The arc spectrum of carbon

It is well known that in the region of observation extending from the red to λ 2000 the spectrum of an ordinary carbon arc shows only a single line, λ 2478. Merton and Johnson, however, have observed a considerable number of lines which they have attributed to the neutral atom of carbon in the spectrum of a vacuum tube containing a trace of an oxide of carbon in helium at 20 to 30 mm. pressure. Another method of developing the lines of C I, in this case almost perfectly free from lines of C II, has recently been described by J. W. Ryde. In these experiments, a carbon arc was fed with currents up to 250 amperes, the potential drop across the arc being 60 to 80 volts. Many of the lines assigned to C I by Merton and Johnson then appeared with great intensity, together with lines of N I, O I, and A I from the atmospheric gases, the lines in question being localised in a region near the negative pole of the arc. With additional observations made during the present investigation, the spectrum of neutral carbon is now sufficiently well known to justify an attempt to extend the analysis of the spectrum, which has already been partially elucidated by Bowen‡ from lines observed in the extreme ultra-violet.