Mr. J. N. Lockyer on Spectrum- Analysis. 409 



results are compared with those obtained with the weak spark; 

 and it is shown that the difference is one of degree : e. g. baric 

 bromide gives 25 lines in the spark ; these are the longest lines. 

 In the flame it gives but one line ; but this is the longest of all 

 the barium hues, and indeed very far exceeds all the others in 

 length. When the flame-spectra are compared with those pro- 

 duced by the low-tension spark, the spectra of the metals in the 

 combination are in the former case invariably more simple than 

 in the latter, so that only the very longest line or lines are left. 



Some experiments made by Mr. E-. J. Iriswell to determine the 

 cause of the similarity of the spectra of the various salts of the 

 same metal observed in air are then given, the conclusion being 

 that the spectrum observed is really that of the oxide. 



Kirchhoff and Bunsen's, Mitscherlich's, and Clifton and Boscoe's 

 prior conclusions on the points investigated are stated at length ; 

 and it is shown that the observations recorded, taken in conjunc- 

 tion with the determination of the long and short lines of metallic 

 vapours, are in favour of the views advanced by Mitscherlich, Clif- 

 ton, and Roscoe. For while the spectra of the iodides, bromides, 

 &c. of any element in air are the same, as stated by Kirchhoff 

 and Bunsen, the fact that this is not the spectrum of the metal is 

 established by the other fact, that only the very longest lines of the 

 metal are present, increased dissociation bringing in the other metallic 

 lines in the order of their length. 



The spectra have been mapped with the salts in hydrogen : 

 here the spectra are different, as stated by Mitscherlich ; arid the 

 metallic lines are represented according to the volatility of the com- 

 pound, only the very longest lines being visible in the case of the least- 

 volatile one. 



The following are the conclusions arrived at : — 



1. A compound body has as definite a spectrum as a simple one ; 

 but while the spectrum of the latter consists of lines, the number 

 and thickness of some of which increase with molecular approach, 

 the spectrum of a compound consists in the main of channelled 

 spaces and bands, which increase in like manner. In short, the 

 molecules of a simple body and of a compound one are affected 

 in the same manner by their approach or recess, so far as their 

 spectra are concerned ; in other words, both spectra have their long 

 and short lines or bands. In each case the greatest simplicity of 

 the spectrum depends upon the greatest separation of molecules, 

 and the greatest complexity (a continuous spectrum) upon their 

 nearest approach* 



2. The heat required to act upon a compound, so as to render 

 its spectrum visible, dissociates the compound according to its 

 volatility : the number of true metallic lines which thus appear is 

 a measure of the dissociation ; and doubtless as the metal lines 

 increase in number the compound bands thin out. 



Mitscherlich's observations, that the metalloids show the same 

 structural spectra as the compound bodies, is then referred to, and 



