TRANSMISSION OF HEAT. 33 



antimony bars contained in a brass cylinder, t, and having the 

 wires from its poles connected with an extremely delicate 

 magnetic galvanometer, n. The extremities of the bars at b 

 being exposed to any source of radiant heat, such as the copper 

 cylinder d y heated by the lamp I, while the temperature of the 

 other extremities of the bars at c is not changed, a current of 

 electricity passes through the wires from the poles of the pile, 

 and causes the magnetic needle of the galvanometer to deflect. 

 The quantity of electricity circulating increases in proportion to 

 the difference of the temperatures of the two ends, b and c, that 

 is in proportion to the quantity of heat falling upon b ; and the 

 effect of this current of electricity upon the needle, or the 

 deviation produced, is proportional to the quantity of electricity 

 circulating, and consequently to the heat itself, at least Melloni 

 finds this correspondence to be exact through the whole arc, 

 from zero to 20, when the needle is truly astatic. 



Melloni proved that heat, which has passed through one plate 

 of glass, becomes less subject to absorption in passing through 

 a second. Thus, of 1000 rays of heat from an oil flame, 451 

 rays being intercepted in passing through four plates of glass of 

 equal thickness 



381 rays were intercepted by the first plate. 

 43 by the second. 



18 by the third. 



. ( > by the fourth. 



451 



The rays appear to lose considerably when they enter the 

 first layers of a transparent medium ; but that portion of heat, 

 which has forced its passage through the first layers, may pe- 

 netrate to a great depth. Transparent liquids are found to be 

 less penetrable to radiant heat than solids. 



The capacity which bodies possess of transmitting heat does 

 not depend upon their transparency ; or bodies are not at all 

 transparent to heat in the same proportion that they are trans- 

 parent to light. Thus, plates of the following transparent 

 minerals, having a common thickness of 0.1031 of an inch, 

 allowed very different proportions of the heat from the flame of 

 .an Argand oil-lamp to pass through them. 



