274 Mr. J. P. Joule on the Heat evolved by Metallic 



62. I now dismissed the narrow electrodes, and substituted 

 for them two pieces of platinum foil, dipping to the bottom 

 of the liquid ; they were one inch apart, and each presented 

 to the dilute sulphuric acid a surface of seven square inches. 

 In this case I used twenty pairs of zinc-iron plates arranged 

 in a series of ten. 



63. The mean of six experiments with this apparatus gave 

 4°*42 of real, and 4 0, 13 of theoretical heat. I have no doubt 

 that the difference is principally occasioned by the formation 

 of the deutoxide of hydrogen, which is known to occur to a 

 considerable extent when oxygen is evolved from an extended 

 surface. Of this we have another instance in the following 

 experiment. 



64-. Exp. 12. — Using the same electrodes, and a battery 

 of ten zinc-iron pairs, I now passed a current of the mean 

 intensity l o, 08 Q through two pounds of dilute nitric acid, 

 sp. gr. 1047, for half an hour. The heat that was thus ge- 

 nerated, when properly corrected, was 3°. 



65. This experiment was, as the others, conducted in the 

 manner described at length under Exp. 7. Water chiefly* 

 was decomposed; and I ascertained, experimentally, that about 



— of the intensity of the battery was expended in overcoming 



3-5 2 + 1-63 

 3-5 



= 2*03, the resistance to conduction ; and hence -7 L 



30' 

 X 2-03 x 7°'56 x ~—T = 2 0, 53, the theoretical heat. 



66. Exp. 13. — Two plates of copper, each of which was two 

 inches broad, were secured at the distance of one inch apart, 

 and immersed in two pounds of a saturated solution of sul- 

 phate of copper. Through this apparatus, a battery often zinc- 

 iron pairs passed the mean current 1° Q during half an hour. 

 The heat thus produced, when properly corrected, was 5 0, 8. 



67. In this case there was no resistance to elect rolysis,and the 

 action may be regarded simply as a transfer of copper from 

 the positive to the negative electrode. All the obstruction, 

 therefore, that was presented to the current, was resistance to 



conduction. Its mean was 5'5, whence we have - — rr\ fl x 5'5 



(1-88) 



30' 

 x 7 0, 56 x -gjy- =5°-88, the theoretical heat. 



» See Faraday on the Electrolysis of Nitric Acid, ' Experimental Re 

 searches,' (752.). 



resistance to electrolysis. Thus I had 3*52 — 



