1 70 Mr. Herschel on certain motions produced in 
globules of that metal towards the negative pole. The rota- 
tion continued with considerable force, when the wires were 
so far withdrawn as to have only their extremities in contact 
with the liquid in the cells, in which case the surface exposed 
by each pair to the action of the acid could not exceed -l 0 f 
a square inch. Nay, so delicate is this indication, that the 
electricity developed by bringing the extremities of a thin 
zinc and copper wire in contact with a glass merely mois- 
tened with the above mentioned dilute acid, is abundantly 
sufficient to cause an immediate and unequivocal rotation in 
an ounce or two of mercury properly exposed to its action. 
By this means, indeed, the feeblest electrical excitement may 
be placed in evidence. I have thus rendered strikingly sen- 
sible the electricity developed by a mere difference in the 
state of the surface of two small portions of copper wire from 
the same coil (one being a little cleaner than the other) not 
above an inch in length of either being immersed ; or that 
set in motion by a copper and zinc wire held near together 
and dipped in common pump water, powers which it is not 
easy to render sensible by other means. For the success of 
these experiments, however, it is not enough merely to plunge 
the extremities of the conducting wires under sulphuric acid. 
The surfaces of contact here require to be greatly increased,* 
* The efficacy of an increase of surface for transmitting electricity info a liquid, 
is remarkable. By bringing the positive pole in contact with a large surface of 
mercury, or still better, of an amalgam of mercury and zinc, over which a saline 
solution is poured, the reduction of the metals of the alkalies and earths at the other 
pole is operated with a degree of facility hardly to be imagined without trial. In 
this way the decomposition of ammonia may be effected with thtee pair of single 
plates of the above dimensions, in very moderate action. 
