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.