-hieid '^di^' /^^^^%^^i<?^ «^ Miscellaneous Articleif^l .-foiJsM 



^O idWflPfiE HEAT ABSORBED IN CHEMICAL DECOMPOSITIONS, j^ 



To the Editors of the Philosophical Magazine and Journal, 

 J ' • Gentlemen, Manchester, Sept. 15, 1856^ 



T AM sorry to have to trouble the patience of your readers on a 

 ^ matter personal between Dr. Woods and myself, but his letter 

 in your last Number leaves me no alternative. My reply shall be a? 

 brief as possible. 



Dr. Woods seems to be ignorant of the necessary and important 

 distinction I have always drawn in my papers between resistance to 

 conduction and resistance to electrolysis. I stated in my last, that 

 I found that the heat evolved was in an electrolytic cell, as well as 

 in a metallic conductor, proportional to the resistance to conduction 

 and the square of the quantity (commonly called intensity) of cur- 

 rent passed in a given time. I therefore still maintain, that his 

 remarks in page 74 involve a mis-statement of my results. The law 

 of which I adduced the example requoted by Dr. Woods, shows 

 clearly the position I had arrived at in 1841, for it proves that I 

 was well aware that resistance to electrolysis reacts upon the inten- 

 sity of the battery, so as to cause a loss of heat to the entire circuit 

 for a given quantity of current passed. I'hat the actual amount of 

 this loss is equal to the heat evolved by the reverse operation of 

 chemical union was proved, as I said in my last, in vol. xix. p. 276 

 of this Magazine. Dr. Woods next says, that I " published a paper 

 in the Philosophical Magazine proving that the heat developed by a 

 galvanic current is proportional to its intensity." I did no such 

 thing. My law was, that the " heat which is produced by any pair 

 is proportional to its intensity and the number of atoms which are 

 electrolysed in it." He goes on to say, that when the current was 

 passed through an electrolyte, I thought part of the intensity was 

 used up in causing electrolysis, and that the remainder only was 

 effective in producing heat. Of course the intensity or electromo- 

 tive force of the battery is so used up ; and hence it arises (as I 

 proved), that after a certain amount of battery action has taken place, 

 a loss of heat evolved is observed, not in the electrolytic cell alone, 

 but in the entire circuit, the amount of such loss being, as I proved, 

 equal to the heat which would be produced by the reunion of the 

 separated elements. 



But Dr. Woods is mistaken if he thinks that my claim rests for 

 support on this individual paper of 1841. He will find in the 

 Memoirs of the Literary and Philosophical Society of Manchester for 

 1846, a paper of mine, " On the Heat evolved during the Elec- 

 trolysis of Water," read before that Society in January 1843, in 

 which, after distinguishing between various kinds of resistance to the"- 

 electrical current, I remark, p. 103, "that in the resistance accom- 

 panied by chemical changes the heat due to its reaction is rendered 

 latent, and is thus lost by the circuit;" and p. 104, " that however 

 we arrange the voltaic apparatus, and whatever cells of electrolysis 



Phil, Mag. S. 4. Vol. 13. No. 79. Oct. 1856. Y 



