.": ^cT. 11. J Ph/ysioLogy. 295 



portional to the quantity of air tliey breathe in a 

 given time. These circumstances are suliicient to 

 establish the flicl, that the heat of animals rlcpunds 

 upon respiration. On tliis subject the phiJosophi- 

 cal world are under strong obligations to Dr. Black, 

 whose doctrine of latait heat offered the first hints 

 towards an explanation of the cause of tempera- 

 ture in breathing animals. It was observed, in a 

 preceding chapter, that the discoveries of this emi- 

 nent chemist place him in a liigh rank, and consti- 

 tute much of the foundation of that chemical phi- 

 losophy which is the boast of modern times, and 

 the source of numberless improvements in the arts 

 and sciences. He early perceived the light which 

 his doctrine of latent heat was calculated to shed on 

 the temperature of animals, and with great sagacity 

 availed himself of the advantage. 



Dr. Black formed the following theory of animal 

 heat. He supposed part of the latent heat of the 

 air received into the lungs to become sensible ; 

 that the temperature of that organ and of the blood 

 passing through it is consequently raised ; and that 

 the blood thus heated communicates its tempera- 

 ture to the whole body. This opinion was plausi- 

 ble, but by no means free from objections; for, ad- 

 mitting the truth of it, the heat of the body ought 

 to be highest in the lungs, and thence gradually to 

 abate in proceeding to the extremities ; which is 

 not the fact. The author's attempts to support 

 this theory were so feeble as to induce the belief 

 that he himself considered it as untenable. 



Lavoisier first announced, in 1777> that animal 

 heat was owing to tlie caloric disengaged from 

 oxygen gas during its decomposition and conden- 



