210 



Scientific Proceedings, Royal Duilin Society. 



Thallium. 

 Both the metal and its chloride give a very intense green luminescence 

 in the tube, and the characteristic lines of thallium show with brilliancy. 

 Thallium has some very strong but discontinuous lines in the spark-spectrum 

 of the metal in air, that is the atoms have vibrations at the electrodes that 

 do not persist as they are shot across the spark gap. In the vacuum tube 

 spectrum these lines do not show at all when no condenser is used, and come 

 out quite strongly wlien a condenser is put in the secondary circuit. 

 This is seen particularly well with the lines A 3091-9, X 2530-0, and \ 2298-2. 

 The rule regarding the non-appearance of lines showing in the arc, but not in 

 the spark, also holds. 



Principal Lines of Thallium. 



Mercury. 

 The vacuum tube spectrum of the vapour of mercury has already been 

 carefully investigated by Eder and Valenta,' and both lines and bands 

 measured, but the photograph and table of wave-lengths (Plate XY.) will be 

 found useful, as these give the lines that are certain to develop with a much 

 shorter exposure than they evidently employed. With excess of vapour, 



1 Penksch. Wien., Bd. Ixi. 1894. 



