OF FISHES IN GENERAL. 
18 
The gills, it would appear, have a power of abforbing 
the air, without giving admiilion to the water which is 
recened into the mouth, and ejefted by the branchia;, at 
each movement of the opercula which cover them ; but 
in what manner the fubtile reparation is performed, no 
nature lift has ever ventured to declare. As the air and 
water pufs quickly through the gills, without any ap- 
parent effort to ieparate them, it is probable, that only 
a fm -ft- quantity of the former is abtorbed, but that, by 
the frequent rranfition of the water, a fufficient quantity 
of it may be admitted to fupply animals whofe blood is 
naturally coid, and not in great abundance *. 
But, however f all a quantity of air may fuflice to 
fupport the life of a fiih, fome portion of it is abfolutely 
neceflary to every living being. Some have been fhut 
up in a narrow mouthed vellel, and have lived there for 
feveral years; but the air is no fooner excluded, by 
flopping up the opening, than the animals are fulfocated 
in the courfe of a few minutes. When this experiment 
was repeated with the veffel filled half with water, and 
half with air, after covering the mouth, as the air below 
began to be exhaufted, the fifties were feen ftruggling 
to rife above one another, and inhale a fmall fupply 
at the furface of the water ; and the fame confequence 
will follow, if you fallen down the opercula of the gills 
with a firing, fo that refpiration cannot be performed f. 
This probably is the caufe why fo many fifties are de- 
ftroyed in rivers by levere froft : When, by the congela- 
tion of the whole furface of the water, the external air 
is exciuded, the animals below mull neceffarily perifh J. 
si e lion 
* Willoughby, lib. i. f Rondelct. lib. iv, 
1 Willoughby, cap. v. 
