metallic contact; he discovered liquids which, though competent
to transmit the feeblest currents--competent therefore to allow the
electricity of contact to flow through them if it were able to form a
current--were absolutely powerless when chemically inactive.

One of the very few experimental mistakes of Faraday occurred in
this investigation. He thought that with a single voltaic cell he
had obtained the spark before the metals touched, but he subsequently
discovered his error. To enable the voltaic spark to pass through air
before the terminals of the battery were united, it was necessary
to exalt the electro-motive force of the battery by multiplying its
elements; but all the elements Faraday possessed were unequal to the
task of urging the spark across the shortest measurable space of air.
Nor, indeed, could the action of the battery, the different metals of
which were in contact with each other, decide the point in question.
Still, as regards the identity of electricities from various sources,
it was at that day of great importance to determine whether or not
the voltaic current could jump, as a spark, across an interval before
contact. Faraday's friend, Mr. Gassiot, solved this problem. He erected
a battery of 4000 cells, and with it urged a stream of sparks from
terminal to terminal, when separated from each other by a measurable
space of air.

The memoir on the 'Electricity of the Voltaic Pile,' published in 1834,
appears to have produced but little impression upon the supporters of
the contact theory. These indeed were men of too great intellectual
weight and insight lightly to take up, or lightly to abandon a theory.
Faraday therefore resumed the attack in a paper, communicated to the
Royal Society on the 6th of February, 1840. In this paper he hampered
his antagonists by a crowd of adverse experiments. He hung difficulty
after difficulty about the neck of the contact theory, until in its
efforts to escape from his assaults it so changed its character as to
become a thing totally different from the theory proposed by Volta. The
more persistently it was defended, however, the more clearly did it
show itself to be a congeries of devices, bearing the stamp of dialectic
skill rather than of natural truth.

In conclusion, Faraday brought to bear upon it an argument which, had
its full weight and purport been understood at the time, would have
instantly decided the controversy. 'The contact theory,' he urged,