On Sal Nitrum and Nitro- Aerial Spirit 179 



of it will never condense into a liquid. It possesses, 

 moreover, as much elastic force as ordinary air, as I 

 have ascertained by the method elsewhere described. 



To this I add, lastly, that air interspersed in water 

 is drawn in by fishes for respiratory purposes. And 

 indeed the gills with which fishes are endowed seem 

 to be formed for this very end, that air (which is ab- 

 solutely necessary for animal life) may be separated 

 from the water by their action and mixed most 

 intimately with the mass of the blood. And the 

 reason that fishes are always engaged in alternately 

 drawing in and expelling water, as terrestrial animals 

 do common air, is that something aerial which is 

 necessary to life may be separated from the water, as 

 in the other case from the air, and passed into the 

 mass of the blood. 



It corroborates this view that most fishes possess a 

 swimming-bladder filled with air. For there can be 

 no doubt that fishes draw this air from the pores of 

 the water. Let me say also in passing that if a fish is 

 placed in water contained in a suitable vessel from 

 which the air is exhausted, it will no longer swim on 

 its belly, but on the contrary on its back. Its belly 

 will also rise to some extent above the surface of the 

 water. And the reason seems to be that the air 

 enclosed in the swimming-bladder expands in virtue 

 of its elastic force as soon as the pressure of the 

 external air is withdrawn, so that the said bladder 

 and also the abdominal cavity in which it lies are 

 inflated by the air, with the result that these swollen 

 parts are forced upwards by the pressure of the water 

 while the back of the fish descends. But if after the 

 fish has died for want of air, external air is again 

 admitted into the glass, the fish will immediately sink 

 to the bottom and its belly will again become flaccid. 



