106 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XVII.) 
much greater dilution effected, I adopted the plan of putting strong acid into the 
tube, and then adding a little water at the top at one of the sides, with the precaution 
of stirring and cooling it previous to the experiment (1973.). 
1979. With iron, the part of the metal in the weaker acid was powerfully positive 
to that in the stronger acid. With copper, the same result, as to direction of the 
current, was produced ; but the amount of the effect was small. With silver, cad- 
mium, and zinc, the difference was either very small or unsteady, or nothing ; so 
that, in comparison with the former cases, the electromotive action of the strong and 
weak acid appeared balanced. With lead and tin, the part of the metal in the strong 
acid was positive to that in the weak acid ; so that they present an effect the reverse 
of that produced by iron or copper. 
1980. Strong and dilute muriatic acid. — I used the strongest pure muriatic acid in 
tube No. 1., and added water on the top of one side for the dilute extremity (1973.), 
stirring it a little as before. With silver, copper, lead, tin, cadmium, and zinc, the 
metal in the strongest acid was positive, and the current in most cases powerful. 
With iron, the end in the strongest acid was first positive : but shortly after, the 
weak acid side became positive and continued so. With palladium, gold, and pla- 
tinum, nearly insensible effects were the results. 
1981. Strong and dilute solution of caustic potassa. — With iron, copper, lead, tin, 
cadmium, and zinc, the metal in the strong solution was positive : in the case of iron 
slightly, in the case of copper more powerfully, deflecting the needle 30° or 38°, and 
in the cases of the other metals very strongly. Silver, palladium, gold, and platinum, 
gave the merest indications (1973.). 
Thus potash and muriatic acid are, in several respects, contrasted with nitric and 
sulphuric acids. As respects muriatic acid, however, and perhaps even the potash, it 
may be admitted that, even in their strongest states, they are not fairly comparable 
to the very strong nitric and sulphuric acids, but rather to those acids when somewhat 
diluted (1985.). 
1982. I know it maybe said in reference to the numerous changes with strong and 
dilute acids, that the results are the consequence of corresponding alterations in the 
contact force ; but this is to change about the theory with the phenomena and with 
chemical force (1874. 1956. 1985. 2006. 2014. 2063.) ; or it may be alleged that it is 
the contact force of the solutions produced at the metallic surfaces which, differing, 
causes difference of effect ; but this is to put the effect before the cause in the order 
of time. If the liberty of shifting the point of efficacy from metals to fluids, or from 
one place to another be claimed, it is at all events quite time that some definite 
statement and data respecting the active points (1808.) should be given. At present 
it is difficult to lay hold of the contact theory by any argument derived from experi- 
ment, because of these uncertainties or variations, and it is in that respect in singular 
