210 RADIANT HEAT. SECT. XXV. 



absorption in each of the succeeding strata, and may 

 therefore be propagated to a greater distance before it 

 is extinguished. The experiments of M. de Laroche 

 show, that glass, however thin, totally intercepts the 

 obscure rays of caloric when they flow from a body 

 whose temperature is lower than that of boiling water ; 

 that as the temperature increases, the calorific rays are 

 transmitted more and more abundantly ; and when the 

 body becomes highly luminous, that they penetrate the 

 glass with perfect ease. The extreme brilliancy of the 

 sun is probably the reason why his heat, when brought to 

 a focus by a lens, is more intense than any that has been 

 produced artificially. It is owing to the same cause 

 that glass screens, which entirely exclude the heat of a 

 common fire, are permeable by the solar caloric. 



The results obtained by M. de Laroche have been 



confirmed by the recent experiments of M. Melloni on 



caloric radiated from sources of different temperatures, 



whence it appears that the calorific rays pass less abun- 



*iy not only through glass, but through rock-crystal, 



spar, and other diaphanous bodies, both solid 



according as the temperature of their origin 



and that they are altogether intercepted 



)erature is about that of boiling water. 



as proved that the heat emanating from 



i a bright flame consists of rays which 



other as much as the red, yellow, and 



,h constitute white light. This ex- 



v f the loss of heat as it penetrates 



H solid mass, or in passing 



for, of the different kinds of 



all are successively 



.re of the substance 



till those homogeneous rays 



ve the greatest facility in passing 



aeuiar substance ; exactly as in a red 



and yellow rays are extinguished, and 



are transmitted. 



Melloni employed four sources of caloric, two of 

 were luminous and two obscure ; namely, an oil- 

 without u glass, incandescent platina, copper 

 to 696, and a copper vessel filled with water at 



