1873.] Mr. J. N. Lockyer on Spectrum- Analysis. 287 



Some experiments made by Mr. E. J. Friswell 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 Eoscoe's prior 

 conclusions on the points investigated are stated at length ; and it is 

 shown that the observations recorded, taken in conjunction with the 

 determination of the long and short lines of metallic vapours, are in 

 favour of the views advanced by Mitscherlich, Clifton, and Eoscoe. 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 ; and the metallic lines are 

 represented according to the volatility of the compound, 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 hues, 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 con- 

 tinuous 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 the question is 

 asked whether the molecules of a metalloid do not in structure he be- 

 tween those of elements on the *one hand and of compounds on the other. 



These considerations are applied to solar and stellar spectra ; the 

 general appearance of the solar spectrum shows that in all probability 

 there are no compounds in the sun. 



Secchi's maps of a large number of stellar spectra are referred to as 

 now indicating beyond all doubt the existence of compound vapours in 



