PHYSICAL BASIS OF SOLAR CHEMISTRY. 259 



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 snapdragon 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 mono- 

 chromatic 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 largo 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 

 yellow 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 spec- 

 trum extends over a sensible space, blending on one side 

 with the orange and on the other with the green. The 

 term "yellow band" is therefore somewhat indefinite. 

 This vagueness may be entirely removed. By dipping the 

 carbon-point used for the positive electrode into a solution 

 of common salt, and replacing it in the lamp, the bright 

 yellow band produced by the sodium vapor stands out 

 from the spectrum. When the sodium flame is caused to 

 act upon the beam it is that particular yellow band that 

 is obliterated, an intensely black streak occupying its 

 place. 



An additional step of reasoning leads to the conclusion 

 that if, instead of the flame of sodium alone, we were to 

 introduce into the path of the beam a flame in which 

 lithium, strontium, magnesium, calcium, etc., are in a 

 state of volatilization, each metallic vapor would cut out a 

 system of bands, corresponding exactly in position with 

 the bright bands of the same metallic vapor. The 



