1879.] On a New Method of Spectrum Observation. 



25' 



give us flame spectra in a Bunsen flame and passing a strong spark 

 through the flame, first during the process of volatilisation, and then 

 after the temperature of the flame has produced all the simplification 

 it is capable of producing. 



The results have been very striking; the puzzles which a com- 

 parison of flame spectra and the Fraunhofer lines has set us find, I 

 think, a solution ; while the genesis of spectra is made much more 

 clear.* 



To take an instance, the flame spectrum of sodium gives us, as its 

 brightest, a yellow line, which is also of marked importance in the 

 solar spectrum. The flame spectra of lithium and potassium give us, 

 as their brightest, lines in the red which have not any representatives 

 among the Fraunhofer lines, although other lines seen with higher 

 temperatures are present. 



Whence arises this marked difference of behaviour ? From the 

 similarity of the flame spectrum to that of the sun in one case, and 

 from the dissimilarity in the other, we may imagine that in the former 

 case — that of sodium — we are dealing with a body easily broken up, 

 while lithium and potassium are more resistant ; in other words, in 

 the case of sodium, and dealing only with lines recognised generally 

 as sodium lines, the flame has done the work of dissociation as com- 

 pletely as the sun itself. Now it is easy to test this point, for if this 

 be so then (1) the chief lines and flutings of sodium should be seen in 

 the flame itself, and (2) the spark should pass through the vapour 

 after complete volatilisation has been effected without any visible 

 effect. 



Observation and experiment have largely confirmed these predic- 

 tions. Using two prisms of 60° and a high-power eyepiece to en- 

 feeble the continuous spectrum of the densest vapour produced at a 

 high temperature, the green lines, the flutings recorded by Roscoe and 

 Schuster, and another coarser system of flutings, so far as I know not 

 yet described, are beautifully seen. I say largely, and not completely, 

 because the double red line and the lines in the blue have not yet 

 been seen in the flame, either with one, two, or four prisms of 60°, 

 though the lines are seen during volatilisation if a spark be passed 

 through the flame. Subsequent inquiry may perhaps show that this is 

 due to the sharp boundary of the heated region, and to the fact that 

 they represent the vibrations of molecular groupings more complex than 

 those which give us the yellow and green lines. The visibility of the 

 green lines, which are short, in the flame, taken in connexion with 

 the fact that they have been seen alone in a vacuum tube, is enough for- 

 my present purpose. 



* I allude more especially to the production of triplets, their change into quartets,, 

 and in all probability into flutings, and to the vanishing of flutings into fines by in- 

 creasing the rate of dissociation. 



