464 Mr. Hunt on Thermography, 



5. The above coins and medals were all placed on the plate, 

 and it was made too hot to be handled, and allowed to cool 

 without their being removed ; impressions were made on the 

 plate in the following order of intensity, — gold, silver, bronze, 

 copper. The mass of the metal was found to influence ma- 

 terially the result ; a large piece of copper making a better 

 image than a small piece of silver. When this plate was ex- 

 posed to vapour, the results were as before (3, 4). On rub- 

 bing off the vapour, it was found that the gold and silver had 

 made permanent impressions on the copper. 



6. The above being repeated with a still greater heat, the 

 image of the copper coin was, as well as the others, most 

 faithfully given, but the gold and silver only made permanent 

 impressions. 



7. A silvered copper plate was now tried with a moderate 

 warmth (3). Mercurial vapour brought out good images 

 of the gold and copper ; the silver marked, but not well de- 

 fined. 



8. Having repeated the above experiments many times with 

 the same results, I was desirous of ascertaining if electricity 

 had any similar effect; powerful discharges were passed 

 through and over the plate and discs, and it was subjected to 

 a long-continued current without any effect. The silver had 

 been cleaned off from the plate (7), it was now warmed with 

 the coins and medals upon it, and submitted to discharges 

 from a very large Leyden jar ; on exposing it to mercurial 

 vapour, the impressions were very prettily brought out, and 

 strange to say, spectral images of those which had been re- 

 ceived on the plate when it was silvered (7) ; thus proving 

 that the influence, whatever it may be, was exerted to some 

 depth in the metal. 



9. I placed upon a plate of copper, blue, red and orange- 

 coloured glasses, pieces of crown and flint glass, mica, and a 

 square of tracing paper. These were allowed to remain in 

 contact half an hour. The space occupied by the red glass 

 was well marked, that covered by the orange was less di- 

 stinct, but the blue glass left no impression ; the shapes of the 

 flint and crown glass were well made out, and a remarkably 

 strong impression where the crown glass rested on the tracing 

 paper, but the mica had not made any impression. 



10. The last experiment repeated, after the exposure to 

 mercurial vapour ; heat was again applied to dissipate it ; the 

 impression still remained. 



11. The experiment repeated, but the vapour of iodine 

 used instead of that of mercury. The impressions of the 

 glasses appeared in the same order as before, but also a very 

 beautiful image of the mica was developed, and the paper well 



