Chemical Affinity in terms of Electromotive Force. 239 

 sition of a gramme-equivalent of water: — 



34101 

 34212 

 32356 



Average . . 33557 



the numbers found by direct determination of the heat of com- 

 bustion of hydrogen in oxygen being close to 34100 gramme- 

 degrees, according to the most trustworthy experiments (vide 



3. Joule further applied this principle to the electrolysis of 

 zinc and copper sulphates, and found that the heat actually 

 developed in the electrolytic cell was in each case less than that 

 which would be developed in a wire " obstructing the current" 

 equally with the cell by a quantity sensibly equal to the heat 

 that would be generated by the oxidation of the metal liberated 

 during the electrolysis by the oxygen simultaneously set free, 

 together with that produced by the solution in the dilute sul- 

 phuric acid also formed of the metallic oxide thus produced. 

 That this should be so is an evident deduction from the prin- 

 ciple of conservation of energy ; for if the total work that 

 could be done in the passage of a current C through a resist- 

 ance R in a time t be expressed by the formula C 2 Bi (Joule's 

 law) , and some of that work be performed not as heat but in 

 undoing chemical combination, it results that the amount of 

 work actually done as sensible heat must be just so much less 

 than C 2 K£ as corresponds to the chemical work performed. 



4. The experiments of Woods were of a far less accurate 

 kind than those of Joule, but were based on much the same 

 principles. A current being passed through an electrolytic 

 cell for a given time, the heat developed was roughly measured 

 and the amount of decomposition ; a wire was then substituted 

 for this electrolytic cell of such resistance that the current 

 passing was the same as before (the electromotor remaining 

 the same). The heat developed in this wire being determined, 

 was found to exceed that produced in the electrolytic cell in 

 the same time by a quantity sensibly equal to that which 

 would be generated by the recombination of the products of 

 electrolysis. 



The formula just deduced for Joule's method of experi- 

 menting then becomes 



w ' 

 for Ci=C 2 and a=l. 



5. In 1865 Joule's results were confirmed by Raoult (Ann. 



S2 



