Studies on the mercury band-spectrum of long duration

Abstract

In a former paper I described a method of observing the band spectrum in luminous mercury vapour distilled away from the arc. The vapour immediately on leaving the arc shows line spectrum exclusively, but as it matures the band spectrum gradually becomes predominant. It was shown by special experiments that the band spectrum derives its energy from the source which maintains the line spectrum. It is also possible, as originally shown by Phillips working in my laboratory at the Imperial College, to excite the band spectrum by exposure of the vapour initially to the right of the resonance line 2537, and to carry it away from the place of excitation with the mercury vapour when the latter is caused to distil. Neither of these methods is very convenient, however, for obtaining a bright spectrum, and an alternative one has been developed which works much better. This is to use a discharge from a Wehnalt cathode to excite the vapour. Under suitable conditions a comparatively bright band spectrum may be obtained initially, with a minimum intensity of line spectrum mixed with it. The vapour distilling from such a discharge shows initially at least the same type of spectrum as the discharge itself. If line spectrum predominates in the discharge it will predominate in the vapour which passes away from it, though, as in the case of the arc above mentioned, it ultimately turns to band spectrum. But if the band spectrum predominate initially, it does so throughout.