HEAT-RADIATIONS. 189 



It is also probable that chemical decomposition is pro- 

 duced by the mere juxtaposition of different bodies. 

 Iodide of gold or silver, perfectly pure, has been placed 

 upon a plate of glass, and a plate of copper covered with 

 mercury suspended over it : a gradual decomposition 

 of those salts is said to have been observed, iodide of 

 mercury to be formed, and the gold or silver salts reduced 

 to a finely divided metallic state. * 



A body whose powers of radiating heat are low, being 

 brought near another whose radiating powers are more 

 extensive, will, in the course of a short time, undergo 

 such an amount of molecular disturbance as will effect 

 n complete change in the arrangement of its surface, 

 and an impression of the body having the highest radi- 

 ating powers will be made upon the other. This im- 

 pression is dormant, but may be developed under the 

 influence of vapour, or of oxidation.f A body, such as 



Kouigsberg ; from PoggendorfFs Annalen, vol. Ivi. p. 569, No. 8. 

 Oft the power which light possesses of becoming latent : by Professor 

 Ludwig Moser, of Konigsberg; from Poggendorff's Annalen, vol. 

 Ivii. No. 9, p. 1. 1842. On certain spectral appearances, and on the 

 discovery of latent light : by J. W. Draper, M.D., Professor of 

 Chemistry in the University of New York; Phil. Mag. p. 348, 

 Nov. 1842. On a new imponderable substance, and on a class of 

 chemical rays analogous to the rays of dark heat : by Professor 

 Draper; Phil. Mag., Dec. 1842. On tha action of the rays of the 

 solar spectrum on the Daguerreotype plate; by Sir J. ; F. W. 

 Herschel, Bart.; Phil. Mag., Feb. 1843. See remarks in this 

 paper on the use which Moser has made of coloured glasses : also 

 a communication by Professor Draper, On the rapid Detithoniziny 

 power of certain gases and vapours, and on an instantaneous means 

 of proditcinif spectral appearances'. Phil. Mag., March 1843 ; and 

 On the causes tohieh concur m the production of the images of Moser : 

 Comptes Rendus, Nov. 1842. See Scientific Memoirs, vol. iii. 



- This fact was first observed by myself, and described in the 

 paper already referred to, Philosophical Magazine, vol. xxii. p. 270. 

 It does not, however, appear to have attracted the attention of any 

 other observer. 



f On Thermography, or the Art of copying Engravings or any 

 printed characters from paper or ptates of metal, and on 'the recent 

 discovery of Moser, relative to the formation of images in - the dark, 



