Chap. xxxv.] EMISSION OF HEAT. 463 



not. A piece of glass and a piece of rock-salt, heated 

 to the same temperature, affect a thermopile in their 

 neighbourhood very differently. The needle of the 

 thermopile is deflected to a large extent when the 

 face of the pile is opposite the glass, and to a much 

 smaller extent when opposite the rock-salt, because 

 the latter, which feebly absorbs, also feebly radiates 

 heat. A metallic surface absorbs feebly, a surface 

 coated with varnish or lamp-black absorbs readily. 

 Accordingly a metallic surface will radiate less than 

 a black-coated surface. Thus, a black kettle will 

 radiate more freely than a bright metallic kettle, and 

 will consequently cool faster. Tyndall has corrobo- 

 rated experiments by Leslie and Melloni, which prove 

 that colour has not the effect on absorption and radia- 

 tion that is generally supposed. The blackened sur- 

 face of a warm cube radiated as much heat to a 

 thermopile as another surface of the same cube coated 

 with whitening, and the bulb of a thermometer coated 

 with alum absorbed more radiant heat than the 

 bulb of another thermometer coated with iodine 

 powder, which was almost black. The white layer of 

 alum was more absorbent than the black layer of 

 iodine. The influence of colour, therefore, is less 

 than is supposed. A white surface, certainly, re- 

 flects the light, while a black surface absorbs it ; 

 but it is not to be supposed that the white similarly 

 reflects the heat rays, since, as has been seen, the 

 most intensely heating rays are invisible rays, and 

 not necessarily thrown off with the visible light rays. 



