THE BLOOD. 7 



verted, the colours are changed, at least that which is now be- 

 come the upper surface assumes a more florid redness. This 

 difference of colour, others have endeavoured to explain from 

 the different proportions of the red particles, or globules as 

 they are called, which, say they, being in a greater proportion 

 at the bottom of the crassamentum, make it appear black ; 

 but, if inverted, the globules then settle from the surface 

 which is now uppermost, and that becomes redder. But this, 

 I think, is not probable ; for the lymph in the crassamentum 

 is so firmly coagulated, as to make it too dense to allow of 

 bodies even heavier than the red particles to gravitate through 

 it; for example, gold. That air has the power of changing 

 the colour of the blood, has been long known ; and the fol- 

 lowing experiment shows it very satisfactorily, and hardly 

 leaves room to refer the appearance to another cause (iv). 



(iv.) Schwenke a entertained the error mentioned in the text, that 

 the heavier corpuscles sink through the clot, while the lighter ones 

 rise to its middle and surface. 



Fracassati 5 noticed the effect of air in making the clot more florid. 

 Harvey c admitted that the blood from an artery of a living animal is 

 more florid than from a vein, which he ascribed to accidental circum- 

 stances, for he believed that the colour of the two kinds of blood is 

 essentially the same. Lower d specially proved the difference of colour, 

 correctly inferring from experiments that the change is effected in the 

 lungs, and further supporting this inference by observing that air pro- 

 duces just the same florid hue on the surface of the blood-clot out of 

 the body. Mayow e fully admitted Lower's views ; and attributed the 

 change of colour suffered by the blood in passing through the lungs 

 to its abstracting from the atmosphere a nitro-aerial spirit, many of the 

 properties of which, as he described them, belong to oxygen. Senac f dis- 

 believed that air affects the colour of the blood; he attributed the change 

 of colour to the more or less density and cohesion of the corpuscles, 

 and treated Mayow with improper contempt. Dr. William Hunter g 

 taught the difference of colour between arterial and venous blood. 

 Haller h opposed Lower's conclusions; and even declared that he had 

 a hundred times assured himself, in dogs, that there is no difference of 



a Haematologia, 8vo, Hagae Com. 1743, f Traite de la Structure du Cceur, torn, ii, 



p. 116. pp. 86-7, ed. 1749. 



b Phil. Trans. 1667, ii, 492. s MS. copy of Lectures, pp. 4-5, 4to, 



c Opera om. 4to, Lond. 1766, p. 115. 1759 ; Press mark, B. i. 17, in Lib. 



d De Corde et de Motu Sang. p. 170 et Med. Chir. Soc. 



seq. 8vo, Lond. 1669. h Elem. Phys. Lib. vi, Sect. HI, xvii; 



e Tractatus quiiique Med. Phys. cap.viii, Primse Lin. Phys. 155, 8vo, Gott. 



p. 114, 8vo, Oxon. 1674. 1780; et Deux Mem. sur le Sang, 



8vo, Lausanne, 1756, pp. 190-1. 



