236 THE ART OP THERMOGRAPHY. SECT. XXV. 



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 thermography, by 

 which he copies prints, wood-cuts, writing, and printing, 

 on copper amalgamated on one surface and highly pol- 

 ished, 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 sub- 

 j ected to the vapor of mercury and afterward t6 that of 

 iodine, when a black and accurate impression of the 

 object comes out on a gray 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 re- 

 ceived 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 

 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 capa- 

 ble of condensing vapor. 



M. Fizean ascribes a numerous class of these phe- 

 nomena to the action of a slight layer of organic or fatty 

 matter on the surfaces, which, being volatile, is trans- 

 ferred to any body near, in a greater or less quantity ac- 

 cording to the distance ; that is, according as the sur- 

 face projects or sinks into hollows. When the different 

 parts of a surface are unequally soiled by extraneous 

 bodies, even in the minutest quantity, the condensation 

 of mercurial vapor is effected in a manner visibly dif- 



