Emission- Spectra of Solid Aromatic Compounds. 621 



individual substance, are, for example, the initial spectra of 

 mesitylenic acid, of metatoluic acid, of the anhydride of 

 benzoic acid, of toluene and of its halogen substituted deri- 

 vates — and of many other substances, especially of those 

 aromatic bodies whose molecules contain a single-ring 

 group. 



In the groups which contain two or even more benzene 

 rings, and especially in condensed substances, one finds also 

 other types ot initial-spectra, all extending from red into 

 the ultra-violet, which I will not speak of in this short 

 report. 



The third spectrum of aromatic compounds is shown in 

 very characteristic forms especially by dissolved compounds 

 of the condensed type ; for instance, by naphthalene and 

 most of its derivates. The chief-spectrum of naphthalene 

 shows the wave-lengths 



^ i 



539 (very bright) 589 (very bright) 

 555 615 (probably a doublet) 



560 630 



573 648 



X 539 and 589 mark sharp boundaries on the violet side, the 

 other wave-lengths belong to the middle of narrow strips. 



The spectrum of the same naphthalene, if dissolved in mono- 

 chlorobenzene (which itself gives only a faint and almost 

 continuous spectrum) shows the following wave-lengths (all 

 for the middle of the narrow strips) : — 



473\i • ,, 505\ rather 51 7 "\ rather 540\557\ rather 

 483 J Dllg l 510 J bright 523 J faint 545 J 565 J faint 



582 faint. 



Beyond this last strip the illuminated ground cannot be 

 separated distinctly into strips. 



One cannot, however, speak of a single solution-spectrum of 

 a body, as the solution-spectrum of the same substance varies 

 greatly with the solvent. 



The solution-spectrum of naphthalene, for example, shows 

 differences, if the naphthalene has been dissolved in metaxylene 

 or in orthoxylene or in paraxylene. Therefore, if one sub- 

 stance shows remarkable differences in isomeric solvents, one 

 cannot wonder that the solution-spectra of the same substance 

 show even much greater differences if more different solvents 

 are used; for instance, if we compare the solution-spectra of 

 the same substance when dissolved either in a xylene or in 

 aniline, pyridine, ethyl-alcohol, and ethyl-ether. 



