ELECTROMOTIVE FORCES IN THE VOLTAIC CELL. 501 



metal and an electrolyte such as water and dilute acid, is small. 1 [Ayrton 

 .jiid Perry, Clifton, Pellat, and probably Sir Wm, Thomson.] 





13. Before proceeding to the statements embodying my own views, it 

 -will be more interesting if I try to explain in a fuller and more connected 

 manner wbat they are. 2 



Let us regard the air as a dielectric bath of oxygen, in which metals 

 are immersed, and picture a piece of zinc surrounded by oxygen molecules 

 which are straining at it, and endeavouring to combine with it. They 

 may indeed partially succeed ; but suppose they do not, we have here a 

 strong potential chemical action or chemical strain, which must probably 

 be accompanied by some physical phenomenon. Now remember that 

 oxygen is an electro-negative element ; and without endeavouring to 

 ■examine too precisely what signification is involved in that statement, it 

 will be not out of accord with orthodox views if we assume that it means 

 that at least any dissociated oxygen atoms are negatively charged, each 

 with the characteristic charge of a free dyad atom. Granting something 

 equivalent to this, without pressing the form of expression too closely, we 

 perceive that the strain of the oxygen towards the zinc will result in 

 what I metaphorically call a slackening up, or attempted compression, of 

 the negative electricity in it, i.e. to a rise of negative potential. We may 

 therefore say that zinc is at a lower potential than the air surrounding it, 

 and that the step of potential in crossing the boundary from zinc to air 

 is closely connected with the chemical affinity between zinc and oxygen. 

 Observe that this step of potential does not obviously nor probably depend 

 on the amount of oxygen present. It is possible that a few million 

 molecules may be as effective as a large number. Note also that the step 

 of potential is by no means caused by actual oxidation : in so far as the 

 iinc surface is tarnished by oxidation the strain will be diminished and 

 the step of potential become less. 



Nothing is said here about the possible effect of the nitrogen, because 

 it is simplest in the first instance to ignore it, though whether experiment 

 will justify this simplicity or not, I do not yet know. 



We may go further and assert that if in general the chemical affinity 

 of two substances can be measured by their energies of combination, then 

 the step of potential in the present case may perhaps be calculable from 

 the heat of combustion of zinc. 



And one may justify this assertion thus. Let an atom of oxygen 

 combine with an atom of zinc ; it will generate an amount of heat h, and 

 its characteristic charge, q, will be given up to the zinc and will thereby 

 fall down the step of potential, v, which separates the zinc from the air. 

 Now if we suppose that the heat h is the representative anal equivalent 



of the fall of energy q v, it follows of course that v = - — . 



q 

 Make the hypothesis and see what comes of it. 



1 The experiments supposed to establish this really prove only that there is very 

 little difference between the air and the water in which a metal is partially immersed. 

 I do not quite know how to understand, on Sir Wru. Thomson's plan, the potential of 

 a metal which is half in one medium and half in another. 



2 The reason I set them forth at length is because I had no time at the meeting 

 both to open the discussion and also to properly express my own ideas, and Sir Wm. 

 Thomson was kind enough to tell me to write out the paper completely, and to 

 explain the position I took up fully. This, therefore, I have endeavoured to do. 



