1893.] 



On the Spectrum of Thallium, Sfc. 



369 



][. " On the Spectrum of Thallium, and its Relation to the 

 Homologous Spectra of Indium and Gallium." By HENRY 

 Wilde, F.R.S. Received March 14, 1893. 



The spectral reactions of thallium have been made the subject of 

 observation by so many distinguished physicists as to leave little to 

 be gleaned by further research of the spectrum of this interesting 

 element. A strong line in the red seems, however, to have been 

 overlooked, with one exception, in all the tables of the spectrum of 

 thallium which have hitherto been published. Huggins would appear 

 to have observed this line in the spark spectrum, and assigned a place 



o 



for it in his table* corresponding to a wave-length of 6547 of Ang- 

 strom's scale.f 



For reasons which will presently appear, the existence of this line 

 has not been confirmed by Thalen in the spark spectrum, nor bv 

 Liveing and Dewar in the arc spectrum,^ nor by other subsequent 

 observers. 



The importance of this line in relation to the homologous spectra 

 of other elements of the same series induced me to undertake ex- 

 periments which have proved, beyond doubt, the existence of this 

 characteristic spectral reaction of thallium. 



When metallic thallium or its chloride is volatilised in the elec- 

 tric arc between carbon points, a red line appears in the spectrum 

 apparently coincident with the hydrogen line C 6562. As the C 

 line invariably appears in the spark spectrum of all metallic sub- 

 stances in moist air, the red thallium line was considered by me, 

 as it would doubtless have been by others, as due to the electro- 

 lysation of aqueous vapour suspended in the atmosphere. It was, 

 however, found that the line did not present itself in the arc 

 spectrum of the alkaline and other metals experimented with under 

 the same conditions. It therefore appeared to me probable that 

 the red line belonged to the spectrum of thallium. There was, 

 moreover, the further fact that, up to the present time, the arc 

 spectrum of thallium below the ultra-violet was limited to the well- 

 known Crookes' green line, and in this respect was anomalous in the 

 simplicity of its spectrum at the high temperature of the electric arc. 



That the red line was not due to electrolytic hydrogen of aqueous 

 vapour was shown by the following experiments : — Two wide- 

 necked phials were partially filled with strong sulphuric acid, 

 leaving an air space at their upper ends of about two cubic inches. 



* ' Phil. Trans.,' vol. 154, p. 152, 1864. 



f Watts' ' Index of Spectra,' 1889. 



X ' Brit. Association Report,' 1885. 

 VOL. LTII. 2 D 



