RADIANT HEAT. 353 



tioned in the last paragraph, it has been shown that 

 good absorbents are good radiators and poor re- 

 flectors, and vice versa ; that the radiating power of a 

 body depends largely upon the nature of its surface ; that 

 smoothing and polishing the surface increases reflecting 

 power, and diminishes absorbing and radiating power; 

 that roughening and tarnishing the surface increases the 

 absorbing and radiating powers, and diminishes the re- 

 flecting power. Tlie powers of absorption and radi- 

 ation go hand in hand. ( 654, 655.) 



(a.) Make a thick paint of a teaspoonful of lampblack and a 

 little kerosene oil. With this, paint the right-hand face of the 

 left-hand bulb (tin can of the differential thermometer described in 

 Appendix M). Provide another oyster can and paint one side with 

 the lampblack. Fill this third can with boiling water and place 

 it on the wooden strips, midway between the two tin bulbs, the 

 two blackened surfaces facing each other. The heat radiated and 

 absorbed by the two blackened surfaces will exceed the heat radi- 

 ated and absorbed by the two equal unpainted surfaces that face 

 each other. The movement of the colored alcohol in the tube will 

 show this to be true. 



56O. Sympathetic Vibrations. The relation 

 between radiation and absorption of heat is closely analo- 

 gous to the relation between the radiation and absorption 

 of sound. If a set of sound waves fall upon a string 

 capable of producing similar waves, the string is set in 

 motion and the sound waves weakened ( 443). When 

 ether waves of a given kind fall upon a body whose mole- 

 cules are able to vibrate at the same rate, and thus to 

 reproduce similar waves, the kinetic energy is transferred 

 from the ether to the molecules, the molecules are heated, 

 the radiant energy absorbed. This ability to absorb wave 

 motion of any particular kind, implies the ability to repro- 

 duce the same kind of waves. It therefore is easily seen 



