Apparent Radiation of CM, 239 



From this equilibrium of temperature preserved among bodies 

 in proximity to each other, however various their surfaces, it may 

 be inferred that the reflecting power of any surface increases, whilst 

 the radiating power diminishes, and vice versa, and that their sum 

 is constant; and also, that the sum of the radiated and reflected 

 caloric proceeding from any surface, is always a constant quantity 

 whilst the temperature of the surface is constant, and that of the 

 surfaces of surrounding bodies equal to its own, whatever change 

 its power of radiation may undergo, by painting, polishing or any 

 other means. Therefore if a metallic body be of the same temper- 

 ature as a thermometer in its vicinity, the latter cannot be affected 

 by merely changing the surface of the former, its temperature re- 

 maining unchanged. 



These principles being premised, suppose experiments to be 

 made simultaneously, and with similar apparatus, in two chambers, 

 A and B, and that metallic bodies, of the same temperature, are 

 employed in both chambers, but that the temperatures of the cham- 

 bers are different. In A suppose the temperature of the air and con- 

 sequently of the thermometer, be equal to that of the metallic body. 

 In B let the air and thermometer be at a higher temperature. Then 

 the body in B is cold, considered with reference to the thermome- 



Experiment 1st. Cover both bodies with a black pigment which 

 increases the radiating and consequently diminishes the reflecting 

 power. Let r represent the quantity of caloric reflected from the 

 body situated in A during the first instant, and p the quantity ra- 

 diated from the same. Letr' be the quantity reflected from that 

 in B, and j' the quantity radiated by it. As the sum of the radia- 

 ted and reflected caloric includes all that can be sent from either 

 body to its thermometer, it follows that r+£ is the amount given 

 off by the painted body in A, and r'-j-f' by that in B. 



Experiment 2d. Remove the paint from both, and also restore 

 to the bodies and the thermometer the temperatures which they re- 

 spectively had in the first experiment. By this operation the quan- 

 tities radiated from both bodies are diminished in some equal ratio ; 

 for their temperatures are unchanged, and their radiating powers 



and unchanged by this operation, the quantity radiated will be as 

 the radiating power. By the same operation, the number of re- 

 fleeted rays from each is increased, hv which means the bodies re- 



