295 
on the mode in which moistened bladders act upon 
the air, may be extended to the action of all other ani- 
mal membranes. In an experiment made by Mr Hun- 
ter, black blood was reddened by the air, when co- 
vered by goldbeater's leaf, which touched its sur- 
face *. But this leaf is an animal substance, and 
when, therefore, it was moistened by coming into con- 
tact with the blood, it acted on the air, like the blad- 
der mentioned above, and to this action the red co- 
lour of that fluid succeeded. Dr Goodwyn, also, 
laid bare the jugular veins of rabbits, and, having in- 
tercepted the blood by ligatures, directed upon the 
coats of the veins a stream of oxygen gas. The 
blood, in some instances, became a little florid ; but, 
in other cases, no change of colour ensued, although 
the stream of oxygen was kept up for two minutes. 
As, however, the change occurred in some instances, 
something, says Dr Goodwyn, must have pervaded 
the vessels, and this, probably, by the force of che- 
mical attraction ; but what that is, he adds, is not 
yet known, whether it be some principle escaping 
from the blood to form fixed air, or whether it be a 
part of the oxygen itself that enters into the blood. 
It is certain, however, that the change of colour 
which the blood undergoes, is occasioned by the che- 
mical action of the air f- 
590. In these experiments of Dr Goodwyn, there 
can be little doubt but that the oxygen gas of the 
air was more or less changed into carbonic acid ; 
* Treatise, on the Blood, p. 62. 
t Connex. of Life with Respiration, p. 6" 
