297 
ed, and the consequent effects on the blood, whether 
it be present in the body, or be withdrawn from it, 
will take place. In every case, however, a due de- 
gree of moisture and heat must be present to favour 
this chemical union ; hence dry bladders produce 
no change in the air, and it remains equally un- 
changed by moistened bladders, if the necessary de- 
gree of heat be withdrawn. 
592. That this change in the colour of blood is 
always accompanied by a corresponding change in 
the air, may be farther inferred from other experi- 
ments of Dr Priestley. He found, that, when the black 
crassamentum of blood was covered by serum or 
milk, it nevertheless acquired a florid hue, on being 
exposed to the air * ; and Dr Wells observed, that 
a covering of albumen, also, did not prevent the ac- 
tion of the air on the blood f. Now we have already 
seen, that serum (97.) and albumen (153.) convert the 
oxygen gas of the air into carbonic acid ; and we found 
by experiment, that the same effect was produced 
by milk, as probably would be the case with most of 
the animal fluids. Hence, it is evident, that, when 
the blood, in the experiments of Dr Priestley, be- 
came florid, through several inches of serum, the 
oxygen gas must have been at once changed by it 
into carbonic acid, and could never therefore, in the 
form of oxygen, be conveyed through this fluid to 
act on the blood. 
593. On the other hand, Dr Priestley found a thin 
* Qbs, on Air abridged, vol. iii. p. 3-70. 
t Phil. Trans. 1797. 
