On experiments relating to the spectrum of nitrogen

Abstract

It is somewhat remarkable that the spectrum of the neutral nitrogen atom, nitrogen arc or NI spectrum, should be the least known of the spectra associated with nitrogen. The positive and negative band spectra, associated with nitrogen molecules, have been the subject of many investigations, and the line spectra which are developed when condensed discharges are passed rough nitrogen at atmospheric pressure or through nitrogen contained in ancuum tubes, the spectrum of singly ionised nitrogen, the NII spectrum, has recently been arranged in series by Fowler ( ‘Roy. Soc. Proc.,' vol. 107, A, p. 31,25), who has in an earlier investigation (‘Monthly Notices R. A. S.,’ vol. 80, 692, 1920) assigned a number of lines which have not yet been arranged in series to the doubly ionised nitrogen atom. It would appear that under ordinary conditions of excitation the lines of the arc or nitrogen I spectrum are not conspicuous, and we are indebted to Hardtke (‘Ann. der Phys.,’ vol. 3, p. 363, 1918) for the information at present available with respect to this Spectrum. Hardtke found that with discharge tubes of special construction containing nitrogen a number of lines were predominant in the spectrum of the positive rays observed in certain regions of the discharge tubes, and that the same lines were relatively enhanced in vacuum tubes of the conventional type when they were excited by condensed discharges of feeble moderate intensities. Hardtke gave approximate measurements of a Number of these lines, which he assigned to the arc spectrum. In a series of previous investigations (Merton, ‘ Roy. Soc. Proc.,’ A, vol. 96, 382, 1920; Merton and Barratt, ‘ Phil. Trans.,’ A, vol. 222, p. 369, 1922; Merton and Johnson, ‘ Roy. Soc. Proc.,’ A, vol. 103. p. 383, 1923) it has been Shown that profound modifications are sometimes observed in the spectrum of a substance when a very small quantity of that substance is present in a discharge tube containing helium at a comparatively high pressure, and the tube is excited by condensed or uncondensed discharges. Thus with uncondensed discharges there is a striking change in the distribution of intensity the lines of the secondary spectrum of hydrogen; a trace of carbon is recognised by the appearance of the ‘‘comet tail” spectrum, first observed by Fowler (‘Monthly Notices R. A. S.,’ vol. 70, p. 484, 1910) at very low pressures and when both carbon and hydrogen are present a new triplet series of band: are developed.