thrown almost entirely out of voltaic action, yet
the acid continues to act upon it and the metal is dissolved, producing
very little more than local effect (947. 996). But when by doubling the
copper, that metal is opposed to the second surface of the zinc plate, then
a great part of the action upon the latter is converted into transferable
force, and thus the power of the trough as to quantity of electricity is
highly exalted.

1150. _First immersion of the plates._--The great effect produced at the
first immersion of the plates, (apart from their being new or used
(1146.),) I have attributed elsewhere to the unchanged condition of the
acid in contact with the zinc plate (1003. 1037.): as the acid becomes
neutralized, its exciting power is proportionally diminished. Hare's form
of trough secures much advantage of this kind, by mingling the liquid, and
bringing what may be considered as a fresh surface of acid against the
plates every time it is used immediately after a rest.

1151. _Number of plates._[A]--The most advantageous number of plates in a
battery used for chemical decomposition, depends almost entirely upon the
resistance to be overcome at the place of action; but whatever that
resistance may be, there is a certain number which is more economical than
either a greater or a less. Ten pairs of four-inch plates in a porcelain
trough of the ordinary construction, acting in the volta-electrometer
(1126.) upon dilute sulphuric acid of spec. grav. 1.314, gave an average
consumption of 15.4 equivalents per plate, or 154 equivalents on the whole.
Twenty pairs of the same plates, with the same acid, gave only a
consumption of 5.5 per plate, or 110 equivalents upon the whole. When forty
pairs of the same plates were used, the consumption was 3.54 equivalents
per plate, or 141.6 upon the whole battery. Thus the consumption of zinc
arranged as _twenty_ plates was more advantageous than if arranged either
as _ten_ or as _forty_.

[A] Gay-Lussac and Thenard, Recherches Physico-Chimiques, tom. i. p. 29.

1152. Again, ten pairs of my four-inch plates (1129.) lost 6.76 each, or
the whole ten 67.6 equivalents of zinc, in effecting decomposition; whilst
twenty pairs of the same plates, excited by the same acid, lost 3.7
equivalents each, or on the whole 74 equivalents. In other comparative
experiments of numbers, ten pairs of the three inch-plates, (1125.) lost
3.725, or 37.25 equivalents upon the whole; whilst twenty pair