Page:The story of the comets.djvu/230

176 Although mention is made in the previous paragraph of a spectrum there called the "Hydro-carbon" spectrum, its real origin is still unsettled. There are four carbon spectra: (1) a simple line spectrum undoubtedly due to carbon itself; (2) a band spectrum given by the base of a candle flame and often called the "Swan" spectrum after the name of its discoverer; (3) a spectrum usually known as the carbonic oxide spectrum, which also is banded; and (4) a banded spectrum due to cyanogen.

Much discussion has raged around the second, the so-called hydro-carbon spectrum, and its real origin is still so uncertain that Frost, in a recent discussion of Morehouse's

Fig. 98.

Comet, refuses to go further than to call it a "carbon" spectrum. Smithells has apparently shown it to belong to carbonic oxide, but the question is too involved for discussion here.

The question however arises, with which of the carbon spectra do the cometary spectra coincide? With regard to the brighter comets the testimony is clear; the spectrum yielded has been that given by the blue base of a candle-flame, or by a Bunsen burner—the spectrum which Lockyer terms that of hot carbon, but which Hasselberg and others consider as characteristic of a hydro-carbon, probably of acetylene. The