TRANSACTIONS OF THE SECTIONS. 



31 



employed. He found that when the negative platinum electrode is covered with a 

 thin film of an oxidised substance, water is decomposed by a current, which under 

 ordinary circumstances could not effect its electrolysis. From these experiments he 

 was led to seek for evidence of a polarised state of the electrolyte, and he came to 

 the conclusion, that the effects produced by oxidising and other substances in the 

 vicinity of the electrodes, are partly due to the chemical action which takes place be- 

 tween the ions of the electrolyte and the matters which surround the electrodes, or 

 the substance of which they are composed, and partly to a depolarization of the 

 electrodes effected by those substances. 



On the Electric Origin of the Heat of Combustion. By J. P. Joule. 



The author had endeavoured, in a former paper (vide Phil. Mag. vol. xx. p. 98), 

 to account for the heat evolved by combustion, on the hypothesis of its arising from 

 resistance to electric conduction, and had shown that the affinity of atoms for one 

 another is the measure of the heat evolved by their combination. In that paper he 

 had introduced his own experiments on the quantity of heat generated by combustion. 

 The simplicity of his apparatus had caused him to suspect that he had not collected 

 all the heat evolved ; but having lately reduced to English measure the results of 

 Dulong's experiments, which were executed in a manner very well adapted to pre- 

 vent loss of heat, he finds them to agree so nearly with his own results as to prove 

 that the method he used was not incapable of accuracy. 



The following is a table of results both of experiment and theory, reduced to de- 

 grees Fahr. per lb. of water: — 



All the theoretical results in column 4, except that for potassium, which was ob- 

 tained by a more complicated process, were calculated in the following manner : — 

 The electromotive forces necessary to separate the elements of the various oxides 

 from the solutions of their sulphates were first ascertained ; and then the quantity of 

 heat which ought to be produced per equivalent of currents of these intensities, ac- 

 cording to the laws which, regulate the heat produced by electricity, was calculated, 

 on the assumption that the intensity necessary to separate oxygen and metal from 

 the sulphate is the same as the intensity of current caused by the union of those 

 bodies in combustion. 



Latterly, however, the author, finding reason to think that this is not the fact, but 

 that part of the force of a current engaged in electrolyzing these compound bodies 

 is used in separating the acid from the base prior to the decomposition of the latter, 

 has endeavoured to obtain the correct results of theory by subtracting from the re- 

 sults in the fourth column the heat due to that extra intensity of current. These 

 corrections were obtained by ascertaining the heat evolved by the solution of the 

 various oxides in dilute sulphuric acid, and when subtracted from the numbers in the 

 fourth column they leave those in the fifth. 



The author is of opinion that he has thus succeeded in rendering evident the fact, 

 that the heat of combustion is an electrical phenomenon, and that the method of its 

 development is by resistance to electric conduction. 



* Prof. Daniell finds that \ of an equivalent of sulphuric acid goes to the positive elec- 

 trode when dilute sulphuric acid is electrolyzed. This demands the further reduction of the 

 theoretical result for hydrogen by about one degree. — August, J. P. J. 



