THE 

 LONDON, EDINBURGH, and DUBLIN 



PHILOSOPHICAL MAGAZINE 



AND 



JOURNAL OF SCIENCE. 



[FIFTH SERIES.] 



APRIL 1880. 



XXXII. On the Determination of Chemical Affinity in terms of 

 Electromotive Force. — Part I. * By C. R. Alder Wright, 

 D.Sc. London, Lecturer on Chemistry and Physics in St. 

 Mary's Hospital Medical School*, 



1. T3ETWEEN the years 1841 and 1846 Joule made a series 

 -LJ of experiments on the development of heat during 

 electrolysis, which led him to the conclusions : — that Avhen 

 a current is passed through a fluid conductor, heat is developed 

 in accordance with the same law as that which he found to 

 obtain with a solid metallic wire, viz. that the heat produced 

 in a given time is proportionate to the square of the current 

 and to the resistance of the conductor — usually known as 

 Joule's law ; that if electrolysis is produced, the actual heat 

 developed in the electrolyte is less than that deducible from 

 this law by a quantity precisely equal to the amount that 

 would be given out in re-forming from its products of decom- 

 position the compound decomposed ; and consequently that it 

 is possible to determine the "heat of formation" of a com- 

 pound by the inverse method of determining the heat absorbed 

 during its electrolysis. The actual results thus obtained, as 

 regards the " heat of formation " of copper sulphate, zinc sul- 

 phate, and water, were communicated to the French Academy 

 in 1846, but were not published in detail until 1852 (Phil. Mag. 

 [4] vol. iii. p. 481). Some few months before they appeared, 

 Dr. Thomas Woods published (ibid. vol. ii. p. 268) the results 

 of experiments showing that in the decomposition of water by 

 electrolysis a quantity of heat is taken up approximately equal 



* Communicated by the Physical Society. 

 Phil Mag. S. 5. Vol. 9. No. 56. April 1880. S 



