

149 



ing experiments, also, where carbonic acid was form- 

 ed when the animals were confined in jars of pure 

 atmospheric air inverted over mercury, the animals 

 must have furnished the carbon, since no other sub- 

 stance was present which could afford it. This car- 

 bon is likewise given out by the blood of animals 

 (100.) after it is withdrawn from the body, in a state 

 .capable of uniting with oxygen gas : and, it is gene- 

 rally admitted, that the animal fluids possess the same 

 power of affording carbon, while retained in the li- 

 ving system ; to the union of which substance, with 

 the oxygen gas of the inspired air, the formation of 

 the carbonic acid of respiration has been ascribed. 



120. Concerning the manner, however, and the 

 place in which the union of these two substances is 

 brought about in the living body, opinions have 

 greatly varied. By some, the carbon of the venal 

 blood has been supposed to attract the oxygen gas 

 of the air through the coats of the pulmonary cells 

 and vessels, and, by uniting with a portion of it, to 

 form the carbonic acid, which is again returned im- 

 mediately through the coats of these same vessels 

 and cells. According to others, the oxygen gas 

 enters into the blood-vessels, where it loosely com- 

 bines with the blood in the capillaries of the lungs, 

 and performs a circulation with it : during this cir- 

 culation, a part of the oxygen unites with the carbon 

 of the blood, so as to form an oxide of that sub- 

 stance, which, on the return of this fluid to the lungs 

 in a venal state, is, by the acquisition of more oxy- 

 gen, transformed into carbonic acid, and afterwards 

 expelled through the coats of these same capillary 

 vessels. Others, again, imagine the air to enter in- 



K 3 



