148 



likewise did the production of carbonic acid, beinp; 

 in that case ^ *. This reciprocal increase and di- 

 minution of these two gases could spring only out 

 of some necessary connexion betwixt them : and 

 the variations occurring so regularly, and in such 

 small quantities, are inconsistent with the supposi- 

 tion that the oxygen gas, which forms the carbonic 

 acid of respiration, is derived from any other source 

 than that of the inspired air. If there were any such 

 source, why should not this acid be expired, in a 

 quantity beyond what the residual air of the lungs 

 will supply (104.), when hydrogen gas is breathed ; 

 or why should it appear only when air containing 

 oxygen gas is respired, and in proportion always to 

 the disappearance of that gas ? Why also should 

 this acid be produced, and life for a time be sustain- 

 ed, when animals are confined in pure oxygen gas : 

 and why should death speedily take place and no 

 acid be formed (beyond what the residual air of 

 the lungs will supply), when they are placed wholly 

 in nitrogen gas ? These facts decisively shew, that 

 the oxygen gas, which composes, in part, the car- 

 bonic acid formed in respiration, is derived from 

 the inspired air alone : and that the production of 

 this acid bears always a constant proportion to the 

 loss of oxygen gas. 



119. But, to constitute this acid, carbon, its 

 other ingredient, must be supplied, which, in the 

 inferior animals, has been shewn (76.) necessarily 

 to proceed from the animal system. In the forego- 



* Nicholson's Journal, May 180K 



