PoLLOK — The Vacuum Tube Sj)ectra of some Metals. 265 



Principal Lines of Strontium. 



Barium. 



It is extremely difficult to get a spectrum with barium chloride, and 

 the strongest heat of a Meker burner must be used ; the luminescence is 

 a brilliant green in the bulb, but does not extend up the capillary tube, 

 and in the photograph only a few of the lines of barium show. Repeated 

 trials were made, and no better results obtained ; possibly if an end-on tube 

 were used, a more complete spectrum would be obtained. The chlorine 

 and water-vapour bands show also a few faint bands, probably due to 

 barium chloride. 



Principal Lines of Barium. 



Magnesium. 



Magnesium chloride gives a brilliant luminescence in the tube, but 



the oxide rapidly deposits in tlie capillary portion and renders it semi-opaque 



if much of the chloride is used. The metal is not sufficiently volatile to give 



a spectrum. The line X 2852-2 is remarkably persistent, and shows quite 



