428 Dr. Wells's Observations and Experiments 
Upon the whole it appears to me, that blood derives its 
colour from the peculiar organization of the animal matter of 
one of its parts ; for whenever this is destroyed, the colour dis- 
appears, and can never be made to return ; which would not, 
I think, be the case, if it depended upon the presence of any 
foreign substance whatsoever. 
I shall conclude this paper with relating several miscellaneous 
facts respecting the colour of blood, and some conclusions 
which may be formed from them. 
Dr. Priestley has jnentioned,* that the only animal fluid, 
beside serum, which he found to transmit the influence of 
common air to blood, was milk. But I have observed, that the 
white of an egg possesses the same property, notwithstanding 
its great tenacity. Now as serum contains’an animal substance 
very similar to the white of eggs, it occurred to me as a ques- 
tion, whether, in transmitting the influence of air to blood, it 
acts by its salts only, or partly by means of the substance of 
which I have just spoken. I took therefore a quantity of urine, 
which is known to contain nearly the same salts as serum, and 
having added to it as much distilled water as rendered its taste 
of the same pungency as that of serum, I poured the mixture 
upon a piece of dark crassamentum of blood. I then put to 
another piece of the same crassamentum an equal quantity of 
serum, and exposed both parcels to the atmosphere. The 
result was, that the blood in the diluted urine did not become 
nearly so florid as that in the serum. I have found also, that a 
solution of sugar in water conveys the influence of air to blood ; 
from which it seems probable, that milk owes its similar pro- 
perty to the saccharine matter which it contains. Black blood 
* Phil. Trans, for 1 776, p. 246. 
