244 2>. Priestley’s Obfer vat ions on 
in a large quantity of ferum, which is a fluid of a quite 
different nature. 
In order to afcertain the effeCt of thefe circumftances, 
I took a large quantity of black blood, and put it into a 
bladder moiftened with a little ferum, and tying it very 
clofe, hung it in a free expofure to the air, though in a 
quiefcent ftate ; and the next day I found, upon exami- 
nation, that all the lower furface of the blood, which had 
been feparated from the common air by the intervention 
of the bladder (which is an animal membrane, fimilar to 
that which conftitutes the veficles of the lungs, and is at 
leaft as thick) and likewdfe a little ferum, had acquired a 
coating of a florid red colour, and as thick, I believe, as 
it would have acquired, if it had been immediately ex- 
pofed to the open air ; fo that this membrane had been no 
impediment to the aCtion of the air upon the blood. In 
this cafe it is evident to obferve, that the change of co- 
lour could not be owing to evaporation , as Mr. cigna con- 
jectures. This experiment I repeated, without previoully 
moiftening the bladder, and with the very fame refult. 
I obferved alfo, that when I cut out a piece of the 
craflamentum, and left the remainder in the veflel 
with the ferum, not only that part of the furface which 
was expofed to the air, but that which was furrounded 
with ferum , and even covered with it to the depth of fe- 
ver al inches, acquired the florid colour; fo that this deep 
covering of ferum, which muft have effectually prevented 
all evaporation, was no more an impediment to the mu- 
tual aCtion of the blood and the air, than the bladder had 
been. 
