in
circumstances connected with its production and appearance (958.). An
ordinary electric spark is understood to be the bright appearance of
electricity passing suddenly through an interval of air, or other badly
conducting matter. A voltaic spark is sometimes of the same nature, but,
generally, is due to the ignition and even combustion of a minute portion
of a good conductor; and that is especially the case when the electromotor
consists of but one or few pairs of plates. This can be very well observed
if either or both of the metallic surfaces intended to touch be solid and
pointed. The moment they come in contact the current passes; it heats,
ignites, and even burns the touching points, and the appearance is as if
the spark passed on making contact, whereas it is only a case of ignition
by the current, contact being previously made, and is perfectly analogous
to the ignition of a fine platina wire connecting the extremities of a
voltaic battery.

1075. When mercury constitutes one or both of the surfaces used, the
brightness of the spark is greatly increased. But as this effect is due to
the action on, and probable combustion of, the metal, such sparks must only
be compared with other sparks also taken from mercurial surfaces, and not
with such as may be taken, for instance, between surfaces of platina or
gold, for then the appearances are far less bright, though the same
quantity of electricity be passed. It is not at all unlikely that the
commonly occurring circumstance of combustion may affect even the duration
of the light; and that sparks taken between mercury, copper, or other
combustible bodies, will continue for a period sensibly longer than those
passing between platina or gold.

1076. When the end of a short clean copper wire, attached to one plate of
an electromotor, is brought down carefully upon a surface of mercury
connected with the other plate, a spark, almost continuous, can be
obtained. This I refer to a succession of effects of the following nature:
first, contact,--then ignition of the touching points,--recession of the
mercury from the mechanical results of the heat produced at the place of
contact, and the electro-magnetic condition of the parts at the moment[A],
--breaking of the contact and the production of the peculiar intense effect
dependent thereon,--renewal of the contact by the returning surface of the
undulating mercury,--and then a repetition of the same series of effects,
and that