The fluorescence of mercury vapour

The recent important and very interesting investigations by Lord Rayleigh of the varied phenomena accompanying the production of the band spectra of mercury by the electrical discharge and optical excitation have added much to our knowledge regarding the emission spectra of this element in the molecular condition, and have shown that the mechanism of emission is even more com­plicated than previously supposed. Lord Rayleigh’s investigations have concerned the after-glow of the electrically excited vapour for the most part, and it appeared to be worth while to renew the attack, upon the fluorescence excited by the light of the aluminium spark, having in mind his recent dis­coveries. In the present paper we shall show that practically all of the stronger arc lines of the mercury atom are emitted by mercury vapour when excited by the light of the aluminium spark, which can be absorbed only by the mercury molecule (band absorption). The intensity of these lines varies as thesquare of the intensity of the exciting light, as was shown recently by one of us in the case ofatomic mercury excited by a two-stage absorption process. The effects of superheating the vapour on the intensities of the various lines and bands has been investigated, as well as the development of other bands and lines due to the presence of impurities, and caused probably by collisions with excited molecules.