d zinc the fall in force soon appeared to become
constant, i.e. to proceed no further. But with the cast zinc plates
belonging to the porcelain troughs, it appeared to continue, until at last,
with the same charge, each plate lost above twice as much zinc for a given
amount of action as at first. These troughs were, however, so irregular
that I could not always determine the circumstances affecting the amount of
electrolytic action.

1148. _Vicinity of the copper and zinc._--The importance of this point in
the construction of voltaic arrangements, and the greater power, as to
immediate action, which is obtained when the zinc and copper surfaces are
near to each other than when removed further apart, are well known. I find
that the power is not only greater on the instant, but also that the sum of
transferable power, in relation to the whole sum of chemical action at the
plates, is much increased. The cause of this gain is very evident. Whatever
tends to retard the circulation of the transferable force, (i.e. the
electricity,) diminishes the proportion of such force, and increases the
proportion of that which is local (996. 1120.). Now the liquid in the cells
possesses this retarding power, and therefore acts injuriously, in greater
or less proportion, according to the quantity of it between the zinc and
copper plates, i.e. according to the distances between their surfaces. A
trough, therefore, in which the plates are only half the distance asunder
at which they are placed in another, will produce more transferable, and
less local, force than the latter; and thus, because the electrolyte in the
cells can transmit the current more readily; both the intensity and
quantity of electricity is increased for a given consumption of zinc. To
this circumstance mainly I attribute the superiority of the trough I have
described (1134.).

1149. The superiority of _double coppers_ over single plates also depends
in part upon diminishing the resistance offered by the electrolyte between
the metals. For, in fact, with double coppers the sectional area of the
interposed acid becomes nearly double that with single coppers, and
therefore it more freely transfers the electricity. Double coppers are,
however, effective, mainly because they virtually double the acting surface
of the zinc, or nearly so; for in a trough with single copper plates and
the usual construction of cells, that surface of zinc which is not opposed
to a copper surface is