PoLLOK — On the Vacuum Tube Spectra of some Metals. 255 



for the loan of instruments, and for a grant which has partly defrayed the 

 expenses of this investigation. 



TjEAD. 



Both the metal and its chloride give a brilliant luminescence in the vacuum 

 tube, and the spectrum is photographed with ease, giving all the principal 

 lines of the metallic spark spectrum of lead, and a number of bands show 

 faintly, especially when no condenser is used. Many of the discontinuous 

 lines of lead do not show, and the introduction of tlie Leyden jar fails to 

 bring them out. Certain lines of lead at the ultra-violet end of the spectrum 

 show with the metal, but not with the chloride ; and lead once introduced in 

 a vacuum tube remains with surprising tenacity ; no amount of boiling with 

 acid, or washing with water, removes the ultimate lines of lead from the 

 photographs of the spectra of otlier materials subsequently examined in the 

 tubes. A very minute quantity of lead must be capable of giving a good 

 spectrum. 



Principal Lines of Lead. 



Seen strongly with the vapour of the metal, but not always with that of the chloride. 



2 R 2 



