242 RADIANT HEAT. [SECT. xxv. 



variable, and those rays alone are transmitted which give 

 the red colour to the liquid. In a similar manner, when 

 plates of the same thickness of any substance, such as glass, 

 are exposed to an argand lamp, a considerable portion of 

 the radiant heat is arrested by the first plate, a less portion 

 by the second, still less by the third, and so on, the quan- 

 tity of lost heat decreasing till at last the loss becomes a 

 constant quantity. The transmission of radiant heat through 

 a solid mass follows the same law. The losses are very con- 

 siderable on first entering it, but they rapidly diminish in 

 .proportion as the heat penetrates deeper, and become con- 

 stant at a certain depth. Indeed, the only difference be- 

 tween the transmission of radiant heat through a solid 

 mass, or through the same mass when cut into plates of 

 equal thickness, arises from the small quantity of heat that 

 is reflected at the surface of the plates. It is evident, there- 

 fore, that the heat gradually lost is not intercepted at the 

 surface, but absorbed in the interior of the substance, and 

 that heat which has passed through one stratum of air ex- 

 periences a less absorption in each of the succeeding strata, 

 and may therefore be propagated to a greater distance be- 

 fore 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 tempe- 

 rature is lower than that of boiling water ; that, as the tem- 

 perature 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 con- 

 firmed by the recent experiments of M. Melloni on caloric 



