hich, being an excellent electrolyte when fluid, was a perfect insulator
when solid, namely, borate of lead, in the form of a glass plate, and
connecting the sides and the edges of this mass with the metallic plates,
sometimes in contact with the poles of a voltaic battery, and sometimes
even with the electric machine, for the advantage of the much higher
intensity then obtained, I passed a polarized ray across it in various
directions, as before, but could not obtain the slightest appearance of
action upon the light. Hence I conclude, that notwithstanding the new and
extraordinary state which must be assumed by an electrolyte, either during
decomposition (when a most enormous quantity of electricity must be
traversing it), or in the state of tension which is assumed as preceding
decomposition, and which might be supposed to be retained in the solid form
of the electrolyte, still it has no power of affecting a polarized ray of
light; for no kind of structure or tension can in this way be rendered
evident.

956. There is, however, one beautiful experimental proof of a state of
tension acquired by the metals and the electrolyte before the electric
current is produced, and _before contact_ of the different metals is made
(915.); in fact, at that moment when chemical forces only are efficient as
a cause of action. I took a voltaic apparatus, consisting of a single pair
of large plates, namely, a cylinder of amalgamated zinc, and a double
cylinder of copper. These were put into a jar containing dilute sulphuric
acid[A], and could at pleasure be placed in metallic communication by a
copper wire adjusted so as to dip at the extremities into two cups of
mercury connected with the two plates.

[A] When nitro-sulphuric acid is used, the spark is more powerful, but
local chemical action can then commence, and proceed without requiring
metallic contact.

957. Being thus arranged, there was no chemical action whilst the plates
were not connected. On _making_ the connexion a spark was obtained[A], and
the solution was immediately decomposed. On breaking it, the usual spark
was obtained, and the decomposition ceased. In this case it is evident that
the first spark must have occurred before metallic contact was made, for it
passed through an interval of air; and also that it must have tended to
pass before the electrolytic action began; for the latter could not take
place until the current passed, and the current could not pass