858 FRAGMENTS OF SCIENCE 



as the waves, tlie motion of tlie waves will be absorbed 

 by the atoms; suppose we send our beam of white light 

 through a sodium flame, the atoms of that flame will be 

 chiefly affected by those undulations which are synchro- 

 nous with their own periods of vibration. There will be 

 on the part of those particular rays a transference of mo- 

 tion from the agitated ether to the atoms of the volatilized 

 metal, which, as already defined, is absorption. 



The experiment justifying this conclusion is now for 

 the first time to be made before a public audience. I pass 

 a beam through our two prisms, and the spectrum spreads 

 its colors upon the screen. Between the lamp and the 

 prism I interpose a snap-dragon light. Alcohol and water 

 are here mixed with common salt, and the metal dish 

 that holds them is heated by a spirit-lamp. The vapor 

 from the mixture ignites and we have a monochromatic 

 flame. Through this flame the beam from the lamp is 

 now passing; and observe the result upon the spectrum. 

 You see a shady band cut out of the yellow — not very 

 dark, but sufficiently so to be seen by everybody present. 



But let me exalt this effect. Placing in front of the 

 electric lamp the intense flame of a large Bunsen's burner, 

 a platinum capsule containing a bit of sodium less than a 

 pea in magnitude is plunged into the flame. The sodium 

 soon volatilizes and burns with brilliant incandescence. 

 The beam crosses the flame, and at the same time the. yel- 

 low band of the spectrum is clearly and sharply cut out, 

 a band of intense darkness occupying its place. On with- 

 drawing the sodium, the brilliant yellow of the spectrum 

 takes its proper place, while the reintroduction of the 

 flame causes the band to reappear. 



Let me be more precise. The yellow color of the 



