Page:The story of the comets.djvu/246

190 observer with eyes sensitive to these more refrangible rays would probably have seen this comet as a brilliant naked-eye object.

The important part played by cyanogen in the spectra of comets, considered in conjunction with solar and laboratory observations of the cyanogen spectrum, led Newall to propound the following interesting questions concerning the nature and origin of cometary radiations. " Is it not possible that the hydrocarbons, nitrocarbons, etc., which seem to be evidenced by the spectra of all comets, are always present in circumsolar space, and rendered incandescent by some processes connected either with the motion of the solid parts (including dust) of the head of the comet through the vapours, or with the emission of some influence from the comet head? Are we to say that all comets, wherever they may come from in the universe, and whatever their main material may be, always bring with them the cyanogen and hydrocarbons which give them luminosity? Or is it not more rational to say that the spectra of all comets are approximately similar, because they always find the same vapours spread in their path as they approach the Sun, and can only elicit the spectra of these vapours?"

The author of this interesting theory has evidence of the existence of cyanogen between the Sun and the Earth's surface, and has worked out at some length the conditions under which the luminosity could be generated. But for the present, despite the delightfully simple way in which such a theory would explain the similarity of cometary spectra in general, we must be content to look upon the comet spectrum as radiation, probably produced by electrical action of some kind, from the particles of the comet itself. That the volatile gases of the carbon compounds should be the first to be excluded is not a matter of wonder, whilst the observation that when the comets attain to lesser distances from the Sun, and therefore become more strongly heated, both by the solar radiation and by the increased number of collisions among their own particles, sodium and iron are vaporised and