RADIATION OF HEAT. 



417 



which the rates of the cooling of bodies under given circumstances might be 

 calculated with precision. Numerous experiments have been made on the 

 rates at which bodies cool in media of lower temperatures, and become hot in 

 media of higher temperatures ; and the results of observation have been found 

 to have 1 a very exact conformity with those which are calculated on the New- 

 tonian law, provided the difference of the temperature does not exceed a cer- 

 tain limit. 



As radiation takes place altogether from the points of a body which are on or 

 very near its surface, it may naturally be expected that the radiating power of 

 bodies will mainly depend on the nature of their surfaces. This idea suggested 

 to Sir John Leslie a series of experiments which led to some of the most re- 

 markable discoveries ever made respecting the radiation of heat. In these ex- 

 periments, cubical vessels, or canisters, of tin were employed, the side of which 

 varied from three inches to ten. These vessels were filled with hot water arid 

 placed before a tin reflector, M, fig. 7, like those already described, in the focus 

 y"of which was placed the focal bnll of a differential thermometer. The face 

 of the canister c containing water being presented to the reflector, rays of heat 

 proceeded directly from it, and striking on the reflector M were collected into 

 the focus /"on the ball of the thermometer. The depression of the liquid in the 

 thermometer furnished a measure of the intensity of the heat radiated. 



The first consequence of these experiments was a verification of the law al- 

 eady mentioned, that, other things being the same, the intensity of the radia- 

 tion was always proportional to the difference between the temperature of the 

 water and the temperature of the air. Thus suppose, the temperature of the 

 air being 50, that of the water 100, that the thermometer fall 20 ; then if 



y the temperature of the air were the same, and the temperature of the water at 

 150, the thermometer would fall 40 ; and again, if the temperature of the 

 water were 200, the thermometer would fall 60, and so on. 



If, while the temperature of the water remains the same, the canister is 

 placed successively at different distances from the reflector, it is found that the 

 thermometer is differently affected ; and that, as the distance of the radiating 

 svrface from the reflector is increased, the intensity of its effect is in the same 

 proportion diminished. It was likewise ascertained, that if the magnitude of 



( the radiating surface were increased, the distance remaining the same, the in- 

 tensity of the radiation would be in the direct proportion of the magnitude of 

 the radiating surface. From this it necessarily follows, that if the magnitude 

 of the radiating surface be increased in the same proportion as the distance is 

 increased, the intensity of the radiation will remain the same ; for as much is 

 gained by the increased magnitude of the radiating surface, as is lost by the 

 increased distance ; and accordingly it was found that the thermometer was 

 equally affected by a surface of double magnitude at a double distance, and of 

 triple magnitude at a triple distance. 



We have hitherto supposed that the face of the canister is placed parallel to 



