374 PHYSIOLOGY CHAP. 



conversion of venous into arterial blood, and that the blood, even 

 through an animal membrane, makes normal air irrespirable and 

 unable to support combustion, by converting it into phlogistic air. 



After Priestley's discoveries it remained for Lavoisier (born in 

 Paris, 1743 ; infamously guillotined March 8, 1794) to earn the 

 glory of rearing a solid and complete edifice on their basis, both as 

 regards the chemical composition of the air, and the phenomena 

 of combustion and respiration. 



In order to refute the cumbrous doctrine of Stahl's phlogiston, 

 it was only necessary for Lavoisier to employ the balance, and to 

 show that the so-called earths or metallic oxides are heavier than 

 metals. Stahl's theory presupposed that metals, when converted 

 into oxides, lost their phlogiston, and therefore lost in weight, the 

 oxides on conversion into metals becoming phlogistic and therefore 

 gaining in weight. Lavoisier (1776) established that the air is 

 not a simple fluid which robs the igneous principle from animals 

 and gives it up to plants, but a mixture of two fluids, one 

 inadequate to support life, which he termed azote, the other 

 eminently respirable, which he called oxygen. The former corre- 

 sponds with Priestley's phlogistic, the latter with his dephlogis- 

 ticated air. 



In 1780 Lavoisier discovered the chemical composition of van 

 Helmont's gas silvestre (the " fixed air " of Black) and showed it 

 to be the result df the combination of carbon with oxygen in 

 definite proportions. He succeeded in correlating the formation 

 of carbonic acid gas in expired air with the synchronous con- 

 sumption of oxygen, and conceived of pulmonary respiration as a 

 phenomenon of combustion, in which, under the influence of life, 

 the oxygen combines with the carbon exhaled from the body, and 

 becomes the principal source of the internal heat generated by the 

 animal. Nor was this all ; by means of the balance he showed 

 that the amount of oxygen consumed in the respiratory work of 

 animals is greater than that contained in the carbonic acid given 

 off. The recent discovery of the chemical composition of water, 

 made by Cavendish (1781), and soon after confirmed by Lavoisier, 

 enabled him to account for this fact and to complete his theory of 

 the chemistry of respiration, which lie conceived as a double 

 combustion, from which are formed carbonic acid and water. He 

 made this deduction from repeated experiments, which enabled 

 him to conclude that " respiration is a slow combustion of carbon 

 and hydrogen, perfectly similar to that which occurs in a burning 

 lamp ; and from this point of view animals which breathe are true 

 combustible bodies that burn and are consumed." 



In 1789 Lavoisier published with Seguin a large number of 

 researches that are of fundamental importance to the theory of 

 respiration. The two experimenters noted that the intensity of 

 respiratory combustion does not vary essentially, whether the 



