THE BEGINNINGS OF LIFE. 25 



c The alimentary substances introduced into the stomach, 

 after being digested and liquified, are absorbed and sent 

 into the vessels, where they mix with the blood ; on the 

 other hand, the air introduced at each inspiration into 

 the pulmonary cavity yields to the blood a part of its 

 oxygen. Struck with this double centripetal move- 

 ment, Lavoisier asked himself what happened to these 

 substances brought into relation with one another 

 within the blood-vessels. Proceeding in this research 

 with all the rigour of a chemical analysis, he showed 

 that the oxygen introduced by the respiratory passages 

 attacks the organic substances furnished by digestion, 

 burns them, combining with their carbon and their 

 hydrogen to form carbonic acid and water. He showed 

 that this slow combustion of the organic materials of 

 the blood is an incessant source of heat 1 .' Lavoisier 

 then instituted experiments to determine the quantity 

 of heat abstracted from the animal by radiation, by 

 contact with air, and by evaporation of fluids from the 

 surface of the body. On the other hand, he measured 

 the quantity of oxygen consumed, calculated the propor- 

 tions of carbonic acid and of water produced by the 

 combination of this oxygen with the materials of the 

 blood, and then estimated the quantity of heat dis- 

 engaged during these reactions. From a comparison 

 of the results thus obtained in these two series of ob- 

 servations, he came to the conclusion that the chemical 

 reactions carried on within the body would furnish 

 1 M&n. de 1'Acad. des Sciences,' 1789. 



