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duction of carbonic acid : the oxygen gas could not 

 vary the result, being in all cases equally abundant. 

 144. Does then this emission of carbon proceed 

 from the fluids circulating in the larger vessels of 

 the animal ? Or is it given out from the exhalent 

 structure of the body ? That these fluids may con- 

 tain carbon as a constituent element, we do not 

 mean to deny : but, that this substance can be emit- 

 ted through that organized structure which carries 

 on the respiratory function, may, we think, be just- 

 ly disputed. This structure has been shewn to em- 

 brace a large portion of the surface of the bodies of 

 insects, which again is occupied only by the termi- 

 nations of exhalent vessels, whose office it is to emit 

 fluids, previously separated from the blood, while 

 the blood itself usually circulates in the deeper seat- 

 ed vessels, or even at the centre of the animal. 

 When also the usual changes on the air are suspend- 

 ed, by placing the animal in a cold medium, the car- 

 bon might be expected to accumulate in the blood, 

 if that were the source from which it is directly de- 

 rived, and carbonic acid to be at that time most 

 abundantly produced : but the preceding facts evince, 

 that it is not to the supposed accumulation of this 

 substance in the blood, but to the state and vigour 

 of the circulating fluids, that the production of this 

 acid is to be ascribed. We have therefore no proof 

 that carbon exists in the mass of blood in a state ca- 

 pable of attracting air, or of being attracted by it 

 through the organized structure of the animal : nor 

 even if these attractions were allowed to take place, 

 do the phenomena at all correspond with what, un- 

 der such a supposition, ought to happen. 



