334 Caufe of the Red Colour [Book IX, 



phial filled with fixed air, but in neither of the expe- 

 riments was the florid colour altered. Thefe expe- 

 riments do not accord with thofe of Dr< Prieftley, but 

 the following is intirely confonant with them. Dn 

 Goodwyn inclofed a quantity of vital air in a glafs 

 receiver inverted in quickfilver, and introduced into it 

 four ounces of blood, frefh drawn from the jugular 

 vein of a ftieep; the blood became inftamly very florid; 

 and after feveral minutes the quickfilver afcended two 

 or three lines, xvhich evidently proved, that while the 

 blood was altered in colour, the air was at the fame 

 time diminimed in quantity. 



It is well known that blood, when it coagulates 

 on being expofed to common air, afiiimes on the fur- 

 face a bright red colour, while the infide is much 

 darker, bordering upon black. 



An objection, however, feems to p.rife to this hy-^ 

 pothefis, viz. that though the blood in the h;ngs is not 

 more than a thoufandih part of an inch from the air, 

 yet it never comes into actual contact with it. In 

 order to examine the foundnefs of this objection, Dr. 

 Prieftley took a large quantity of black blood, and 

 put it -into a bladder moiftened with a little ferum, 

 and tying it very clofe, hung it in a free expofure to 

 the air, though in a quiefcent ftate, and next day found, 

 upon examination, that all the lower furface of the 

 blood, which had been feparated from common air 

 only by the intervention of the bladder, had acquired 

 a coating of a florid red colour, and as thick appa- 

 rently as it would have acquired if immediately ex- 

 pofed to the open air. In this cafe it is evident, that 

 the change of colour could not have been owing to 

 evaporation, as fome have imagined. A piece of the 

 eraffarnentuffij furrounded by ferum, acquired (ndt 



only 



