Mr. Hunt, observing that a black substance leaves a stronger impression on a metallic surface than a white, applied the property to the art of copying prints, woodcuts, writing, and printing, on copper amalgamated on one surface and highly polished, merely by placing the object to be copied smoothly on the metal, and pressing it into close contact by a plate of glass: after some hours the plate is subjected to the vapour of mercury, and afterwards to that of iodine, when a black and accurate impression of the object comes out on a grey ground. Effects similar to those attributed to heat may also be produced by electricity. Mr. Karsten, by placing a glass plate upon one of metal, and on the glass plate a medal subjected to discharges of electricity, found a perfect image of the medal impressed on the glass, which could be brought into evidence by either mercury or iodine; and, when several plates of glass were interposed between the medal and the metallic plate, each plate of glass received an image on its upper surface after the passage of electrical discharges. These discharges have the remarkable power of restoring impressions that have been long obliterated from plates by polishing—a proof that the disturbances upon which these phenomena depend are not confined to the surface of the metals, but that a very decided molecular change has taken place to a considerable depth. Mr. Hunt’s experiments prove that the electro-negative metals make the most decided images upon electro-negative plates, and _vice versâ_. M. Matteucci has shown that a discharge of electricity does not visibly affect a polished silver plate, but that it produces an alteration which renders it capable of condensing vapour. The impression of an engraving was made by laying it face downwards on a silver plate iodized, and placing an amalgamated copper plate upon it; it was left in darkness fifteen hours, during which time an impression of the engraving had been made on the amalgamated plate _through the paper_. An iodized silver plate was placed in darkness with a coil of string laid on it, and with a polished silver plate suspended one-eighth of an inch above it: after four hours they were exposed to the vapours of mercury, which became uniformly deposited on the iodized plate, but on the silver one there was a sharp image of the string, so that this image was formed in the dark, and even without contact. Coins or other objects leave their impressions in the same manner with perfect sharpness and accuracy, when brought out by vapour without contact, in darkness, and on simple metals.