ELECTROMOTIVE FORCES IN THE VOLTAIC CELL. 525 



•take the plates several atoms thick, but we must suppose films of air of 

 sufficient thickness to preserve their normal activities in the way of chemical 

 strain to be shut up with the plates. Given these, the amount of work 

 which he has calculated would certainly be done in shutting the book, 

 and a corresponding amount of heat generated. Bat would this heat have 

 anything to do with the making of brass ? So far as I can see, nothing 

 whatever. _ < 



If we intend to make brass, must we not regard the air surrounding 

 the plates as a simple accident, and imagine all air-films removed before 

 be<nnnin°" the operation ? Work with the zinc and copper plates in 

 absolute vacuum, where (on my hypothesis at any rate) the only 

 difference of potential between them is a minute thermoelectric one ; 

 there will be an attraction caused by this difference of potential, and 

 work will be clone in shutting the book ; but to get any appreciable 

 amount of heat the plates must be terribly thin. How much heat is 

 really produced in the formation of brass I do not believe anyone knows ; 

 but if it be enough to warm the metals sixty degrees, the lower limit to 

 the size of atoms becomes greatly depressed. 



In a note at the end of this paper I show that a rise of from £ to 

 2 degrees is all that is probable, on the usual estimate of atomic 

 dimensions ; the smaller evolution of heat being caused by alloying the 

 metals at 10° C, the larger being produced by alloying them at. 400° C. 



26. Is there much heat produced in the formation of brass ? Is there 

 any way of attacking the question simply ? The only way which has 

 occurred ' to me is to dissolve brass in acid, and to see whether one gets 

 appreciably less heat than by dissolving its constituent copper and zinc 

 separately. "When an alloy is dissolved, I suppose the affinities of its con- 

 stituents are unloosed, or the combination undone ; hence the heat developed 

 during the solution of an alloy, subtracted from that produced during the 

 solution of its constituent metals and mixing of those solutions, ought to 

 measure the heat of formation of the alloy. Dr. Forster Morley, of 

 University College, London (also on the boat), said he might be willing to 

 • undertake this observation, which is doubtless a delicate one, for he was 

 engaged in some thermo-chemical researches. It may not be practicable 

 for the actual case of brass, because of the complication and uncertainty 

 introduced by secondary products, but a better pair of metals may no 

 doubt be readily found. 



Adhering to zinc and copper as convenient for explanation, the argu- 

 ment, though obviously not the order of experiment, will stand as follows : 

 Take definite weights of zinc and copper, dissolve them separately, getting 

 heats H^ and H 2 respectively, then mix the solutions, getting a possible 

 further heat production h. This is one plan of passing from separate zinc 

 and copper to a solution of a salt of brass. 



Next take the same weights of zinc and copper as before and alloy 

 them, getting heat H, then dissolve the brass in the same acid as before, 

 getting heat H 3 . This is another plan of passing from separate zinc and 

 copper to a solution of a salt of brass. 



Now, unless external work and secondary products are different in the 

 two cases, we are justified in writing the heats evolved in the two cases 

 equal : — ■ 



H, + H 2 + 7( = H + H 3 . 



1 It. occurred in conversation with Professor S. P. Thompson and Dr. Fleming on 

 board the Quebec excursion steamer Canada, and I am unable to say who suggested it. 



