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There is one observation made by Bunsen which appears to stand as an exception to the rule that only bodies in the gaseous state give, when luminous, discontinuous spectra. Bunsen discovered that solid erbia, when heated to incandescence, gives a spectrum containing bright bands. It is therefore conceivable, though all the evidence we possess from experience is opposed to the supposition, that carbon might exist in some form in which it would possess a similar power of giving a discontinuous spectrum without volatilization. There is the further objection to this hypothesis, that the telescopic phenomena observed in comets appear to show that vaporization does usually take place.
However this may be, a state of gas appears to accord with the very small power of reflexion which the matter of the coma of this comet possessed, as was shown by the great faintness of the continuous spectrum.
A remarkable circumstance connected with comets is the great transparency of the bright cometary matter. The most remarkable instance is that of Miss Mitchell’s comet in 1847, which passed centrally over a star of the fifth magnitude. “ The star’s light appeared in no way enfeebled, yet such a star would be completely obliterated by a moderate fog extending a few yards from the surface of the earth”*. We do not know what amount of transparency is possessed by the vapour of carbon, but the absence of a continuous spectrum seems to show that, as it existed in the comet, it was almost perfectly transparent. The light of a star would suffer, therefore, only that kind and degree of absorption which corresponds with its power of radiation, as shown by its spectrum of bright lines. As these occur in the brightest part of the spectrum, we should expect a noticeable diminution of the star’s light, if it were not for the luminous condition of the gas, in consequence of which it would give back to the beam light of precisely the same refrangibilities as it had taken, and so enable the part of the field occupied by the image of the star to appear of its original brightness, or nearly so. This state of things would not prevent an apparent diminution of the star’s light from the effect upon the eye of the brightness of the surrounding field. In the case of the tails of comets, the great transparency observed is more probably to be referred to the widely scattered condition of the minute particles of the cometary matter.
I may be permitted to repeat here a paragraph from my paper on the Spectrum of Comet I., 1866f.
“ Terrestrial phenomena would suggest that the parts of a comet which are bright by reflecting the sun’s light, are probably in the condition of fog or cloud.
“We know, from observation, that the comse and tails of comets are formed from the matter contained in the nucleus^.
* Outlines of Astronomy, p. 373. + Proceedings of the Royal Society, vol. xv. p. 5.
t The head of Halley’s comet in 1835 in a telescope of great power “ exhibited the appearance of jets as it were of flame, or rather of luminous smoke, like a gas fan-light. These varied from day to day, as if wavering backwards and forwards, as if they were thrown out of particular parts of the internal nucleus or kernel which shifted round, or to or fro by their recoil like a squib not held fast. The bright smoke of these jets, however, never seemed to be able to get far out towards the sun, but always to be driven back and forced into the tail as if by the action of a violent wind setting against them (always from the sun), so as to make it clear that the
“ The usual order of the phenomena which attend the formation of a tail appears to be that, as the comet approaches the sun, material is thrown off, at intervals, from the nucleus in the direction towards the sun. This material is not at once driven into the tail, but usually forms in front of the nucleus a dense luminous cloud, into which for a time the bright matter of the nucleus continues to stream. In this way a succession of envelopes may be formed, the material of which afterwards is dissipated in a direction opposite to the sun, and forms the tail. Between these envelopes dark spaces are usually seen.
“ If the matter of the nucleus is capable of forming by condensation a cloud-like mass, there must be an intermediate state in which the matter ceases to be self-luminous, but yet retains its gaseous state, and reflects but little light. Such a non-luminous and transparent condition of the cometary matter may possibly be represented by some at least of the dark spaces which, in some comets, separate the cloud-like envelopes from the nucleus and from each other.”
Now considerable differences of colour have been remarked in the different parts of some comets. The spectrum of this comet would show that its colour was bluish green. Sir W. Herschel described the head of the Comet of 1811 to be of a greenish or bluish-green colour, while the central point appeared to be of a pale ruddy tint. The representations of Halley’s comet at its appearance in 1835, by the elder Struve, are coloured bluish green, and the nucleus on October 9 is coloured reddish yellow. He describes the nucleus on that day, thus:—“ Der Kern zeigte sich wie eine kleine, etwas ins gelb-liche spielende, gliihende Kohle von langlicher Form”*. Dr. Winnecke describes similar colours in the bright comet of 1862. “Die Farbe des Strahls erscheint mir gelbrothlich; die des umgebenden Nebels (vielleicht aus Contrast) mattblaulich.” “ Die Farbe der Ausstromung erscheint mir gelblich; die Coma hat blauliches Licht”f.
Now carbon, if incandescent in the solid state, or reflecting, when in a condition of minute division the light of the sun, would afford a light which, in comparison with that emitted by the luminous vapour of carbon, would appear as yellowish or approaching to red.
The views of comets presented in this paper do not, however, afford any clue to the great mystery which surrounds the enormous rapidity with which the tail is often projected to immense distances. There are not any known properties peculiar to carbon, even when in a condition of extremely minute division, which would help to a solution of the enigma of the violent repulsive power from the sun which appears to be exerted upon cometary matter shortly after its expulsion from the nucleus, and upon matter
tail is neither more or less than the accumulation of this sort of luminous vapour darted off, in the first instance towards the sun, as it were something raised up, and, as it were, exploded by the sun’s heat out of the-kernel, and then immediately and forcibly turned back and repelled from the sun.”—Sir John Herschel. Familiar Lectures on Scientific Subjects, p. 115.
* Beobachtungen des Halleyschen Cometen, s. 41.
t Memoires de PAcademie Imperiale des Sciences de St. Petersbourg, tome vii. No. 7.