GENERAL PRINCIPLES 45 



force can become the calorific force and the latter work. In 

 animals, he added, the production of work is due to the chemical 

 operations of the organism which develop heat, so that the re- 

 actions being constant, the ascent of a mountain will tend to 

 cool the body. (*) 



Robert Mayer, more of a physiologist than Joule, examined 

 this last problem in an admirable paper, ( 2 ) which appeared in 

 1845, Pie said : ' The chemical energy contained in the food 

 consumed and the oxygen breathed, is the source of two kinds 

 of forces : movement and heat ; and the sum of the physical 

 forces produced by an animal is the equivalent of the total sum 

 produced by the chemical process which has taken place at the 

 same time. If all the mechanical work performed by a horse 

 during a certain time after being converted into heat is added to 

 the heat produced simultaneously in its body, the sum will be 

 equal to the quantity of heat evolved in the corresponding chemical 

 reaction." 



Helmholtz (1847) generalised the doctrine of equivalence, and 

 from that time, adopting the term Energy, all the natural phil- 

 osophers were agreed that Energy is that which is conserved in 

 the following transformations : work, heat, light, electricity, 

 etc. There is undoubtedly a relation between work and heat ; 

 but chemical energy is first converted into heat energy and then 

 into work. The same applies to luminous or electric energy, 

 etc. There is no doubt about the truth oi the principle of equiva- 

 lence or of the conservation of energy. 



A body, therefore, possesses the power to furnish a deter- 

 mined quantity of energy, in one of the preceding forms, that 

 power being analogous to the fortune of a man, a fortune of 

 which the precise nature is not specified. Energy is thus 

 manifested as an invisible and universal entity capable of changing 

 its aspect without change of magnitude. It is an invariant, as 

 the geometricians say. 



34. Interpretation and Application of the Principle of Equival- 

 ence. The principle of equivalence, in relation to mechanical 

 and thermal energy only, is shown by the relation : 



T= EQ; 



in mechanical units (kilogrammetres) or 



T 



in calories. 



(*) Joule, Philos. Mag., 1843, ser. III., vol. xxiii. 



( 2 ) Jules-Robert Mayer, Die Organische Bewegung in ihrem Zusammenhang 

 mit dem Stoffwechsel, 1845 (trans, by Perard in 1872 ; sub. tit. Du Mouve- 

 ment Organique}. 



