183 



145. If, then, the carbon in question be not emit- 

 ted from the blood while yet in the larger vessels, 

 nor after it has ceased to be in motion, it must be 

 given out by that fluid while in circulation, and after 

 it has entered into the minuter vessels : and thus it 

 becomes an animal excretion, derived, like other ex- 

 cretions, from the blood, and emitted, like them, by 

 some appropriate structure from the surface of the 

 body. Hence any cause, as cold, checking the cir- 

 culation, restrains the production of this carbon ; or, 

 although the circulation be not checked, the emis- 

 sion of this substance is prevented by smearing the 

 bodies of insects (51.) over with unctuous matter, 

 which in consequence causes their death. All that 

 has now been said of insects and snails, applies 

 equally to the marmot and other torpid animals ; for 

 the emission of carbon in all, is obedient to the same 

 laws, except that, in the former case, it is given out 

 by the exhalent surface of the body, while, in the 

 latter, it proceeds from the exhalent structure of the 

 lungs. For these reasons, we consider the emission 

 of carbon in these animals to be truly an excretion 

 dependent on the due circulation of their blood, and 

 partaking of all its variations. The experiments of 

 Spallanzani, it may be observed, prove likewise, that 

 under decomposition, animal as well as vegetable 

 substances form carbonic acid in air where no oxy- 

 gen gas was originally present *, a fact with which 

 the experiments of Priestley had long before made 

 us acquainted f. 



* Memoirs on Respiration, p. 346. 

 f Priestley on Air, vol. iii. p. 34?0. 

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