2:38 Apparent Radiation of Cold. 



in other words, must produce the greatest cold. A blackened sur- 

 face, we have seen, is that which, at a given temperature, radiates 

 the largest quantity of caloric, and a metallic surface, that which 

 radiates least. Were Prevost's explanation just, therefore, the 

 blackened surface is the one which, in the experiment on radiant 

 cold, ought to produce the least cooling effect, and the metallic 

 surface the greatest, because the former gives off more caloric by 

 radiation than the latter. But the fact is the reverse ; the cold be- 

 ing greatest when the blackened surface, and least when the me- 

 tallic surface, is opposed to the mirror." 



In examining the objection, we may observe that the two mir- 

 rors subtend equal angles at the bodies, in their respective foci, 

 and consequently intercept equal proportions of the caloric radiated 

 from the nearest surfaces of these bodies ; and from their form and 

 position, and the known laws of reflection, they condense upon the 

 body situated in either focus, all the rays which the mirror most 

 remote from it receives from the body in its own focus, with the ex- 

 ception of the rays absorbed by the reflectors and those intercepted 

 by the air. Therefore the ratio between those portions of caloric, 

 which the two bodies receive from each other in this experiment, 

 is the same as would exist in a case of direct interchange by radia- 

 tion, when no mirrors are employed : and if the cold body have its 

 whole surface alike painted or alike metallic, the same proportion 

 of all the rays which emanate from it, will be received by the ther- 

 mometer, as the thermometer will receive of all those emanating 

 from the cold body. Therefore, by supposing the experiments to 

 be made in this manner, with a thermometer in one focus, and a 

 metallic body in the other, which has in one experiment its whole 

 surface painted, and in another its whole surface unpainted, and by 

 assuming that all the rays emanating from this body, reach the 

 thermometer, and all those from the thermometer reach the metal- 

 lic body, I shall add to the simplicity, without impairing the strict- 

 ness of the demonstration, in proving that the above mentioned ex- 

 periments confirm instead of subverting the theory against which 

 they are alleged. 



In any chamber where no fire or other source of heat or cold ex- 

 ists, to disturb the equilibrium, every article of furniture, and in 

 short, all inanimate objects within it, (whatever be their difference 

 of radiating power resulting from color, texture or any other cause) 

 are of the same temperature. 



