269 
this superior attractive power which water possesses 
for oxygeo, the air dissolved in it is fouruji to con- 
tain always a due proportion of that gas. But the 
experiments of Scheele (554.) also shew, that, as the 
oxygen is thus attracted, the carbonic acid is expelled, 
so that that gas never exceeds a certain quantity in the 
water, either of springs or of rivers. In like manner, 
Dr Priestley observes, that hydrogen, during its solu- 
tion in water, seems to expel nitrogen, while nitrogen 
and oxygen are capable of existing together in that 
fluid *. By this difference in attractive power which 
water possesses for the different gases, connected 
with the expulsive force which they seem to exert 
tov/ards each other, the noxious gases, formed in wa- 
ter by the exercise of the animal functions, and by 
the decomposition of organic bodies, are regularly 
expelled ; and thus the air, destined to support the 
living functions of aquatic animals, like that of the 
atmosphere which we breathe, is maintained nearly 
in an uniform state of composition and purity. 
559. The last class of inferior animals, in the ar- 
rangement which we have now followed, is denomi- 
nated Reptiles, in which are included all the animals 
that formed the Linnsean class of Amphibia. The 
experiments already given (60 et seq.), clearly prove, 
that frogs and toads, which belong to this class, en- 
tirely convert, by respiration, the oxygen gas of the 
air into nearly an equal bulk of carbonic acid, with- 
out producing any change in its nitrogenous portion. 
* Obs. on Air, vol. i. p. :>9- 
