Local Chemical Action

199

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.

694. With reference to the other set of cases, namely, those
of local action (682) in which chemical affinity being exerted
causes no transference of the power to a distance where no
electric current is produced, it is evident that forces of the
most intense kind must be active, and in some way balanced in
their activity, during such combinations; these forces being
directed so immediately and exclusively towards each other,,
that no signs of the powerful electric current they can produce
become apparent, although the same final state of things is
obtained as if that current had passed. It was Berzelius, I
believe, who considered the heat and light evolved in cases of
combustion as the consequences of this mode of exertion of the
electric powers of the combining particles. But it will require
a much more exact and extensive knowledge of the nature of
electricity, and the manner in which it is associated with the
atoms of matter, before we can understand accurately the action
of this power in thus causing their union, or comprehend the
nature of the great difference which it presents in the two
modes of action just distinguished. We may imagine, but such
imaginations must for the time be classed with the great mass
of doubtful knowledge (611) which we ought rather to strive to
diminish than to increase; for the very extensive contradictions
of this knowledge by itself shows that but a small portion of it
can ultimately prove true.

695. Of the two modes of action in which chemical affinity is
exerted, it is important to remark, that that which produces
the electric current is as definite as that which causes ordinary
chemical combination; so that in examining the 'production orire of com-