334 LECTURE XV. 



calories; 3.99 for vegetable albumin; and 4.1 as an average value for 

 proteins as a class. 



Before discussing the significance of these figures, we must consider 

 whether the law of the conservation of energy 1 applies entirely to the 

 animal organism. We have seen that plants, with the assistance of the 

 kinetic energy of the sun's rays, are able to liberate oxygen from water 

 and carbon dioxide. They use up kinetic energy and form potential 

 energy. 2 The reverse process takes place in the animal organism. In it the 

 oxygen unites with the compounds poor in oxygen, the end products being 

 water and carbon dioxide again. This applies, at least as indicated above, 

 to the fats and carbohydrates. Potential energy is utilized and kinetic 

 energy takes its place. This appears partly in the form of heat, partly 

 as mechanical work. We may expect that the sum of the energies of the 

 metabolized food materials will be exactly equivalent to the energy pro- 

 duced by the animal organism. 



The first experiment in this direction was carried out by Lavoisier, 3 as 

 early as 1780, with, to be sure, rather primitive methods. Neither he 

 nor the two later investigators Despretz 4 and Dulong 5 were able to 

 establish a satisfactory agreement between the amounts of energy received 

 and that produced. We owe to Max Rubner 6 the first exact proof of 

 this relation, while more recently W. O. Atwater 7 has repeated the exper- 

 iments, eliminating all sources of error. Atwater compared the amounts 

 of potential energy in the substances which were actually oxidized in the 

 body with the amount of kinetic energy evolved by the latter. This 

 appears in the form of heat in the rest experiments, and as heat and mus- 

 cular work in the work experiments. Even in the latter case this was 

 measured in heat units. The following table gives a summary of some of 

 Atwater's results, the experimental details of which we shall discuss in 

 another place. 8 



1 Cf. R. Mayer: Die Mechanik der Warme. Stuttgart 1867 (2d ed. 1874); Die 

 Erhaltung der Energie. Berlin, 1889. 



2 This, of course, only applies to the principal activity of the parts of the plants 

 containing chlorophyll. These, also, require oxygen and give off carbon dioxide (Cf. 

 Lecture IV). 



Lavoisier et de la Place: Mem. Acad. roy. sciences, p. 355 (1780). 



Despretz: Ann. chim. phys. 27, 337 (1824). 



Dulong: ibid. (3) 1, 440 (1841). 



Max Rubner: Z. Biol. 30, 73 (1894). 



W. Atwater: Ergeb. Physiol. (Asher and Spiro) Jg. Ill, 1 Abt. p. 497 (1904). 



See Lecture XXVII. 



