Mr. Hunt on Thermography. 463 



tium when the sun is shining brightly on the flower, and carry 

 it into a dark room, we shall still be enabled to see it by the 

 light which it emits. 



The human hand will sometimes exhibit the same phseno- 

 menon, and many other instances might be adduced in proof 

 of the absorption of light; and, I believe, indeed of the prin- 

 ciple that light is latent in bodies. I have only to show that 

 the conclusions of M. Moser have been formed somewhat 

 hastily, being led, no doubt, by the striking similarity which 

 exists between the effects produced on the Daguerreotype 

 plates under the influence of light, and by the juxtaposition 

 of bodies in the dark, to consider them as the work of the 

 same element. 



1 . Dr. Draper, in the Philosophical Magazine for Septem- 

 ber 1 840, mentions a fact which has been long known, " That 

 if a piece of very cold clear glass, or what is better, a cold po- 

 lished metallic reflector, has a little object, such as a piece of 

 metal, laid on it, and the surface be breathed over once, the 

 object being then carefully removed, as often as you breathe 

 again on the surface,,a spectral image of it may be seen, and this 

 singular phenomenon may be exhibited for many days after 

 the first trial is made." Several other similar experiments 

 are mentioned, all of them going to show that some mysterious 

 molecular change has taken place on the metallic surface, 

 which occasions it to condense vapours unequally. 



2. On repeating this simple experiment, I find that it is ne- 

 cessary, for the production of a good effect, to use dissimilar 

 metals ; for instance, a piece of gold or platina on a plate of 

 copper or of silver will make a very decided image, whereas 

 copper or silver on their respective plates gives but a very 

 faint one, and bodies which are bad conductors of heat, placed 

 on good conductors, make decidedly the strongest impressions 

 when thus treated. 



3. I placed upon a well-polished copper plate, a sovereign, 

 a shilling, a large silver medal, and a penny. The plate was 

 gently warmed by passing a spirit lamp along its under sur- 

 face ; when cold, the plate was exposed to the vapour of mer- 

 cury ; each piece had made its impression, but those made 

 by the gold and the large medal were most distinct; not only 

 was the disc marked, but the lettering on each was copied. 



4. A bronze medal was supported upon slips of wood, 

 placed on the copper, one-eighth of an inch above the plate. 

 After mercurialization, the space the medal covered was well- 

 marked, and for a considerable distance around the mercury 

 was unequally deposited, giving a shaded border to the image ; 

 the spaces touched by the [mercury?] were thickly covered with 

 the vapour. ' 



