Light from a mercury arc in quartz, operating at atmospheric pressure, and excited by various electrical means was photographed through a quartz spectrograph.
Line Spectra: With very high currents through the arc some thirteen lines were found to be self-reversed, two of which are spark lines. The reversal of one of these (λ1942) supports the view of Carrol, Turner and Compton that it is a member of the first doublet of the principal series of Hg+. Lines of the diffuse series of Hg appear to be the most readily reversed.
Continuous Spectra: With relatively low currents through the arc the continuous spectrum appears in four discrete bands, three of which have intensity maxima lying close to the three spectral series limits: λ4580; λ3320; λ2240. A modification of Bohr’s theory, which attributes bands of continuous radiation to the recombination of electrons with atomic or molecular ions, is found to agree very well with the observed facts. From theoretical considerations it is shown that a velocity distribution of the free electrons in the arc, corresponding to a temperature 2500°K, gives an intensity distribution of continuous spectrum in agreement with that observed. Certain observations by Rayleigh are found to be compatible with this theory.
W. H. CREW and L. H. DAWSON, "SPECTRA OF MERCURY AT ATMOSPHERIC PRESSURE," J. Opt. Soc. Am. 17, 261-270 (1928)