154 Royal Institution : — 



scatters it also in all other directions round the room. Thus the 

 rays which, were the glass not interposed, would be shot directly 

 against your person, are for the most part diverted from their original 

 direction, and you are preserved from their impact. 



Now for our experiment. I pass the beam from the electric lamp 

 through the two prisms, and the spectrum spreads its colours upon 

 the screen. Between the lamp and the prism I interpose this snap- 

 dragon light. Alcohol and water are here mixed up with a quantity 

 of common salt, and the metal dish that contains them is heated by 

 a spirit-lamp. The vapour from the mixture ignites, and we have 

 this monochromatic flame. Through this flame the beam from the 

 lamp is now passing; and observe the result upon the spectrum. 

 You see a dark band cut out of the yellow, — not very dark, but suf- 

 ficiently so to be seen by everybody present. Observe how the 

 band quivers and varies in shade as the amount of yellow light cut 

 off by the unsteady flame varies in amount. The flame of this mono- 

 chromatic lamp is at the present moment casting its proper yellowlight 

 upon that shaded line ; and more than this, it casts in part the light 

 which it absorbs from the electric lamp upon it ; but it scatters the 

 greater portion of this light in other directions, and thus withdraws 

 it from its place upon the screen ; as the glass, in the case above sup- 

 posed, diverted the heat of the fire from your person. Hence the 

 band appears dark; not absolutely, but dark in comparison with the 

 adjacent brilliant portions of the spectrum. 



But let me exalt this effect. I place in front of the electric lamp 

 the intense flame of a large Bunsen's burner. I have here a platinum 

 capsule into which I put a bit of sodium less than a pea in magni- 

 tude. The sodium placed in the flame soon volatilizes and burns 

 with brilliant incandescence. Observe the spectrum. The yellow 

 band is clearly and sharply cut out, and a band of intense obscurity 

 occupies its place. I withdraw the sodium, the brilliant yellow of 

 the spectrum takes its proper place : I reintroduce the sodium, and 

 the black band appears. 



Let me be more precise : — The yellow colour of the spectrum ex- 

 tends over a sensible space, blending on one side into orange and on 

 the other into green. The term " yellow band" is therefore some- 

 what indefinite. I want to show you that it is the precise yellow 

 band emitted by the volatilized sodium which the same substance 

 absorbs. By dipping the coal-point used for the positive electrode 

 into a solution of common salt, and replacing it in the lamp, I ob'.ain 

 that bright yellow band which you now see drawn across the spec- 

 trum. Observe the fate of that band when I interpose my sodium 

 light. It is first obliterated, and instantly that black streak occupies 

 its place. See how it alternately Hashes and vanishes as I withdraw 

 and introduce the sodium flame ! 



And supposing that instead of the flame of sodium alone I intro- 

 duce into the path of the beam a flame in which lithium, strontium, 

 magnesium, calcium, &c. are in a state of volatilization, each metallic 

 vapour would cut out its own system of bands, each corresponding 

 exactly in position with the bright band which that metal itself would 



