Prof. R. Bunsen's Spectral- Analytical Researches. 529 



of lines, and when the induction-current used for the produc- 

 tion of Leyden-jar sparks is of such a strength as to cause the 

 formation of sparks between thick pointed platinum wires from 

 I to 2 centimetres apart. 



In our first memoir on spectral analysis we stated that the 

 spectra of the elements are by no means always identical with 

 the spectra of their compounds. Such an identity, it is true, is 

 often noticed, especially in the case of those elements which, 

 when in the state of glowing vapour, give out homogeneous 

 light ; yet it appears to me that the question as to the apparent 

 or real nature of this identity has not as yet been decided, in 

 any instance, with strict scientific accuracy. 



Although sodium vapour, as I have before pointed out, exhi- 

 bits dark lines at the temperature of boiling mercury (much 

 below a red heat), and although these lines are observed, reversed 

 as bright lines, in the glowing vapour of all volatile sodium com- 

 pounds, nevertheless we cannot as yet finally decide whether 

 these similar spectra are derived from the element as well as 

 from its compounds, or whether they are derived from the 

 element alone, set free by the decomposition of the compound 

 bodies. How this question may be solved is of little importance 

 in the practical application of spectral analysis. It concerns us 

 merely to remember that the spectrum of an element is not 

 always independent of the compound in which it occurs, and 

 that it therefore appears necessary to choose certain fixed com- 

 pounds for the recognition of the various elements. 



The chlorides were employed in these researches, as in our 

 first investigation, on account of their volatility, and of the ease 

 with which they may be prepared. The order in which the spectra 

 appear when a mixture of chlorides is vaporized by the flame 

 or spark, depends upon the relative quantities and volatility of 

 the chlorides. If a substance, volatile only at high tempera- 

 tures and giving a well-marked line-spectrum, be mixed with 

 increasing proportions of another, volatile at low temperatures 

 but giving no spectrum, the spectrum of the former is gradually 

 rendered less marked until at last it can scarcely be recognized. 

 If both substances give line-spectra, the spectrum of the more 

 volatile is generally first observed ; and it is only when this has 

 become very faint by repeated moistening with hydrochloric acid 

 and strong heating, that the spectrum of the more difficultly vola- 

 tile substance come3 into view. This phenomenon finds its 

 explanation in the circumstance that the temperature of the 

 heated mixture is always equal to the volatilization temperature 

 of the least volatile of the substances contained in it, so that 

 those substances which are volatile at a higher temperature 

 cannot as vet be vaporized. 



Phil. Mag. S. 4. No. 334. Suppl. Vol. 50. 2 M 



