A curious experiment showing that a photographic dry-plate can be
otherwise affected than by light, so as to form an image upon it, is
the following:
An image of copper in relief is necessary--a penny will do for this
purpose. Place an unexposed dry-plate in a normal pyro developer, and
on it lay the copper coin. After about five minutes or so, remove the
penny, fix and wash the plate, when a perfect image of the penny will
be found on it.