'After many fruitless attempts, and after having sent to the
press the preceding part of this volume, I at last hit upon a
method of producing electricity by the action of metallic
substances upon one another, and apparently without the
interference of electric bodies. I say apparently so, because
the air seems to be in a great measure concerned in those
experiments, and perhaps the whole effect may be produced by
that surrounding medium. But, though the irregular,
contradictory, and unaccountable effects observed in these
experiments do not as yet furnish any satisfactory theory, and
though much is to be attributed to the circumambient air, yet
the metallic substances themselves seem to be endowed with
properties peculiar to each of them, and it is principally in
consequence of those properties that the produced electricity
is sometimes positive, at other times negative, and various in
its intensity.'

Cavallo then proceeds to describe the experiments on contact
electrification which were described in the previous paper
referred to at the beginning of the article.

Cavallo's experiments were evidently made in 1795. In the
following year Volta announced the discovery of the electrical
current. In a letter written to Gren's Neues Journal der
Physik, August, 1796, Volta says:

'The contact of different conductors, particularly the
metallic, including pyrites and other minerals as well as
charcoal, which I call dry conductors, or of the first class
with moist conductors, or conductors of the second class,