ELECTROMOTIVE FORCES IN THE VOLTAIC CELL. 



503 



The agreement of the numbers, though not exact, seems to me too close 

 to be the result of accident. One may, I think, claim that the hypothesis 

 whence the calculated numbers are obtained is justified by the figures as 

 far as they go. It is not put forward as a completed theory, but only as a 

 first step to such a theory. I believe it to be a step in the direction of the 

 truth, but it requires working out and elaborating by a scientific chemist. 



18. Not many measurements of metal pairs have been made even in air, 

 for mere permutations snch as copper-tin, tin-silver, &c, follow at once 

 from the numbers given above, by Volta's series law ; but in gases other 

 than air one has at present no experimental guidance, beyond the barest 

 qualitative one given by Mr. Brown, that copper-nickel reverses its sign 

 when changed from air to hydrochloric acid, and that copper-iron is 

 reversed in sulphuretted hydrogen. But satisfactory observation in these 

 gases is difficult, because they not only tend to attack the plates, but they da 

 attack them; and so a film is formed and everything is rendered uncertain. 



Another complication results from the fact that when metals are 

 taken out of air and put into a foreign gas, they are already coated with 

 a film of oxygen, and it is not clear in what way this will affect the action 

 of the new gas. It may have to be replaced almost by substitution ; the- 

 affinity to be considered in chlorine, for instance, being something like 

 M,C1 2 — M,0. In a compound like HC1, the hydrogen also may have to 

 be provided for, the resulting chemical strain being, for instance, M,C1 9 — 

 M,0 + H 2 ,0 — 2 (H,C1) ; but the consideration of the hydrogen affinities 

 •will not affect differences, and therefore will leave comparisons with ex- 

 periment unaffected. Taking the metals as clean, however, and without 

 air films, we must suppose the following series to be right : — 



Energies of Combination of Metals with Chlorine; and Calculated 

 Volta Series in that Gas. 



This series will hold, as far as differences are concerned, lor hydro- 



