352 RADIANT HEAT. 



the focus of the lens, the galvanometer needle will at once 

 turn aside, showing a rise of temperature. If the face of 

 the pile be placed anywhere else than at the focus, there 

 will be no such deflection of the needle. 



557. Change of Radiant into Sensible Heat. 



Of all the rays falling upon any substance, only those 

 that are absorbed are of effect in heating the body upon 

 which they fall. The motion of the ether waves may be 

 changed into vibrations of molecules of ordinary matter, 

 and thus produce sensible heat, but the same energy can- 

 not exist in waves of ether and in ordinary molecular 

 vibrations at the same time. 



(.) Phosphorus or gun-cotton may be ignited by solar rays at 

 the focus of a lens made of clear ice. The heat rays pass through 

 the ice without melting it. It is only when the radiation is stopped 

 that the energy of the ray can warm anything. 



558. Determination of Absorbing, Reflecting and 

 Radiating Powers. For experiments in determining the 

 absorbing, reflecting and radiating powers of solids, the apparatus 

 generally used consists of a Leslie's cube, concave mirrors of 

 different materials, and a differential thermometer or a thermopile. 

 The Leslie's cube is a box about three inches on each edge, the 

 sides being made of, or covered with, different materials, to show 

 their differences in radiating power. The cube filled with hot water 

 is placed before the reflector, and a bulb of the thermometer is 

 placed at the focus. By turning different faces of the cube toward 

 the mirror, the relative radiating powers are determined. By using 

 different mirrors, the reflecting powers are determined. By coating 

 the bulb with different substances, their absorbing powers are 

 determined. The relative radiating powers of several common 

 substances are as given below : 



Lampblack 100 



Paper 98 



Crown glass 90 



Tarnished lead 45 



Mercury 20 



Gold, silver, copper 12 



559. Mutual Relations of Absorption, Re- 

 flection and Radiation. By means like those men- 



