Chap. 32.] of the Blood. 335 



only on that part of the furface which was expofed to 

 the air, but in thofe parts which were covered feveral 

 inches with ferum) a florid red, fo that the deep co- 

 vering of ferum, which muft have prevented all eva- 

 poration, was no more an impediment to the action of 

 the air than the bladder. That it is really the air, 

 acting through the ferum, and not the ferum itfelf, 

 which gives the florid colour, is clearly afcertained by 

 the following experiment : two equal portions of black 

 blood were put into two cups, containing equal quan- 

 tities of ferum, which covered the blood in both to the 

 depth of half an inch. One of the cups being left in 

 the open air, and the other being placed under the 

 jexhaufted receiver of art air pump, the former pre- 

 fently acquired a florid colour, while the other con- 

 tinued twelve hours as black as. at firft. In another 

 experiment, the. reverfe of the former, the influence 

 of the air upon the blood was no lefs decifively proved j 

 for red blood became black through the depth of two 

 inches of ferum, when the veiTel containing it was ex- 

 pofed to azotic gas. Putiefaction, however, will pro- 

 duce a fimilar effect ; for a frnall quantity of perfectly 

 florid blood being put into a glafs tube fealed herme- 

 tically, and thus cut off from all communication with 

 external fubftances, became black in a few days. Ex- 

 cept ferum, milk is the only animal fluid, through 

 which the air can act upon blood. . By fome fubfe- 

 quent experiments made by Dr. Prieftley, he found 

 that the intervention of a bladder by no means pre- 

 vents the action of fome airs on each other, as the ni- 

 trous on common air, &c. 



The fame induftrious philofopher found alfo, that 

 the air and blood employed in the above experiments 

 undergo oppofite changes ; for vital air was vitiated 

 by expofure to venous blood, and, on the contrary, 



inflammable- 



