its velocity as the anal; alfo, in contrary currents, they 
counterbalance the effeft of the anal fins, and keep the 
animal in equilibrio; but very frequently they would 
only tend to deftroy that equilibrium, and to turn the 
fifh over, if the anirnal had not the power of contrafting 
all or any of the rays of thole fins, and of laying them 
flat on his back, either wholly or in part; and fome fpecies 
have a furrow on the back, into which they may be drawn 
quite out of fight. 
It is eafy to conceive that the a£lion of the tail and fins 
may very much affift the fifh in rifing or finking in the 
water, independent of any fwelling-up of the belly, or 
dilatation of the fwimming-bladder; accordingly feveral 
flat fifh, as the rays and foies, are totally unprovided with 
this organ. The principal organ for fwiftnefs or fwim- 
ming is the tail ; and the chief impediments are the length 
of the head and prolongations of the jaws or fnout; fo 
that the accelerating powers lie in the pofterior parts, the 
obftrudions in the anterior; and according to the propor¬ 
tions of thefe will the filh be enabled to move with more 
or Ids velocity. On this proportion will confequently de- 
$ C peud 
