33 3 Birds hotter than other Animals. [Book 1 JG 



fiderably higher than the fur rounding atmofphere 5 but 

 fuch animals as are not furnifhed with refpiratory or- 

 gans, are very nearly of the fame temperature with 

 the medium in which they live. Among the hot 

 animals, thofe are the warmed whkh have the largeft 

 refpiratory organs in proportion to the bulk of their 

 bodies. This is particularly the cafe with birds, which 

 have the greatefl degree of animal heat. In the fame 

 animal, the degree of heat is in fome meafure propor- 

 tionable to the quantity of air refpired in a given time. 

 Thus we find, that animal heat is increafed by exer- 

 cife and whatever accelerates refpi ration. By the word 

 heat I do not in this -place mean a fenfarion, but calo- 

 ric, or a peculiar fluid, having attractions for other fub- 

 ftances, with which it is capable of forming combinations, 

 und producing many important phenomena. We muft 

 carefully avoid eftimating the quantity of heat or caloric 

 contained in a body by its temperature, as indicated by 

 the fenfes, or even by a thermometer ; for water, wax, 

 metallic bodies, &c. in paffing from a fluid to a folid 

 ftate, lofe a vaft quantity of heat without any diminu- 

 tion of temperature ; and it is well known, that a 

 quantity of wax, &c. partly folid and partly fluid, is 

 always of the fame temperature, whether it tends to- 

 wards the folid or the fluid ftate j that is, whatever ii 

 the temperature of the furrounding medium. If the 

 wax is in a melting ftate, it abforbs the fuperfluous 

 caloric, which becomes latent ; if on the other hand ic 

 is congealing, its latent heat is continually extricated, 

 and fuftains its temperature at a fixed point. Caloric 

 may therefore exifl in a latent ftate, in which it Cannot 

 affect the thermometer. 



There is ffill another difference with refpedt to the 

 quantity of heat or caloric contained in bodies, indepen- 

 dent of any change of form, ftmilajr to. that which takes 



place 



