3. The phenomena of striations athwart the tail.
It is evident that all these phenomena are such as we might fairly expect if a comet’s tail is caused by the sun’s repulsive action on molecules, raised by his heating action on the head. The matter thus swept away would resemble smoke, driven upwards from the funnel of a moving steamer, and then swept in any given direction by a steady wind; we should see a curved train of such matter just as we see a curved streak of smoke. If the matter raised from the head is not all of one kind (and it is antecedently unlikely that it should be), there would be more than one trail of matter, if the sun’s repulsive action were different on these different kinds of matter. Lastly, the striations seen athwart the tail, as in the well known case of Donati’s great comet, would be explained, either as due to the observed pulsational manner in which the envelopes are raised (if matter were raised uniformly from the head there could be no formation of successive envelopes), or else as due to the carrying off into the main tail, where alone such striations are seen, of matter which, had it freed itself at the beginning, would have been swept off into the smaller tails, but being as it were entangled in the great outflow of matter forming the large tail, escapes later, and when it does, gets swept off at its own more rapid rate, and there forms a streak lying at an angle with the direction of the principal tail.
Bredichin has shown that where there are three tails to a comet, their forms correspond with the theory that the envelopes raised from the head are principally formed of hydrogen, carbon and iron, but this, which, if established, would be the most important physical discovery yet made respecting comets, seems open at present to considerable doubt, though confirmations seem to be given to it, in some respects, by the results of spectroscopic analysis.
To spectroscopic analysis we must in all probability look for such information respecting comets, as may hereafter enable us to understand their nature. On this point let us consider what is said by one who, if not the greatest living astronomical spectroscopist, is facile princeps in this country—Dr. W. Huggins. First, however, we must consider the past of this method of research as applied to comets.
The first successful application of the spectroscope to comets was made by Donati in 1864—the light of the comet being then divided into three bright bands, whose position, however, was not exactly determined. In 1866 Dr. Huggins obtained two kinds of light from a telescopic comet, part of the comet’s light giving a continuous spectrum, probably reflected sunlight, the other a spectrum of three bands. In 1868 a comet was observed (Brorsen’s) with more success. Three bands were seen in the spectrum of the light from the comet’s head, and a comparison of these with measures of similar bright bands belonging to the spectra of various combinations of carbon, showed, or rather seemed to suggest, that “combinations of carbon might be present in the comet.”
“In conjunction with my friend, the late Dr. W. Allen Miller,” says Dr. Huggins, “I confronted directly with the spectroscope attached to the telescope, the comet’s light with that from inductive sparks passing in olefiant gas. The sensible identity of the two spectra left no doubt of the essential oneness of the cometary stuff with the gas composed of carbon and hydrogen that was employed for comparison.” “Since that time,” proceeds Dr. Huggins, “the light from some twenty comets has been examined by different observers. The general close agreement in all cases, notwithstanding some small divergences, of the bright bands in the cometary light with those seen in the spectra of hydrocarbons, justifies us fully in ascribing the original light of these comets to matter which contains carbon in combination with hydrogen.”
Last year photography was applied to this spectroscopic work. The spectrum of the brightest comet of that year was partly continuous, and on this continuous spectrum many of the well known Fraunhofer lines could be traced. This made it certain that part of the comet’s light was reflected sunlight; though Dr. Huggins considers also that a part of the continuous spectrum of every comet is due to inherent light. On this point some doubts may be permitted. It is one thing for special bands to show themselves, for some substances may become self-luminous under special conditions at very moderate temperatures; it is quite another thing that the solid parts of a comet’s substance should become incandescent. I venture to express my own belief that this can scarcely happen except in the case of comets which approach very near to the sun. Besides the continuous spectrum with dark lines, the photograph showed also a spectrum of bright lines.
“These lines,” says Dr. Huggins, “possessed extreme interest, for there was certainly contained within this hieroglyphic writing some new information. A discussion of the position of these new lines showed them to be undoubtedly the same lines which appear in certain compounds of carbon. Not long before, Professors Liveing and Dewar had found from their laboratory experiments that these lines are only present when nitrogen is also present, and that they indicate a nitrogen compound of carbon, namely—cyanogen. Two other bright groups were also seen in the photograph, confirming the presence of hydrogen, carbon, and nitrogen.”
It is worthy of notice that, only a few days later, Dr. H. Draper succeeded in obtaining a photograph of the same comet’s spectrum. It appeared to him to confirm Dr. Huggins’ statements, except only that the dark Fraunhofer lines were not visible—the photograph having probably been taken under less favorable conditions.
So far, then, it seems clear that comets shine in part by reflecting sunlight, partly with light of their own; the part of the cometic substance which certainly shines with its own light is gaseous, and this gas in most comets “contains carbon, hydrogen, and nitrogen, possibly also oxygen, in the form of hydrocarbons, cyanogen, and possibly oxygen compounds of carbon.”