110 DR. FARADAY’S EXPERIMENTAL RESEARCHES IN ELECTRICITY. (SERIES XVII.) 
acid, but iron in weak acid was very positive to silver in strong acid. Generally the 
metal, usually called positive, was most positive in the weak acid ; but that was not 
the case with lead, tin, and zinc. 
1996. Two metals in strong and weak nitric acid. — Here the degree of change pro- 
duced by difference in the strength of the acid was so great, as to cause not merely 
difference in degree, but inversions of the order of the metals, of the most striking 
nature. Thus iron and silver being in tube No. 2 (1974.), whichever metal was in 
the weak acid was positive to the other in the strong acid. It was merely requisite to 
raise the one and lower the other metal to make either positive at pleasure (19/5.). 
Copper in weak acid was positive to silver, iron, lead, or tin, in strong acid. Iron 
in weak acid was positive to silver, copper, lead, zinc, or tin, in strong acid. Lead 
in weak acid was positive to copper, silver, tin, cadmium, zinc, and iron in strong 
acid. Silver in weak acid was positive to iron, lead, copper, and, though slightly, 
even to tin, in strong acid. Tin in weak acid was positive to copper, lead, iron, zinc, 
and silver, and either neutral or a little positive to cadmium in strong acid. Cad- 
mium in weak acid is very positive, as might be expected, to silver, copper, lead, iron, 
and tin, and, moderately so, to zinc in the strong acid. When cadmium is in the 
strong acid it is slightly positive to silver, copper, and iron in weak acid. Zinc in 
weak acid is very positive to silver, copper, lead, iron, tin, and cadmium in strong 
acid : when in the strong acid it is a little positive to silver and copper in weak acid. 
1997- Thus wonderful changes occur amongst the metals in circuits containing 
this acid, merely by the effect of dilution ; so that of the five metals, silver, copper, 
iron, lead, and tin, any one of them can be made either positive or negative to any 
other, with the exception of silver positive to copper. The order of these five metals 
only may therefore be varied above one hundred different ways in the same acid, 
merely by the effect of dilution. 
1998. So also zinc, tin, cadmium, and lead ; and likewise zinc, tin, iron, and lead, 
being groups each of four metals ; any one of these metals may be made either posi- 
tive or negative to any other metal of the same group, by dilution of this acid. 
1999. But the case of variation by dilution may, as regards the opposed theories, 
be made even still stronger than any yet stated ; for the same metals in the same acid 
of the same strength at the two sides may be made to change their order, as the che- 
mical action of the acid on each particular metal is affected, by dilution, in a smaller 
or greater degree. 
2000. A voltaic association of iron and silver was dipped, both metals at once, into 
the same strong nitric acid ; for the first instant, the iron was positive ; the moment 
after, the silver became positive, and continued so. A similar association of iron and 
silver was put into weak nitric acid, and the iron was immediately positive, and con- 
tinued so. With iron and copper the same results were obtained. 
