2 Dugald Clerk, The Work and Discoveries of Joule 



This statement was made by the great physicist a year after 

 his first acquaintance with Joule at the British Association 

 Meeting- of 1847 hdd in Oxford. At the Oxford Meeting 

 Joule gave an account of his determinations of the mechanical 

 equivalent of heat by the friction of fluids. Thomson took 

 part in the discussion, and he stated in 1882 that he then made 

 Joule's acquaintance and that it quickly ripened into a life- 

 long friendship. At the meeting, however, he said that he 

 felt strongly impelled to rise and say that Joule must be 

 wrong, because he knew from Carnot's law that the amount 

 of work obtainable from a given quantity of heat varied with 

 the temperature differences between source and refrigerator. 

 He became convinced later that Joule had discovered an 

 important truth, and that there must be some means of recon- 

 ciling the laws of Joule and Carnot. It took Kelvin some 

 four years to abandon the errors of the material theory, and 

 he did not abandon that idea till after the mathematical work 

 of Macquorn Rankine in Scotland and Clausius on the Con- 

 tinent had provided the necessary reconciliation between the 

 Joule and Carnot laws by showing that heat is converted into 

 work and accordinglv disappears to the extent of the perform- 

 ance of the work, but to do this it requires a certain proportion 

 of the total heat dealt with to be discharged from the engine 

 by conduction at the lowest temperature of the cycle of 

 operations. 



Joule and Thomson were agreed as to the difficulty of 

 mentally accepting both laws, and Joule proposed that the 

 Carnot idea should be abandoned. Thomson fortunately was 

 convinced of the essential truth of Carnot, and ultimately the 

 two great laws of thermodynamics were accepted. The first 

 law was Joule's, and stated the existence of a mechanical 

 equivalent of heat. The second law was Carnot's, and stated 

 that between given temperature limits the heat which it was 

 possible to convert into mechanical work varied with the range 

 of the temperature limits. Both laws were determined quan- 

 titatively when the absolute zero* of temperature was 

 discovered, first by Joule on one train of reasoning and later 

 bv Thomson on another. At the time we are discussing- — - 

 1850 — the bare conception of the idea of an absolute zero of 

 temperature was one which was startling in its boldness, and 

 it must have been difficult indeed then to imagine anv definite 

 line of proof which could be followed to establish the real 

 existence of such a physical limit. We are so familiar with 

 the existence of high temperatures, vastly transcending the 

 temperatures in which we personally exist, that we can hardly 



