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opening of living animals *; and as a fimilar change is 
produced by air applied to blood out of the body, 
it is p re fumed that the air in the lungs is the imme- 
diate caufe of this change; but how it effects it, is 
not yet determined. 
As the blood is changed to a more florid red in 
pa fling through the lungs, or from the venous to the 
arterial fyflem, fo it lofes. that colour again in palling 
from the arteries to the veins in the extreme parts, 
efpecially when the perfon is in health ; but every 
now and then we oblerve the blood in the veins more 
florid than is ufual, and it likewife frequently hap- 
pens in blood-letting, that the blood which comes 
firft out is blackifh, but afterwards it becomes more 
florid : in thefe cafes, the arterial blood pafl'es into the 
veins without undergoing that change which is na- 
tural to it. 
Some of the neutral falts have a fimilar effect on 
the colour of the blood to what air has, particularly 
nitre ; thence l'ome have attributed the difference of 
colour in the arterial and venous blood to nitre, which 
they fuppofed was abforbed from the air whilft in the 
lungs. But we know that this is a mere fuppofition, 
for air contains no nitre. Indeed nitre is far from 
* That this change is really produced in (he lungs, I ant 
perfuaded from experiments, in which I have diftinffly feen the 
blood of a more florid red in the left auricle, than it was in the 
right. But fome authors of the greateft authority fay, that 
they could not obferve any fuch difference in a great number of 
experiments which they made; but this I fltould attribute to 
their having been later ;n opening the left auricle after the col- 
lapfing of the lungs than I was ; for it feems probable, that 
whatever is the alteration produced on the blood in its circula- 
tion through this organ, after it is collapfed, this change can- 
not take place. 
being 
