ANIMAL HEAT. 179 



composes a part of the carbonic acid gas than when it is 

 pure and uncombined, and therefore holds less heat, ( 416, p. 

 177,) and must give out some when it enters the compound. 

 From both of these causes, heat is derived from fire of every 

 kind. Whatever may be the theory or explanation, the fact 

 is evident, that heat is evolved from the union of oxygen 

 with fuel carbon or hydrogen. This is what we call com- 

 bustion or jire* The. amount of heat thrown out from this 

 union or combustion is always in proportion to the amount 

 of material consumed. A pound of wood in a solid block 

 gives out the same quantity of heat, in burning slowly, as a 

 pound of shavings of the same wood, in burning rapidly. 



419. Upon these principles, it will now be easy to under- 

 stand how the warmth of the animal body is obtained. The 

 particles of our flesh are continually changing. ( 242 244, 

 pp. Ill, 112.) The old ones are going away, and new ones 

 taking their places. The principal components of these old 

 particles are carbon, nitrogen, and hydrogen. ( 290, p. 132.) 

 When the air is received into the lungs, and brought into 

 contact with the old and venous blood, it is decomposed, or 

 divided into its two elements ; the oxygen is separated from 

 the nitrogen, and united with the blood. ( 297, p. 135.) 

 The blood, at the same time, throws out into the air carbonic 

 acid gas and vapor, (297, 303, pp. 135, 137;) and then, 

 being relieved of these impurities, it is returned to the heart, 

 and thence it is circulated throughout the body, carrying 

 the newly-acquired oxygen with it. 



420. It was once generally believed, by chemical physi- 

 ologists, that the oxygen does not enter the blood, but 

 that these dead particles are brought unchanged to the 

 lungs, and there the carbon and the hydrogen, meeting the 

 oxygen, combine with it, and form carbonic acid gas and 

 water, which are given out with the returning air. 



421. But it is now, with better reason, supposed, that 

 the oxygen of the air enters the minute arteries in the lungs, 

 and is there mingled with the blood. It is then carried 

 with this blood to the heart, and thence sent through the ar- 

 teries all over the body. When this blood, and the oxygen 



