265 
inspired and expired volumes of air has increased, 
so as, in most cases, to have reached, at length, an 
almost perfect equality. If, however, these diffi- 
culties exist, when we operate with air alone, how 
much more may they be expected to attend experi- 
ments on air, dissolved in water ? In the experi- 
ments which have now been detailed, it may be 
said, that, if the same degree of ebullition were kept 
up, the same quantity of air might be expected 
to be obtained from the water, after the fishes 
had breathed in it, as before, provided this quan- 
tity really existed in it ; but as, by the act of re- 
spiration, the composition of the air is changed, this 
circumstance may, perhaps, vary the quantity re- 
tained by the distilled water through which it is sub- 
sequently passed, and thus occasion a variation in 
the result. Farther, in the act of transferring the 
water from the glass-bell into the balloon, a consider- 
able portion of the acid gas might escape ; for, ac- 
cording to Scheele, the carbonic acid, formed by a- 
quatic animals, must always separate from the water, 
since, as carbonic acid, it does not remain with water 
in the open air * ; a fact which is evinced by the 
small portion of carbonic acid contained in river-wa- 
ter, notwithstanding the great quantities that must 
be constantly forming in it both by the living func- 
tions of animals, and by the decomposition of dead 
organised matter. 
555. That these, or other sources of fallacy, exist 
in these experiments, may be farther assumed from 
On Air and Fire, p. 105. 
