150 THE BLOOD. 



salts, make venous blood florid ; and this, if added in consider- 

 able quantities, even when the blood is exposed, not to oxygen 

 or air, but to a blackening gas. 



Dr. Stevens seems to have proved that the colouring matter of 

 the blood is really black, and acquires redness only by the action 

 of the salts upon the hematosine ; and that venous blood is of a dark 

 red through the presence of carbonic acid, and but for the salts 

 would be black. If blood is black from want of salts, very little 

 of them will make it florid ; if it is black from the presence of 

 carbonic acid, azote, &c., the quantity of salts requisite will be 

 proportionate to the quantity of the blackening agent. Oxygen in- 

 directly renders blood florid, by removing the carbonic acid gas, 

 and thus allowing the salts of the serum to brighten it; for if 

 these are washed away, we see that oxygen has no effect, and we 

 see that in proportion to the disappearance of oxygen is carbonic 

 acid evolved. The same interchange of carbonic acid takes place 

 in hydrogen, and the blood remains black; but then hydrogen 

 blackens blood as well as carbonic acid. It is possible that oxygen 

 may have the property of making blood florid, just as hydrogen, 

 nitrogen, carbonic acid, &c. have to make it dark. But if it have, 

 still it does not make the blood florid unless salts be present, 

 and carbonic acid always appears ; and when blood is darkened by 

 putrefaction, so that air will not make it florid, the addition of 

 most neutral salts instantly brightens it. Such are the state- 

 ments of Dr. Stevens. 



Berzelius finds the colouring particles only concerned in these 

 changes. Prevost and Dumas found more fibrin and red parti- 

 cles in arterial than in venous blood; and the venous must contain 

 a larger quantity of carbonic acid, and the arterial an abun- 

 dance of oxygen : Macaire and Marcet, on ultimate analysis, 

 find about five per cent more oxygen in arterial, and five per 

 cent more carbon in venous, blood. d 



It is in the red covering of the particles, or hematosine, as the 

 colouring matter is now called, that the iron of the blood exists. 

 Berzelius informs us that serum, although able to dissolve a 

 small portion of the oxides, not indeed of the phosphates, of 

 iron, does not acquire a red colour by their addition, and that 

 he has never discovered iron nor lime in the entire blood, al- 

 though both are so abundant in its ashes. He concludes that 



c Boyle, Ph. Tr. 1666-7. Haller, El. Phys. lib. v. 1757. Hewson, Ph. Tr. 

 1770. Dr. Priestley, Ph. Tr. 1776. He adds that the urine makes blood florid 

 because of its saline 'nature. Dr. Stevens, more minutely, 1. c. 1832. 



d Mm. de la Soc. Phys. et d'Hist. Nat. de Geneve, t. v. p. 400. 



