4 2 4 Dr. Wells’s Observations and Experiments 
appear florid ; but the same acids, added to a solution of the red 
matter in water, do nothing more than destroy its colour. 
Upon examining the crassamentum, a reason for this difference 
of effect is discovered ; for the spots, upon which the acid was 
dropped, are found covered with whitish films. From which it 
seems evident, that the acid had occasioned an increase of opa- 
city in the crassamentum, more quickly than it had destroyed 
its colour ; and that the red matter, from having been in con- 
sequence seen by a greater quantity of light, had in that short 
interval appeared more florid than formerly. 
The change which, I think, I have proved to take place in 
blood, when its colour is brightened by common air and the 
neutral salts, is similar to that which occurs to cinnabar, in the 
making of vermilion. This pigment, it is known, is formed 
from cinnabar, merely by subjecting it to a minute mechanical 
division. But the effect of this division is, to interpose among 
its particles, an infinite number of molecules of air, which, now 
acting as opake matter, increase the reflection of light from 
the interior parts of the heap, and by this means occasion the 
whole difference of appearance which is observed between those 
two states of the same chemical body. 
I expect, however, it will be said, in opposition to what I have 
advanced, that, granting an increased reflection of light takes 
place from the interior parts of blood, in consequence of the ap- 
plication of common air and the neutral salts, still this is not a 
sufficient cause for the production of the colour which they occa- 
sion ; for the colour of blood, after those substances have acted 
upon it, is a scarlet, which, agreeably to the observation of a 
learned and ingenious Fellow of this Society, Dr. G. Fordyce,* 
* Elements of the Practice of Physic, p. 13. 
