COMPOUNDS OF CARBON Cyanogen. The bands headed at 3885, 4215, have been attributed [180] to the CN or the C₂N₂ radical, or to the molecule of nitrogen. The assignment to a particular atom is essentially a question for the terrestrial physicist, and to discuss it here would be out of place.[181] The bands are universally known as the “cyanogen bands,” and this designation will therefore be adopted. The 3885 and 4215 bands are conspicuous in and stars of low density,[182] and furnish a valuable method for the measurement [Pg 62] of absolute magnitude—a method which has been used both at Mount Wilson[183] and at Harvard.[184] The band at 3885 is largely responsible for cutting off the ultra-violet light of the cooler stars.[185] Cyanogen absorption has been reported as early[186] as , and according to Lindblad[187] it reaches a maximum at . The cyanogen bands reach great intensity in the stars, and are indeed the most conspicuous feature of these spectra. Shane[188] places the maximum in Class , and is doubtless correct in so doing. The maximum given by Lindblad refers to the series , and the stars are notoriously not members of that sequence.[189] Cyanogen is also a typical constituent of the comet-head spectrum.[190][191][192] Carbon Oxides. The band spectrum attributed to the CO molecule,[193][194] is a strong feature of the spectra of and stars.[195][196] It is also the chief component of the spectrum of the comet tail, which has been reproduced in the laboratory, at very low pressures, by A. Fowler.[197] Swan Spectrum. The bands of the Swan spectrum are clearly to be assigned to some compound of carbon,[198][199] but the source is not as yet certainly established. They are characteristic of the comet head.[200] Another band, presumably to be associated with the Swan spectrum, was identified in the heads of nine comets by Baldet.[201] [Pg 63] The ordinary Swan bands are also identified in the comet tail.[202] Hydrocarbon. The identity of the “” band with the 4314 hydrocarbon group was pointed out by Newall, Baxandall and Butler.[203] The strength of the band is increased, in the stellar spectrum, by the superposition of the lines of calcium, and by the lines of titanium, as well as other metallic lines, but the presence of the hydrocarbon band is certain, and is of the highest interest. The “” band is first seen in some spectra[204] of Class , and it attains a maximum at or . The number of carbon compounds which occur lends plausibility to the suggestion that much of the stellar carbon is in combination at temperatures below 5000°. NITROGEN (7)