i8o 



NATURE 



\7tily 9, 1874 



consists of carbon ; that the vapour is carbon vapour 

 driven into incandescence by a temperature high enough 

 to volatilise carbon, and not the vapour of a volatile 

 hydrocarbon. 



Such is not M. Vogel's view, and I confess it is not 

 mine. After giving details of the observations of the nine 

 comets examined between 1864 and 1871, M. Vogel thus 

 analyses them : — 



" Of these nine comets, there is only one (1S70) for which 

 we have no observations as to the position of the bright 

 bands. Of the remaining eight, the spectra of five (i, 2, 

 4, 7, and 9) have shown no agreement with the hydrocarbon 

 spectrum. As regards the Comet 11. 1S67 the supposition 

 is offered that its spectrum was similar to the spectrum 

 named; as to Encke's Comet 111. 1S71, it remains un- 

 certain in which class it is to be reckoned (Huggins' 

 observations being at variance v/ith tliose of Young and 

 myself). There remains only the Comet 11. 1868, for 

 which Huggins' and Secchi's observations assert a proba- 

 biUty of coincidence of the lines in its spectrum with 

 those in the spectra of volatile hydrocarbons. 



" It thus appears a somewhat questionable view, that 

 the comets consist of such matter ; and we should, I 

 think, content ourselves with the deduction that a portion 

 of the light emitted by the comet is its own light, and 

 very probably from glowing gas." 



Hence, then, the whole question of the true materia] 

 of which that part of the comet consists, the spectrum of 

 which has been already observed, must be acknowledged 

 as being still sub jndice: and this is a matter of the first 

 order of importance, on which the present comet may 

 throw much light. 



But one of the most hopeful points is this : the comets 

 up to the present time have been cither so small or so 

 distant that the record of aigrettes or envelopes on the 

 spectrum has not been deteimincd ; nay, the comets 

 might have been deprived of those appendages, hence 

 the statement concerning the spectrum is a very general 

 one ; there has been no sufficient opportunity of localising 

 the spectrum-giving region or regions. 



What a glorious harvest will be reaped should the jets 

 appear as decided as in the comet of 1861, or in Halley's 

 comet at its return in 1835 ; "jets, as it were, of flame, or 

 rather of luminous smoke, like a gas fan-light," which, as 

 described by Sir John Herschel, " varied from day to day 

 as if waving backwards and forwards, as if they were 

 thrown out of particular parts of the internal nucleus or 

 kernel, which shitted round, or to and fro, by their recoil, 

 like a squib not held fast." 



Or again, suppose the system of concentric envelopes ig 

 developed to the same extent as in Donati's comet, in 

 which the action at all points of the nucleus, to follow Sir 

 John Herschel's reasoning, was probably more general, a 

 result due to a more uniform chemical constitution. 



Hence the comet may leave us a rich inheritance in the 

 shape of " spectrum of jets," or " spectrum of envelopes ;" 

 and from what I have already seen dimly (for such observa- 

 tions are beyond my instrumental power), the former is the 

 more probable, and in the nucleus we may have the equiva- 

 lent of the sun, or the carbon pole of an electric lamp, with a 

 continuous spectrum, and in the jets phenomena identical 

 with those presented by solar storms, or the electric arc, 

 that is, lines of various Icngthsindicating various vapours, 

 shooting out or extending to various distances according 

 to their volatilities, or vapour densities. 



We seem, indeed, to have got a true physical approxima- 

 tion to this state of things in the comet of 1868, for Mr. 

 Huggins observed that while some of the lines thinned 

 out as one sees them do in the ordinary spark by using a 

 lens, quite independently of the general visibility of the 

 vapour, others did not so thin out, but retained their 

 breadth till they disappeared altogether. 



The extent to which this action will go on will obviously 

 depend upon two things, first the temperature and secondly 

 the materials of the comet ; and this raises an important 

 question, which perhaps is easier of solution than the 

 determination of the materials ejected, should that phe- 

 nomenon be spectroscopically recognisable. 



I have already communicated to Natuke the fact that 

 to me the continuous spectrum of the nucleus appears 

 deficient in blue rays. The eflect of this upon the colour 

 of the nucleus would be to give it a yellowish tinge like 

 that of a candle flame, and for the same reason. 



Dr. Vogel, in the paper to which I have already referred, 

 deals with this Cjuestion of colour, stating that : — ' 



" Dr. Zenker arrives at the conclusion that there must 

 be water-vapour in the comets ; since they have, according 

 to Schmidt, a yellowish-red colour, and the sun's rays, 

 when they pass through a considerable thickness of 

 aqueous vapour, are coloured thus. But apart from the 

 consideration that sunlight has a yellowish-red colour 

 on passing through other vapours as well as acjueous, I 

 would remark that we must take the proper light of the 

 comet, which appears from spectral analjtic observations 

 to be generally more intense than the reflected light, as 

 determining its colour. According to ihe observations 

 made, we should expect that the comet is, on the whole, 

 of greenish or greenish-blue colour, since all the spectra 

 consist, as we have seen, of two or three bands of light, 

 of which one is in the yellow, the second and brightest in 

 the green, and the weakest in the beginning of the blue. 

 Of the (generally very faint) continuous spectrum, only the 

 brightest part— yellow, green, and commencement of blue 

 — is visible. The entire image, therefore, even where the 

 weak continuous spectrum appears, will seem of greenish 

 colour. Colour-data have been furnished by other ob- 

 servers besides Schmidt ; and the head of the Comet 181 1, 

 e.g. had, according to Herschel, a greenish or bluish 

 colour ; the nucleus was slightly red. The colour of 

 Halley's comet, at its return in 1825, was a bluish-green 

 (Struve). Winneckc says of the comet of 1862, 'The 

 colour of the neck appears to me yellowish ; the coma 

 has bluish light.' " 



It will be seen that these remarks are quite in accordance 

 with the suggestion. Dr. Zenker attributes to absorption 

 the effect which I ascribe to defective radiation, and if it 

 should be determined that the spectrum of the nucleus 

 is truly deficient in blue rays, then a great point will be 

 gained,/;^/' its icmpcratuyc jnust be loin. 



Angstiom, whose death the world of science is now de- 

 ploring, lived to say that he conceded that different mole- 

 cular arrangements of the same element might give us 

 different spectra ; and Roscoe and Schuster have recently 

 placed beyond all doubt that, besides the well-known 

 high temper.iture spectra of sodium and potassium, there 

 are other spectra appertaining to the vapour of these 

 elements .at a lower temperature. 



Now these spectra are cliaiinelled-space specti-a, that is 

 similar in character to the spectrum which has already 

 been observed in the case of comets ; and if such spectra 

 be obtained for all elements (and I have already added to 

 the list), if a comet be a body at a low temperature, it is 



