270 THE ART OF THERMOGRAPHY. [SECT. xxv. 



plate where the diffused light which always accompanies the 

 solar spectrum had fallen ; on the obscure space occupied by 

 the maximum heating power of Sir William Herschel, and 

 also the great heat spot in the thermic spectrum of Sir John 

 Herschel, the condensation of the mercury was so thick that 

 it stood out a distinct white spot on the plate, while over the 

 whole space that had been under the visible spectrum the 

 quantity of vapour was much less than that which covered 

 the other parts, affording distinct evidence of a negative effect 

 in the luminous spectrum, and of the power of the calorific 

 rays, which is not always confined to the surface of the metal, 

 since in many instances the impressions are formed to a con- 

 siderable depth below it, and consequently are permanent. 



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 amalga- 

 mated 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 gray 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 



