But the latest comet has brought with it fresh news. Its spectrum is not like that given by the comets we have been considering. The bright lines of sodium are seen in it, and also other bright lines and groups of lines, which have not yet been shown to be identical with any belonging to the hydrocarbon groups, but probably are so. Dr. Huggins’ photograph shows, he considers, “that the original light of the comet, which gives a continuous spectrum (he means that portion of the original light which does so), was too strong to allow of the Fraunhofer lines being recognized in the reflected solar light.” We demur to this as being shown, it may fairly be said to be suggested. The cyanogen groups are not seen.
Such is Dr. Huggins’ account; but it is manifest that this comet underwent important changes, of which—we are surprised to note—Dr. Huggins has taken no account. Thus, in April, Professors Tacchini and Vogel found simply a faint continuous spectrum. In May, Vogel found that the three bands associated with carbon were present, though faint, while there was no trace of the sodium band. On the contrary, on the nights of June 4, 5, and 7, Dr. B. Hasselberg, of the Observatory of Pulkowa, found that the nucleus of the comet gave a very strong and extended continuous spectrum, with an “excessively strong” bright line in the orange yellow, proved by micrometrical measurement to be identical with the D line—the well known double sodium line of the solar spectrum. The observation was confirmed by Dunér, Bredichin, and Vogel. On this Mr. Hind remarks, “It is necessary to conclude that, during the last fortnight of May, the spectrum of Wells’ comet had changed in a manner of which the history of science furnishes no precedent.” It must, however, be remembered that as yet no comets have been examined under sufficiently favorable conditions, to enable us to say whether the change thus observed was really exceptional, or only exceptional in being for the first time noted. Whenever such a comet as Donati’s comes favorably under spectroscopic scrutiny, we shall probably learn something about these changes which will throw more light than anything yet discovered on the physical economy of these mysterious bodies.
What, then, do we know certainly respecting comets? What may we surmise with more or less probability? And in what direction may we look with most hope for future information? We know certainly that, in whatever way they are formed, the sun excites intense disturbance in them as they approach him. Prof. Stokes remarks that these effects, so much greater at a first view than we might fairly expect in the case of many of the comets observed, which have approached the sun no nearer than our own earth does, or not so near, may be accounted for by the circumstance that comets travel in what may be regarded as, to all intents and purposes, a vacuum. From Dr. Crooke’s experiments on very high vacua, we may infer that there is very little loss of heat, except by radiation. Thus the heat received by the meteoric components of a comet would be much greater than might otherwise be expected. Dr. Huggins mentions, in the same connection, the remarkable persistence of the bright trains of meteors in the rare upper air, which sometimes remain visible for three-quarters of an hour before the light fades, as the heat is gradually radiated away. “Our reasoning on these points,” he remarks, in his dry way, “would undergo considerable modification if we accept the views as to the condition of interplanetary space and of the sun’s action which have been recently suggested by Dr. Siemens in his solar theory”—but of course we do not.
Bredichin’s researches, showing that three distinct curvatures in comets’ tails correspond to the winnowing out by solar repulsive action of (1) hydrogen, (2) carbon, and (3) iron, seem worthy of careful study and investigation. It accords well with spectroscopic evidence as to the condition of the matter raised in gaseous form from the nucleus; and if as yet we have had no direct spectroscopic evidence of the existence of iron in comets, we know that meteors are closely connected with comets, and that many meteors contain iron. Moreover, as unexpected spectroscopic evidence of the presence of the substance sodium, common in so many meteors, has been found in the case of one comet, we may fairly hope that under yet more favorable conditions, the presence of iron also may be recognized in the same way.
How far electricity may be looked to for an explanation of cometic phenomena, is a doubtful point among astronomers and physicists. For my own part, I must confess I share the strong objections which many physicists have expressed against the mere vague suggestion that perhaps this is an electrical phenomenon, perhaps that other feature is electrical too, perhaps all or most of the phenomena of comets depend on electricity. It is so easy to make such suggestions, so difficult to obtain evidence in their favor having the slightest scientific value. Still, I hold the electrical idea to be well worth careful study. Whatever credit may hereafter be given to any electrical theory of comets, will be solely and entirely due to those who may help to establish it upon a basis of sound evidence—none whatever to the mere suggestion, which has been made time and again since it was first advanced by Fontenelle. Dr. Huggins says that he finds there is a rapidly growing feeling among physicists that both the inherent light (which he prefers to call the self-light) of comets and the phenomena of their tails belong to the order of electrical phenomena. An American astronomer recently wrote to him, as to American views of the self-light of comets, “I can not speak with authority for anyone but myself; still I think the prevailing impression amongst us is that this light is due to an electric, or, if I may coin the word, (far better not) an electric-oid action of some kind.” On this Dr. Huggins himself remarks:
“The spectroscopic results fail to give conclusive evidence on this point; still, perhaps, upon the whole, especially if we consider the photographs of last year, the teachings of the spectroscope are in favor of the view that the self-light of comets is due to electric discharges. Those who are disposed to believe that the truth lies in this direction, differ from each other in the precise modes in which they would apply the known laws of electric action to the phenomena of comets. Broadly, the different applications of principles of electricity which have been suggested, group themselves about the common idea, that great electrical disturbances are set up by the sun’s action in connection with the vaporization of some of the matter of the nucleus, and that the tail is probably matter carried away, possibly in connection with electric discharges, under an electrical influence of repulsion exerted by the sun. This view necessitates the supposition that the sun is strongly electrified, either negatively or positively, and further, that in the processes taking place in the comet, either of vaporization or of some other kind, the matter thrown out by the nucleus has become strongly electrified in the same way as the sun—that is, negatively if the sun’s electricity is negative, or positively if the sun’s is positive. The enormous disturbances which the spectroscope shows to be always at work in the sun must be accompanied by electrical changes of equal magnitude, but we know nothing as to how far these are all, or the great majority of them, in one direction, so as to cause the sun to maintain permanently a high electrical state, whether positive or negative.”
Unless some such state of things exist, Sir John Herschel’s statement, “That this force” (the repulsive force forming the tail) “can not be of the nature of electric or magnetic forces,” must be accepted, for, as he points out, “the center of gravity of each particle would not be affected; the attraction on one of its sides would precisely equal the repulsion on the other.” Repulsion of the cometary matter would only take place if this matter, after it has been driven off from the nucleus and the sun, have both high electric potentials of the same kind. Further, it is suggested that luminous jets, streams, halos, and envelopes belong to the same order of phenomena as the aurora, the electrical brush, and the stratified discharges of exhausted tubes.
All this, it will be noticed, is at present merely hypothetical. It is, however, worthy of notice that outside of electricity there is nothing known to physicists which seems to afford even a promise of explanation, so far, at least, as the grander and more striking (also the most mysterious) of cometic phenomena are concerned. It may well be that with our advancing knowledge of meteors and meteor systems, the spectroscopic analysis of the next few comets of the larger and completer types—comets like Donati’s comet, the great comet of 1811, and the comet of 1861—may throw unexpected light on mysteries which still remain among the most profound and unpromising problems presented to modern science.—The Contemporary Review.
