CIRCULATING SYSTEM. 351 



be necessary to attend, in the first place, to the changes 

 produced in the air itself. 



In man, regarding whose respiration the greatest num- 

 ber of experiments have been performed, it has been ob- 

 served, that the air which is alternately inspired and ejected, 

 becomes unfit for future use ; and is likewise rendered in- 

 capable of supporting combustion. The analysis of this 

 altered air indicates the change to have taken place in its 

 oxygenous portion *. A part thereof has disappeared, and 

 an equal bulk of carbonic acid is found occupying its place. 

 The quantity of oxygen in this carbonic acid is equal to 

 that which has been abstracted from the air. In this case, 

 either carbonic acid escapes from the blood, and an equiva- 

 lent bulk of oxygen is absorbed ; or, the blood furnishes 

 the carbon only, with which the oxygen of the air unites. 

 The former supposition was long countenanced by chemists; 

 the latter is at present the prevailing opinion. In the 

 adoption of the former, many difficulties present them- 

 selves. There is no apparent cause to produce the expul- 

 sion of the carbonic acid, as there is no substance in the 

 blood with which it could be combined, and from which it 

 could so easily escape ; neither is there the slightest reason 

 to suppose, that it could be displaced by the same bulk of 

 oxygen, since these two gases have very different combin- 

 ing values. But when it is considered, that, in all cases 

 where carbon unites with oxygen, the carbonic acid pro- 

 duced is equal in bulk to the oxygen consumed, we are 



There is no reason to believe that any change is produced on the azotei 

 In some experiments on the respiration of fishes, by HUMBOLDT and PEO* 

 TEN$ A L, a loss of azote was indicated. But the sources of error in perform- 

 ing such experiments are so many, and the results which were obtained 

 differ so much from one another, that no confidence can be placed in the 

 conclusions which have been drawn from them. 



