$40 FRAGMENTS OF SCIENCE. 



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 volatilises 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 occupy- 

 ing its place. On withdrawing 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 colour of the 

 spectrum 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 indefi- 

 nite. 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 

 vapour 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 con- 

 clusion 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, &c., are 

 in a state of volatilisation, each metallic vapour would 

 cut out a system of bands, corresponding exactly in 

 position with the bright bands of the same metallic 

 vapour. The light of our electric lamp shining through 

 such a composite flame would give us a spectrum cut 

 up by dark lines, exactly as the solar spectrum is cut 

 up by the lines of Fraunhofer. 



Thus by the combination of the strictest reasoning 



