on the Colour of Blood. 423 
could not arise from any change in the reflective power of 
their surfaces; for bodies reflect light from their surfaces in 
proportion to their density and inflammability ; and neither of 
those qualities, in the reddened pieces of crassamentum, can 
be supposed to have been augmented by common air, or a 
solution of a neutral salt in water. The increased reflection 
must, consequently, have arisen from some change in their 
internal parts, by means of which much of the light which had 
formerly been suffocated, was now sent back through their 
anterior surfaces, tinged with the colour of the medium through 
which it had passed. 
The precise nature of the change which is induced upon 
blood by the neutral salts, is made manifest by the following 
experiment. I poured upon a piece of printed card as much 
serum, rendered very turbid with, red globules, as barely al- 
lowed the words to be legible through it. I next dropped upon 
the card a little of a solution of nitre in water ; when I obser- 
ved, that, wherever the solution came in contact with the 
turbid serum, a whitish cloud was. immediately formed. The 
two fluids were then stirred together; upon which the mixture 
became so opake, that the printed letters upon the card could 
no longer be seen. I. have not. hitherto been able to devise any 
experiment, which shews the exact change induced by common 
air ; but it is evident that air must also, in some way, increase 
the opacity of blood, since it can, by no other means, increase 
the reflection of light from the interior parts of that body. 
This theory explains another fact respecting the colour of 
blood, which might otherwise seem unaccountable. If a. small 
quantity of a concentrated mineral acid.be applied to a piece of 
dark crassamentum, the parts touched by it will for an instant 
