268 
contains less than -rl>-t> of that gas. This small quan- 
tity of oxygen gas in water might lead to the belief 
that it is not so necessary to the life of fishes as of 
other animals ; but these animals soon suffer from 
the smallest suspension of their respiration ; and this 
distress seems to arise more from the deficiency of 
oxygen, than from the presence of carbonic acid *. 
For when water is completely deprived of its oxygen, 
fishes die in it in a few minutes ; but if oxygen be 
present, they live very well in water that contains 
more than one-eighth of its volume of carbonic 
acid f 
558. To maintain the respirability of the air dis- 
solved in water, ample means are provided. Dr 
Priestley long since remarked, that boiled water de- 
composed air by attracting its oxygen J. And Scheele, 
also, observed, that water attracted oxygen, but not 
nitrogen ||. M. de Marty, however, observes, that 
nitrogen is attracted by water in limited quantity, and 
that it afterwards attracts oxygen, so as accurately to 
analyse atmospheric air. To saturate water complete- 
ly with air, exposure to the atmosphere seems to be 
necessary \ for M. Humboldt remarks, that water, 
which has been deprived of air, does not recover 
either the same volume of air, or the same propor- 
tions of oxygen and nitrogen under close vessels, as 
when it is exposed to the free atmosphere . From 
* Mem. d'Arcueil, t. ii. p. 379. f Ibid. p. 380. 
J Phil. Trans, vol. Ixii. p. 247. 
|| On Air and Fire, p. l6'3. 
} Mem. d'Arcueil, torn. ii. p. 370. 
