The spectra of metallic arcs in an exhausted globe

Working on the supposition that the bands which occur in the arc spectrum of magnesium are due to possible combinations of the metal with gases in the outer part of the arc, we have recently attempted to obtain the line spectrum free from bands by photographing the spectrum when the arc is passed in an exhausted globe. It is found, however, that while the band beginning at A, 5007·5 is sometimes greatly reduced in intensity under these conditions, the bands attributed to “magnesium hydride” by Messrs. Liveing and Dewar are very strongly developed. Besides this, the characteristic spark, or “enhanced” line at λ 4481·3, which is almost invisible in the ordinary arc in air, is one of the strongest lines in the spectrum.