CHROMATICS SPECTRA, 



421 



such bodies without having their peculiar wave-motion 

 transferred to the molecules of the body through which 

 they pass. When a ray of light or heat, in passing through 

 a substance, gives its energy to the molecules between 

 which it is passing in the ether, the ray is absorbed. It 

 no longer exists as radiant energy ; it has become absorbed 

 heat, and warms the body. It is no longer a motion of the 

 ether ; it has become a motion of ordinary matter. As in 

 the case of radiant heat, so with light ; the best absorbents 

 are the best radiators. A piece of transparent, colorless 

 glass will absorb very little light; heat it intensely, and 

 it will radiate very little light. On 

 the other hand, a piece of opaque 

 glass will absorb a great deal of 

 light ; when heated intensely, it will 

 radiate a great deal of light. 



FIG. 324. 



(a.) If an intensely heated pot of melted 



lead, tin or plumber's solder be carried 



into a dark place and the dross skimmed 



aside by a red-hot iron ladle, the liquid 



metal (which in sunlight 

 would reflect rather than 

 absorb the light) will ap- 

 pear less bright than the 

 surrounding dross. If a 

 piece of platinum- foil bear- 

 ing 'an ink-mark be heat- 

 ed to incandescence and 

 viewed in a dark room, the 

 ink-mark will radiate more 

 light than the metal. Ex- 

 posed to sunlight, the ink- 

 mark will absorb more 

 light than the metal. If a 

 chalk-mark be made on a 

 black poker, the poker 

 FIG. 325. heated red-hot and viewed 



