SECT, xxv.] AND OF ME. HUNT. 269 



pend on difference of temperature, and that, in order to ob- 

 tain good impressions, dissimilar metals must be used. For 

 example, gold, silver, bronze, and copper coins were placed 

 on a plate of copper too hot to be touched, and allowed to 

 remain till the plate cooled ; all the coins had made an im- 

 pression, the distinctness and intensity of which was in the 

 order of the metals named. When the plate was exposed 

 to the vapour of mercury, the result was the same, but, when 

 the vapour was wiped off, the gold and silver coins only had 

 left permanent images on the copper. These impressions 

 are often minutely perfect whether the coins are in actual 

 contact with the plate or ^ of an inch above it. The mass 

 of the metal has a material influence on the result ; a large 

 copper coin makes a better impression on a copper plate 

 than a small silver coin. When coins of different metals 

 are placed on the same plate, they interfere with each other. 



When, instead of being heated, the copper plate was 

 cooled by a freezing mixture, and bad conductors of heat 

 laid upon it, as wood, paper, glass, &c., the result was simi- 

 lar, showing that the phenomena could be produced by any 

 disturbance of the caloric latent in the substances. 



There can be no doubt that these phenomena are universal, 

 since all substances are more or less sensitive to light, which 

 must produce innumerable changes in the nature of terrestrial 

 things, especially in the vegetable tribe, by the power it gives 

 of condensing vapour and consequently the deposition of 

 dew. 



Red and orange-coloured media, smoked glass, and all 

 bodies that transmit or absorb the calorific rays freely, leave 

 strong impressions on a plate of copper, whether they be in 

 contact or of an inch above the plate. The strongest proof 

 that heat is concerned in some at least of these phenomena is 

 evident. For instance, a solar spectrum concentrated by a lens 

 was thrown on a polished plate of copper, and kept on the 

 same spot by a heliostat for one, two, or three hours ; when 

 exposed to mercurial vapour, a film of the vapour covered the 



