on the Colour of Blood. 425 
differs not only in brightness, but also in kind, from the ordi- 
nary colour of that fluid, which is a Modena red. 
My answer is, that there are examples, beside that to which 
the objection is made, of dark blood appearing florid, merely 
from its colouring matter being seen by means of an increased 
quantity of light. One is afforded by rubbing a piece of the 
darkest crassamentum with a proper quantity of serum ; for a 
mixture is thus formed, in a few seconds, possessing a colour 
similar to that which is given to crassamentum by common air. 
But here we certainly do nothing more, than interpose among 
the red globules a number of the less dense particles of serum ; 
which, in their present situation, act as opake matter, and con- 
sequently increase the internal reflections. A second example 
occurs, when we view, by transmitted light, the fine edges and 
angles of a piece of crassamentum in water ; for, in this si- 
tuation, their colour appears to be a bright scarlet, though all 
the other parts of the same mass are black. These facts seem 
sufficient to prove, that the immediate cause I have assigned 
for the production of the florid appearance in blood, which has 
been exposed to the action of common air and neutral salts, is 
adequate to the effect ; but I shall advance a step further, and 
shew how the Modena red is converted into a scarlet. 
Blood, as I have found by experiment* is one of those fluids 
which Sir Isaac Newton has observed appear yellow,* if 
viewed in very thin masses. When, therefore, a number of 
opake particles are formed in it, by the action of common air 
and the neutral salts* many of them must be situated imme- 
diately beneath the surface. The light reflected by these will 
consequently be yellow ; and the whole effect of the newly- 
* Book i. Part II. Prop. iq. 
