SECT. XXIV. EFFECT OF ELECTRICITY. 221 



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 im- 

 pression of the object comes out on a grey ground. Effects 

 similar to those attributed to heat may also be produced by elec^ 

 tricity. 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 elec- 

 trical 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 versa. 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 impres- 

 sions in the same manner with perfect sharpness and accuracy, 

 when brought out by vapour without contact, in darkness, and 

 on simple metals. 



