568 THE ANIMAL BODY AS A MACHINE 



species of intuitive sympathy with the order of nature, as in his purely 

 intellectual endowments as these are ordinarily understood. There 

 can be no question at all that both Lavoisier and Faraday, without 

 ever having formulated it in so many words, and certainly without 

 adequate proof of its validity, nevertheless assumed the truth of the 

 law of the conservation of energy and were guided in their investiga- 

 tions by this assumption. 



Lavoisier had shown in 1790 that the oxygen absorbed and trans- 

 formed into other substances by a man or animal is increased by the 

 performance of Muscular Work and by exposure to a low temperature. 

 Work and the production of Bodily Heat were thus correlated with the 

 occurrence of chemical reactions which were known to liberate energy, 

 i. e., combustions. The next step was to institute a direct comparison 

 between the heat of combustion of a carbonaceous material and the 

 heat-evolution of an animal, a comparison which has since then been 

 repeated many times, and with ever-increasing exactitude. The 

 material chosen by Lavoisier as a standard for comparison was pure 

 carbon. He measured the amount of heat evolved in the conversion 

 of the carbon into carbon dioxide, and he then measured the amount 

 of heat and carbon dioxide given off by a guinea-pig in a period of 

 ten hours. The heat-evolution was estimated from the latent heat of 

 ice which was melted by the heat of the burning carbon in the one 

 experiment and by the heat of the animal's body in the other. It was 

 found that the guinea-pig communicated 31.8 calories to the ice, while 

 25.4 calories were yielded by burning enough carbon to furnish the 

 amount of carbon dioxide exhaled by the animal in the same period. 

 The figures are not equal and we now know why. Apart from experi- 

 mental errors arising from the unavoidably imperfect technic of 

 the estimation, the animal burnt, not only carbon during the period 

 of its incarceration in the ice-chamber, but also hydrogen. Were 

 Carbohydrates, in which the hydrogen is fully neutralized by oxygen 

 already present in the molecule, the sole source of energy, then the 

 comparison instituted by Lavoisier would have been adequate, but the 

 Fats and Proteins contain an excess of hydrogen, of which the heat of 

 combustion must be added to that of the carbon in order to establish 

 the chemical origin of animal heat and work. Nevertheless the figures 

 obtained by Lavoisier were sufficiently comparable to afford decided 

 encouragement to the view which he himself expressed : " La vie est 

 une fonction chimique." 



In 1793 Lavoisier was condemned to death and executed by the 

 apostles of Liberty, Equality and Fraternity. His crime appears to 

 have consisted in his being a man of superior intellect and education 

 who had dared to express his opinion that the French Academy of 

 Sciences should be preserved and not suppressed, as the National 

 Convention desired. His appeal for liberty to live and serve was thus 

 answered by the president of the tribunal which condemned him: 



