232 PHILOSOPHICAL TRANSACTIONS. N [ANNO 1797. 



latter. Now this increased reflection of light, in the reddened pieces, 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 nei- 

 ther of those qualities, in the reddened pieces of crassamentnm, can be supposed 

 to have been augmented by common air, or a solution of a neutral salt in water. 

 The increased reflexion must, consequently, have arisen from some change in their 

 internal parts, by means of which much of the light which had formerly been suf- 

 focated, 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 on blood by the neutral salts, 

 is made manifest by the following experiment. I poured on a piece of printed card 

 as much serum, rendered very turbid with red globules, as barely allowed the words 

 to be legible through it. I next dropped on the card a little of a solution of nitre 

 in water; when I observed, that wherever the solution came in contact with the 

 turbid serum, a whitish cloud was immediately formed. The 2 fluids were then 

 stirred together; on which the mixture became so opake, that the printed letters on 

 the card could no longer be seen. I have not hitherto been able to devise any ex- 

 periment which shows 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 reflexion 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 in- 

 stant appear florid; but the same acid, added to a solution of the red matter in 

 water, do nothing more than destroy its colour. On examining the crassamentum, 

 a reason for this difference of effect is discovered; for the spots, on which the acid 

 was dropped, are found covered with whitish films. From which it seems evident, 

 that the acid had occasioned an increase of opacity in the crassamentum, more 

 quickly than it had destroyed its colour; and that the red matter, from having been 

 in consequence seen by a greater quantity of light, had in that short interval ap- 

 peared 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 vermillion. 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 dif- 

 ference of appearance observed between those 2 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, 

 iu consequence of the application of common air and the neutral salts, still this is 



