185 



withstand equally, the destructive influence of an excessive degree of 

 cold*? 



LXXIX. Caloric, in a latent state, or in combination with bodies, is 

 disengaged from them, whenever they assume a different state 5 when, 

 from a gaseous form they become liquid ; or, when from being liquid, 

 they become solid. No\v, living bodies are a kind of laboratories in 

 which all these changes are perpetually going on ; the blood which cir- 

 culates in every part of the human frame, is constantly receiving supplies 

 of fresh materials; from the thoracic duct which pours into it the chyle, 

 abounding in nutritious particles ; from respiration which imparts it to an 

 aeriform principle obtained from the atmosphere ; and even, in some 

 cases, from cutaneous absorption, through which different elements are 

 received into it. All these different substances carry along with them 

 into the blood, a certain quantity of caloric, which is- combined with 

 them, and which is disengaged during the changes which they undergo, 

 from the influence of the action of the organs, and gives out its caloric 

 to the parts among which it is disengaged. Of all the principles in the 

 blood, which have the power of communicating heat to the organs, none 

 furnishes a greater quantity than oxygen, which during respiration, com- 

 bines with the blood in the lungs. Gaseous substances, it is well known, 

 contain most combined caloric; their state of elastic fluidity, is entirely 

 owing to the accumulation of that principle, and they part with it, when 

 from any cause whatever, they become liquid. It is on that account, that 

 the heat of the bodies is greater, the more they have the power of im- 

 pregnating their fluids with a considerable quantity of oxygen from the 

 atmosphere. For the same reason, as was already observed, in animals 

 that have cellular lungs, and a heart with two ventricles, the blood is of 

 the same temperature, as in man ; and such animals belong as well as 

 man, to the great class of warm red-blooded animals; a class in which 

 birds occupy the first place, from the vast extent of their lungs, which 

 reach into the abdomen and communicate with the principal bones of the 

 skeleton. The capacity of the pulmonary organ of birds, is not the only 

 cause why their temperature is eight or ten degrees higher than that of 

 man : this increase* of temperature depends, likewise, on the greater 

 frequency of their respiration, and on the velocity of their pulse ; on 

 the quickness and multiplicity of their motions, and on -the vital activity 

 which animates them. In reptiles which have vesicular lungs, and a 

 heart with a single ventricle, whose respiration is slow, and performed 

 at distant intervals, the blood, though red, is of very inferior tempera- 

 ture to that of man. They have, from that circumstance, been called 

 cold red-blooded animals ; this numerous class includes fishes, which pos- 

 sess an organ supplying but imperfectly the office of lungs. In fishes, 

 the heart which has but a single ventricle, sends, it is true, to the 

 gills (the organ supplying the place of lungs is so called) the whole 

 of the blood ; that fluid, however, is but imperfectly vivified in the 

 gills, on account of the small quantity of air which can be taken 

 in during the act of respiration. Lastly, in white-blooded animals and 

 in plants, the combinations with the air being more difficult, the vital 

 energy less, marked, the temperature differs only by a few degrees, from 



* See the remarks, in Api>*jir\', Note Y, on the production of Animal Ifeat. 



2 A 



