VOL. LXXXVII.] PHILOSOPHICAL TRANSACTIONS. 22$ 



what is taken for granted by him in the whole of his inquiry, that the alteration in- 

 duced on the colour of blood, both by common air and the neutral salts, is alto- 

 gether independent of any change effected by them on its colouring matter. 



I infused a piece of black crassamentum of blood in distilled water, and imme- 

 diately after covered the containing vessel closely, to prevent the access of air* 

 Having obtained by this means a transparent solution of the red matter of blood, 

 nearly free from serum and coagulable lymph, I exposed a quantity of it to the 

 open air in a shallow vessel, and poured an equal quantity into a small phial, which 

 was then well closed. When the 1st portion of the solution had been exposed to 

 the air for several hours, I decanted it into a phial, of the same size and shape as 

 that which contained the 2d portion, and having added to it as much distilled 

 water as was sufficient to compensate the loss it had suffered by evaporation, I now 

 compared the 2 together, and found them to be exactly of the same colour, with 

 regard both to kind and degree. I afterwards poured 2 other equal quantities of 

 the red solution into 2 phials of the same size and shape. To l I added a little of 

 a solution of nitre in water, and to the other as much distilled water. On com- 

 paring the 2 mixtures together, I found that they also possessed precisely the same 

 colour. Lastly, I cut a quantity of dark crassamentum of blood into thin slices, and 

 exposed them to common air. When they became florid, I put them into a phial 

 containing distilled water. I then took as much of the same crassamentum, which 

 was still black, and infused it in an equal quantity of distilled water, contained in a 

 phial similar in size and shape to the former. The 2 solutions which were thus 

 obtained, 1 from florid blood, the other from black blood, were notwithstanding of 

 precisely the same colour. These experiments were frequently repeated, and were 

 attended with the same results, as often as I used certain precautions, which shall 

 be mentioned hereafter, as the reasons for them will then be more readily under- 

 stood than they can be at present. Assuming therefore as proved, that neither 

 common air, nor the neutral salts (for all those I have tried are similar to nitre in 

 this respect) change the colour of the red matter of blood ; I shall now attempt to 

 explain the manner in which those substances give, notwithstanding, to black blood 

 a florid appearance ; premising however some observations on the colours of bodies 

 in general. 



It was the opinion of Kepler *, that light is reflected without colour from the 

 surfaces of bodies ; which he says is easily proved, by exposing to the sun's light a 

 number of cups filled with transparent liquors of different colours, and receiving 

 the reflexions from them on a white ground in a dark place. Zucchius, who was 

 younger than Kepler, but for some time his cotemporary, taught more explicitly *J-, 

 that the colours of bodies depend, not on the light which is reflected from their 

 anterior surfaces, but on that portion of it which is received into their internal 

 parts, and is thence sent back through those surfaces. The following are some 

 of the experiments on which he founded this doctrine. He exposed small round 

 * Paralipomena in Vitellionem, p. 23 et 436. — t Optica Philos. pars I, p. 278 et seq. — Orig. 



