SECT. XXV. LATENT CALORIC. 235 



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 similar, showing that the phenomena could be pro- 

 duced 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 vege- 

 table tribe, by the power it gives of condensing vapor 

 and consequently the deposition of dew. 



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

 bodies that transmit or absorb the calorific rays freely, 

 leave strong impressions on a plate ef 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 pol- 

 ished plate of copper and kept on the same spot by a 

 heliostat for one, two, or three hours ; when exposed 

 to mercurial vapor a film of the vapor covered the plate 

 where the diffused light which always accompanies the 

 solar spectrum had fallen ; on the obscure space occu- 

 pied 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 vapor 

 was much less than that which covered the other parts, 

 affording distinct evidence of a negative effect hi 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 considerable depth below it, and consequently are 

 permanent. 



