AEROSTATICS. 229 



320. Heat escapes from bodies which are heated 

 above a certain temperature, by radiation, or by 

 passing in straight lines through the air with great 

 rapidity. 



The heat thus emitted from bodies, is reflected from 

 the surfaces of metallic specula, like the rays of 

 light. 



A body heated, though not so as to shine, and placed 

 before a concave speculum of metal, communicates 

 heat instantaneously to a thermometer in the cor- 

 responding focus. A cold body does the same ; 

 and it is remarkable, that an effect so difficult to be 

 explained, is, nevertheless, perfectly consistent with 

 the law of continuity. The experiment of the re- 

 flection of cold was first made by M. PICTET of 

 Geneva. Essai sur le Feu, par M. A. PICTET, 

 69. 



Radiant heat from a luminous body, is refracted as 

 well as reflected ; the rays from a candle or a fire 

 produce heat in the focus of a lens. It does not 

 appear that the same happens in the case of obscure 

 heat, except when it comes from the sun, which, 

 according to the experiments of Dr HERSCHELL, is 

 concentrated in the focus of a lens. Phil. Trans. 

 1803, p. 298, 304. 



Dr HERSCHELL, in a series of very interesting experi- 

 ments, separated the caloric from the light of the 

 sun's rays, and shewed that the focus of the former 

 was not the same with that of the latter. Phil. 

 Trans, ibid. 



321. The 



