120 Dr. Faraday’s Experimental Researches in Electricity. 
used, which is proportionate to the diminution of the zinc dis- 
solved. x. The acid also is more easily exhausted, and is 
in such small quantity that there is never any occasion to re- 
turn an old charge into use. Such old acid, whilst out of use, 
often dissolves portions of copper from the black flocculi usu- 
ally mingled with it, which are derived from the zinc; now any 
portion of copper in solution in the charge does great harm, 
because, by the local action of the acid and zinc, it tends 
to precipitate upon the latter, and diminish its voltaic efficacy 
(1145.). xi. By using a due mixture of nitric and sulphuric 
acid for the charge (1139.), no gas is evolved from the troughs; 
so that a battery of several hundred pairs of plates may, with- 
out inconvenience, be close to the experimenter. xii. If, dur- 
ing a series of experiments, the acid becomes exhausted, it can 
be withdrawn, and replaced by other acid with the utmost fa- 
cility ; and after the experiments are concluded, the great ad- 
vantage of easily washing the plates is at command. And it 
appears to me, that in place of making, under different circum- 
stances, mutual sacrifices of comfort, power, and economy, to 
obtain a desired end, all are at once obtained by Dr. Hare’s 
form of trough. 
1133. But there are some disadvantages which I have not 
yet had time to overcome, though I trust they will finally be 
conquered. One is the extreme difficulty of making a wooden 
trough constantly water-tight under the alternations of wet 
and dry to which the voltaic instrument is subject. To remedy 
this evil, Mr. Newman is now engaged in obtaining porcelain 
troughs. The other disadvantage is a precipitation of copper 
on the zinc plates. It appears to me to depend mainly on the 
circumstance that the papers between the coppers retain acid 
when the trough is emptied; and that this acid slowly acting 
on the copper, forms a salt, which gradually mingles with the 
next charge, and is reduced on the zinc plate by the local ac- 
tion (1120.): the power of the whole battery is then reduced. 
I expect that by using slips of glass to separate the coppers at 
their edges, their contact can be sufficiently prevented, and the 
space between them be left so open that the acid ofa charge can 
be poured and washed out, and so be removed from every part: 
of the trough when the experiments in which it is used are 
completed. 
1134. The actual superiority of the troughs which I have 
constructed on this plan, I believe to depend, first and prin- 
cipally, on the closer approximation of the zinc and copper 
surfaces ;—in my troughs they are only one tenth of an inch 
apart (1148.) ;—and, next, on the superior quality of the rolled 
zinc above the cast zinc used in the construction of the ordi- 
nary pile. It cannot be that insulation between the contigu- 
