Manchester Memoirs, Vol. Ixv. (192 1), No. 3 



III. — The Work and Discoveries of Joule. 



By Sir Dugald Clerk, K.B.E., D.Sc, LL.D., F.R.S. 



First Joule Memorial Lecture, December 14th, igzo. 



The greatest generalisation in the early history of physical 

 science was made late in the seventeenth century by Sir Isaac 

 Newton when he enunciated the laws of motion and deduced 

 from them the existence in space of attraction between planets 

 and the sun. Mechanical science has been built up on 

 Newton's fundamental propositions and discoveries. 



The discovery by Joule in the middle of the nineteenth 

 century of the mechanical equivalent of heat and his sugges- 

 tion and determination of the existence of an absolute zero, 

 together with the adaptation of Carnot's cycle of 1824 to the 

 theory of heat as a mode of motion, provide generalisations 

 of equal importance to Newton's law of gravitation; and from 

 them fundamental thermodynamic laws are deduced — the 

 equivalence of energy in different forms ; conservation of 

 energy and dissipation of energy. 



Joule's discovery in fact called the modern science of 

 thermodynamics into existence. 



It is true that men of the first rank in intellect — Newton, 

 Cavendish, Rumford, Young and Davy — had long before 

 expressed the opinion that heat was not material in its nature, 

 but was a mode of motion ; but their opinions, although to 

 some extent supported by experiment, made little impression 

 upon the scientific world ; and as late as 1850 we still find most 

 distinguished physicists adhering to the "Caloric" or material 

 theory of heat. 



Sir William Thomson, for example, believed in the 

 material theory as late as 1848, as he then declared in a paper 

 read at the Cambridge Philosophical Society : — 



" the conversion of heat (or caloric) into mechani- 

 cal effect is probablv impossible, certainly undiscovered. 

 In actual engines for obtaining mechanical effect through 

 the agency of heat we must consequently look for the source 

 of power, not in any absorption or conversion, but merely 

 in a transmission of heat." 



May 1 2 th, 102 1. 



