108 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XVII.) 
1988. I prepared also three sulphuric acids: 
E was strong oil of vitriol ; 
F one volume of E and two volumes of water; 
G one volume of E and twenty volumes of water. 
Lead in F was well negative to lead either in E or G. Copper in F was also nega- 
tive to copper in E or G, but in a smaller degree. So here are two cases in which 
metals in an acid of a certain strength are negative to the same metals in the same 
acid, either stronger or weaker. I used platinum wires ultimately in all these cases 
with the same acids to check the interference of the combination of acid and water 
(1973.) ; but the results were then almost nothing, and showed that the phenomena 
could not be so accounted for. 
1989. To render this complexity for the contact theory still more complicated, 
we have further variations, in which, with the same acid strong and diluted, some 
metals are positive in the strong acid and others in the weak. Thus, tin in the 
strongest sulphuric acid E (1988.) was positive to tin in the moderate or the weak 
acids F and G : and tin in the moderate acid F was positive to the same metal in G. 
Iron, on the contrary, being in the strong acid E was negative to the weaker acids 
F and G ; and iron in the medium acid F was negative to the same metal in G. 
1990. For the purpose of understanding more distinctly what the contact theory 
has to do here, I will illustrate the case by a diagram. Let fig. 1 1 represent a circle 
of metal and sulphuric acid. If A be an arc of iron or copper, and B C strong oil of 
vitriol, there will be no determinate current : or if B C be weak acid, there will be no 
such current : but let it be strong acid at B, and diluted at C, and an electric current 
will run round A C B. If the metal A be silver, it is equally indifferent with the 
strong and also with the weak acid, as iron has been found to be as to the production 
of a current ; but, besides that, it is indifferent with the strong acid at B and the 
weak acid at C. Now if the dilution of the electrolyte at one part, as C, had so far 
increased the contact electromotive force there, when iron or copper was present, as 
to produce the current found by experiment; surely it ought (consistently with any 
reasonable limitations of the assumptions in the contact theory,) to have produced 
the same effect with silver: but there was none. Making the metal A lead or tin, the 
difficulty becomes far greater ; for though with the strong or the weak acid alone any 
effect of a determinate current is nothing, yet one occurs upon dilution at C, but now 
dilution must be supposed to weaken instead of strengthen the contact force, for the 
current is in the reverse direction. 
1991. Neither can these successive changes be referred to a gradual progression in 
the effect of dilution, dependent upon the order of the metals. For supposing dilution 
more favourable to the electromotive force of the contact of an acid and a metal, in 
proportion as the metals were in a certain order, as for instance that of their efficacy 
in the voltaic battery ; though such an assumption might seem to account for the 
