THE PROGRESS OF PHYSICS. 141 



While we have given the foremost place to Joule 

 in connection with the doctrine of energy, we must 

 also recognise the genius of Helmholtz, as expressed 

 in his work on Die Erhaltung der Kraft (the persis- 

 tence of force), published in 1847, in which he 

 showed that this great conclusion follows from New- 

 ton's second interpretation of the third law of motion, 

 if we make the postulate (sufficiently justified exper- 

 imentally) of the impossibility of " perpetual motion." 



SUMMARY. " In his determination of the me- 

 chanical equivalent of heat, James Prescott Joule 

 gave to the world of science the results of experiments 

 which placed beyond reach of doubt or cavil the 

 greatest and most far-reaching scientific principle of 

 modern times, namely, that of the conservation of 



HEAT AS A MODE OF ACTION. 



Old Theory of Heat as a Kind of Matter. The 

 theory that heat is a subtle kind of matter was sug- 

 gested by some of the Greek philosophers, and it was 

 a dominant theory in the eighteenth century. In 

 the interpretation of combustion defended by Stahl 

 (1660-1734) a burning body was supposed to give 

 off a substance called " phlogiston." Lavoisier in- 

 cluded heat in his list of elements. 



Seventeenth Century Theories of Heat as a Mode 

 of Motion. A more remarkable fact, however, is 

 that in the seventeenth century the modern view was, 

 to say the least, clearly hinted at. As Cajori notes 

 in his History of Physics, " We are surprised to 

 find that Newton's immediate predecessors had antici- 

 pated our modern theory of heat. Heat a Mode of 



* Sir Henry Roscoe, Pres. Address, Rep. Brit. Ass.. 1887, 

 p. 4. 



