SECT. XXVII. ABSORPTION OF HEAT. 259 



bodies, no relation has been traced between that power and the 

 crystalline form. 



The transmission of radiant heat is analogous to that of light 

 through coloured media. When common white light passes 

 through a red liquid, almost all the more refrangible rays, and a 

 few of the red, are intercepted by the first layer of the fluid ; fewer 

 are intercepted by the second, still less by the third, and so on : 

 till at last the losses become very small and invariable, 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 con- 

 siderable 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 

 quantity 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 consider- 

 able on first entering it, but they rapidly diminish in proportion 

 as the heat penetrates deeper, and become constant at a certain 

 depth. Indeed, the only difference between 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, therefore, that the heat gradually lost is not in- 

 tercepted at the surface, but absorbed in the interior of the sub- 

 stance, and that heat which has passed through one stratum of 

 air experiences a less 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 heat 

 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 artifi- 

 cially. It is owing to the same cause that glass screens, which 

 entirely exclude the heat of a common fire, are permeable by the 

 solar heat. 



The results obtained by M. de Laroche have been confirmed 



