SICT. XXV. TRANSMISSION OP HEAT. 209 



remarkable difference in the transmissive power of sub- 

 stances having the same- appearance, is attributed by M. 

 Melloni to their crystaline form, and hot to the chemical 

 composition of their molecules, as the following experi- 

 ments prove. A block of common salt cut into plates, 

 entirely excludes calorific radiation ; yet when dissolved 

 in water, it increases the transmissive power of that 

 liquid : moreover, the transmissive power of water is 

 increased in nearly the same degree, whether salt or 

 alum be dissolved in k ; yet these two substances 

 transmit very different quantities of heat in their solid 

 state. Notwithstanding the influence of ciy stall zation 

 on the transmissive power of bodies, no relation has 

 been traced between that power and the crystaline form. 

 The transmission of radiant heat is analogous to that 

 of light through colored media. When common white 

 light, consisting of blue, yellow, and red rays, passes 

 through a red liquid, almost all the blue and yellow rays, 

 and a few of the red, are intercepted by the first layer 

 of the fluid ; fewer are intercepted by the second, 

 less by the third, and so on : till at last the losses ' 

 very small and invariable, and those rays 

 transmitted which give the red color to t 1 

 a similar manner, when plates of the sair 

 any substance, such as glass, are exposet 

 lamp, a considerable portion of the rad 

 rested by the first plate, a less port^- 

 still less by the third, and so on, ' ' 

 heat decreasing till at last the loss^ 

 quantity. The traowmssion 

 solid mass follows t ' @g f fie 

 considerable on first 

 ish in proportion as 

 become constant at a certain 

 difference between the transm> 

 through a solid mass, or through the 

 cut into plates of equal thickness, arises . 

 quantity of heat that is reflected at the surface 

 plates. It is evident, therefore, that the heat 

 ually lost is not intercepted at the surface, but ab 

 in the interior of the substance, and that heat 

 has passed through one stratum of air experiences 

 14 s2 



