before the
spark appeared. Hence I think there is sufficient proof, that as it is the
zinc and water which by their mutual action produce the electricity of this
apparatus, so these, by their first contact with each other, were placed in
a state of powerful tension (951.), which, though it could not produce the
actual decomposition of the water, was able to make a spark of electricity
pass between the zinc and a fit discharger as soon as the interval was
rendered sufficiently small. The experiment demonstrates the direct
production of the electric spark from pure chemical forces.

[A] It has been universally supposed that no spark is produced on
making the contact between a single pair of plates. I was led to
expect one from the considerations already advanced in this paper. The
wire of communication should be short; for with a long wire,
circumstances strongly affecting the spark are introduced.

958. There are a few circumstances connected with the production of this
spark by a single pair of plates, which should be known, to ensure success
to the experiment[B]. When the amalgamated surfaces of contact are quite
clean and dry, the spark, on making contact, is quite as brilliant as on
breaking it, if not even more so. When a film of oxide or dirt was present
at either mercurial surface, then the first spark was often feeble, and
often failed, the breaking spark, however, continuing very constant and
bright. When a little water was put over the mercury, the spark was greatly
diminished in brilliancy, but very regular both on making and breaking
contact. When the contact was made between clean platina, the spark was
also very small, but regular both ways. The true electric spark is, in
fact, very small, and when surfaces of mercury are used, it is the
combustion of the metal which produces the greater part of the light. The
circumstances connected with the burning of the mercury are most favourable
on breaking contact; for the act of separation exposes clean surfaces of
metal, whereas, on making contact, a thin film of oxide, or soiling matter,
often interferes. Hence the origin of the general opinion that it is only
when the contact is broken that the spark passes.

[B] See in relation to precautions respecting a spark, 1074.--_Dec.
1838._

959. With reference to the other set of cases, namely, those of local
action (947.) in which chemical affinity being exerted causes no
transference of the power to a