192 Faraday's Researches

the alkali,, i.e. the electric current tended to pass through the
vessels in the direction of the arrow,, being the reverse direction
of that which the acid in A would have produced alone: but
the effect instantly ceased, and the action of the plates in the
vessels was so equal, that, being contrary because of the con-
trary position of the plates, no permanent current resulted.

671. Occasionally a zinc plate was substituted for the plate
P P, and platina plates for the plates Z Z; but this caused no
difference in the results: nor did a further change of the middle
plate to copper produce any alteration.

672. As the opposition of electro-motive pairs of plates pro-
duces results other than those due to the mere difference of
their independent actions (747, 781), I devised another form
of apparatus, in which the action of acid and alkali might be

2 F

Fig. 40.

Fig. 41.

Fig. 42.

more directly compared. A cylindrical glass cup, about two
inches deep within, an inch in internal diameter, and at least a
quarter of an inch in thickness, was cut down the middle into •
halves, fig. 41. A broad brass ring, larger in diameter than
the cup, was supplied with a screw at one side; so that when
the two halves of the cup were within the ring, and the screw
was made to press tightly against the glass, the cup held any
fluid put into it. Bibulous paper of different degrees of per-
meability was then cut into pieces of such a size as to be easily
introduced between the loosened halves of the cup, and served
when the latter were tightened again to form a porous division
down the middle of 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. 42. This instrument I have found of most general appli-
cation in the investigation of the relation of fluids and metals
amongst themselves and to each other. By combining its usent