THE REIGN OF LAW 167 



For James Prescott Joule (1818-1889), who came 

 of a family of brewers ami was early engaged him- 

 self in the brewing industry, was reserved, however, 

 the distinction of discovering the exact relation be- 

 tween heat and mechanical energy. After having 

 studied chemistry under Dal ton at Manchester, he 

 became engrossed in physical experimentation. In 

 1843 he prepared a paper On the Calorific Effects 

 of Magneto-Electricity and on the Mechanical Value 

 of Heat. In this he dealt with the relations between 

 heat and the ordinary forms of mechanical power, 

 and demonstrated that the mechanical energy spent 

 " in turning a magneto-electrical machine is converted 

 into the heat evolved by the passage of the currents 

 of induction through its coils ; and, on the other hand, 

 that the motive power of the electro-magnetic engine 

 is obtained at the expense of the heat due to the 

 chemical reactions of the battery by which it is 

 worked." In 1844 he proceeded to apply the prin- 

 ciples maintained in his earlier study to changes of 

 temperature as related to changes in the density of 

 gases. He was conscious of the practical, as well as 

 the theoretical, import of his investigation. Indeed, 

 it was through the determination by this illustrious 

 pupil of Dalton's of the amount of heat produced by 

 the compression of gases that one of the greatest im- 

 provements of the steam engine was later effected. 

 Joule felt that his investigation at the same time con- 

 firmed the dynamical theory of heat which originated 

 with Bacon, and had at a subsequent period been 

 so well supported by the experiments of Rumford, 

 Davy, and others. 



Already, in this paper of June, 1844, Joule had 



