the cup, sufficient to keep any two fluids on opposite sides of the paper
from mingling, except very slowly, and yet allowing them to act freely as
one _electrolyte_. The two spaces thus produced I will call the cells A and
B, fig. 82. This instrument I have found of most general application in the
investigation of the relation of fluids and metals amongst themselves and
to each other. By combining its use with that of the galvanometer, it is
easy to ascertain the relation of one metal with two fluids, or of two
metals with one fluid, or of two metals and two fluids upon each other.

938. Dilute sulphuric acid, sp. gr. 1.25, was put into the cell A, and a
strong solution of caustic potassa into the cell B; they mingled slowly
through the paper, and at last a thick crust of sulphate of potassa formed
on the side of the paper next to the alkali. A plate of clean platina was
put into each cell and connected with a delicate galvanometer, but no
electric current could be observed. Hence the _contact_ of acid with one
platina plate, and alkali with the other, was unable to produce a current;
nor was the combination of the acid with the alkali more effectual (925.).

939. When one of the platina plates was removed and a zinc plate
substituted, either amalgamated or not, a strong electric current was
produced. But, whether the zinc were in the acid whilst the platina was in
the alkali, or whether the reverse order were chosen, the electric current
was always from the zinc through the electrolyte to the platina, and back
through the galvanometer to the zinc, the current seeming to be strongest
when the zinc was in the alkali and the platina in the acid.

940. In these experiments, therefore, the acid seems to have no power over
the alkali, but to be rather inferior to it in force. Hence there is no
reason to suppose that the combination of the oxide formed with the acid
around it has any direct influence in producing the electricity evolved,
the whole of which appears to be due to the oxidation of the metal (919.).

941. The alkali, in fact, is superior to the acid in bringing a metal into
what is called the positive state; for if plates of the same metal, as
zinc, tin, lead, or copper, be used both in the acid or alkali, the
electric current is from the alkali across the cell to the acid, and back
through the galvanometer to the alkali, as Sir Humphry Davy formerly stated
[A]. This current is so powerful, that if amalgamated zi