Q34 , Transmission of Radiant Heat 



same manner as luminous rays are acted upon by miscroscopes, tele- 

 scopes and optical instruments in general. 



In speaking of the independence which exists between the trans- 

 parency of bodies with respect to hght and heat, we cited examples 

 which shew the little influence which the thickness of the plate pos- 

 sesses on the quantity of heat transmitted ; but that applies only in 

 the case then under consideration, namely when operating with the 

 flame of a, lamp. Melloni has made similar experiments with other 

 sources, whence it results that the influence of thickness on the phe- 

 nomena of transmission is greater as the temperature of the source 

 is low. It becomes very great in low temperatures. This proposi- 

 tion is intimately connected with the law of Delaroche, for the dif- 

 ferences between the quantities of heat transmitted through the same 

 plate of glass exposed in succession to various sources, diminishes 

 in proportion as the plate becomes less thick, and is completely effa- 

 ced at a certain limit of thickness ; so that in presenting a plate in 

 a certain state of exiguity to two sources of very different tempera- 

 tures, it transmits the same quantity of heat from each. The author 

 proved this by an extremely thin plate of mica exposed to incandes- 

 cent platina and to a mass of iron heated to 360° C. 



Melloni has also studied the effect which the color of bodies has 

 on the transmission of radiant heat, as well as the condition of the 

 surface, and the resistance of the successive layers which compose it. 

 He has ascertained, 1st, that all the coloring materials which enter 

 into the composition of colored glass, green excepted, act upon rays 

 of heat, as dark substances do upon hght when introduced into a dia- 

 phanous medium ; 2d, that the more polished the diathermanous 

 surface, the more it facilitates the transmission of rays of heat; 3d, 

 that the loss experienced by rays in traversing one of tlie thin lay- 

 ers into which we may conceive the medium to be divided, is so 

 much the less, the more distant the layer is from the surface on 

 which the rays fall. 



In what we have thus far considered, terrestrial heat alone has 

 been the subject of experiment ; in applying his methods to solar 

 heat Melloni arrived at this result, that each ray of the solar spec- 

 trum acts like the terrestrial rays derived from different sources, so 

 that the most refrangible rays may be compared to the heat from a 

 very hot focus, and the least refrangible to that from one of low 

 temperature. 



