[ 4i« ] 
XIX. Observations and Experiments on the Colour of Blood. 
By William Charles Wells, M. D. F. R. S. 
Read July 6, 1797. 
Dr. Priestley is, I believe, the only person who has hi- 
therto attempted to shew by what means common air brightens 
the colour of blood, which has been for some time exposed to 
it.* His opinion is, that the air produces this effect by de- 
priving the blood of its phlogiston ; for blood, according to the 
same author, is wonderfully fitted both to imbibe and to part 
\vith phlogiston, becoming black when charged with that 
principle, but highly florid when freed from it. Various ar- 
guments may be brought to prove that this opinion is erro- 
neous, even upon the admission of such a principle of bodies 
as phlogiston. It may be said, for instance, that it is contrary 
to the laws of chemical affinity, that the same mass should, at 
one time, convert pure into phlogisticated air, by giving out 
its phlogiston, and immediately after reconvert phlogisticated 
into pure air, by imbibing that principle ; both which changes 
are supposed by Dr. Priestley to be induced by blood upon 
those airs. Again; it may be urged, that, since the neutral 
salts, and the different alkalis, when saturated with fixed air, 
produce the same effect as common air upon the colour of 
blood, if common air acts by attracting phlogiston, those other 
-bodies must have a similar operation. But surely it cannot be 
Phil. Trans, for 1776. 
