RADIATION OF HEAT. 29 



and certain other transparent media, with the velocity of light. 

 It is not necessary that a body be heated to a visible redness to 

 enable it to discharge heat in this manner, Rays of heat, unac- 

 companied by light, continue to issue from a hot body through 

 the whole process of its cooling, till it sinks to the actual tem- 

 perature of the air or surrounding medium. The circumstance 

 that bodies suspended in a perfect vacuum cool rapidly and com- 

 pletely, without the intervention of conduction, places the fact 

 of the dissipation of heat by radiation, at low temperatures, 

 beyond a doubt. 



The most valuable observations which we possess on this 

 subject, were published by Sir John Leslie in his Essay on Heat, 

 in 1804. Leslie proved that the rate of cooling of a hot body 

 is more influenced by the state of its surface than by the nature 

 of its substance. He filled a bright tin globe with hot water, 

 and observed its rate of cooling in a room of which the air was 

 undisturbed. A thermometer placed in the water cooled half 

 way to the temperature of the apartment in 15G minutes. The 

 experiment was repeated, after covering the globe with a thin 

 coating of lamp-black. The whole now cooled to the same 

 extent as In the first experiment in 8 1 minutes ; the rapidity of 

 cooling being nearly doubled, merely by this change of sur- 

 face. 



An experiment of Count Rumford is even more singular. 

 Water, of the same temperature, was allowed to cool in two 

 similar brass cylinders, one of which was covered by a tight 

 investiture of linen, and the other left naked. The covered 

 vessel cooled 10 in 36j minutes, while the naked vessel re- 

 quired 55 minutes ; or the covering of linen, like the coating of 

 lamp-black greatly expedited the cooling, instead of retarding 

 the escape of heat, as might have been expected. The cooling 

 was accelerated in the same manner, when the cylinder was 

 coated with black or white paint, or smoked by a candle. 



In determining the radiating power of different surfaces, 

 Leslie generally made use of square tin canisters, of which the 

 surfaces were variously coated, and which he filled with hot 

 \vater. Instead of watching the rate of cooling, as in the expe- 

 riments already mentioned, he presented the side of a canister, 

 having its surface in any particular condition, to a concave 

 metallic mirror, which concentrated the heat falling upon it 

 into a focus, where the bulb of an air- thermometer was placed 



