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III. Letter from JAMES P. JOULE, Esq. F.R.S. to Prof. STOKES, 

 in reference to the Paper of Dr. WOODS read on the 10th 

 of January 1856. Received February 22, 1856. 



Manchester, February 21st, 1856. 



In the abstract of Dr. "Woods' paper printed in the ' Proceedings ' 

 for January 10th, the following remark occurs: "Mr. Joule published 

 in the Philosophical Magazine for June 1852, a memoir proving 

 exactly the same proposition, but giving me the merit of priority in 

 a preliminary remark." In justice to myself I must state that my 

 actual words were "I observe with pleasure that Dr. Woods has 

 recently arrived at one of the results of the paper, viz. ' that the 

 decomposition of a compound body occasions as much cold as the 

 combination of its elements originally produced heat,' by the use 

 of an elegant experimental process described in this Magazine for 

 October 1851. I ought, however, to remark, that previous to the 

 year 1843 I had demonstrated 'that the heat rendered latent in the 

 electrolysis of water is at the expense of the heat which would other- 

 wise have been evolved in a free state by the circuit.' " 



The memoir referred to by Dr. Woods was acknowledged by the 

 French Academy in its ' Comptes Rendus ' for Feb. 9, 1846, and 

 according to established rule dates from that period. I may how- 

 ever observe that the law he claims was published by me in the Philo- 

 sophical Magazine for October 1841, where I pointed out that the 

 heat evolved by the combination of oxygen and hydrogen is equal to 

 that due to the electrical intensity required to separate water into its 

 elements. The same fact was reiterated in various subsequent 

 papers, in which it is also proved that "the quantities of heat which 

 are evolved by the combustion of the chemical equivalents of bodies 

 are proportional to the intensities of their affinities for oxygen" 

 (Phil. Mag. xx. p. 1 1 1), a proposition which is given as his own by 

 Dr. Woods, and considered by him as " an original idea." 



JAMES P. JOULE. 



