x80



NA TURE



consists of carbon; that the vapour is carbon vapour
driven into incandlescence by a temperature high enough
to volatilise scarboon, aund not the vapour of a volatile
hydrocarbon.
  Such is not M\l. Vogcl's view, and I confess it is not
mine.   After giving, details of the observations of the nine
comets Cexam aindcd between i864 and 1871, M. Vogel thus
analyses them
  " Of these nine comets, there is only one (1 870) for which
w\,e 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 ulm agreement with the hydrocarbon
spectrum. As regards the Comet II. i867 the supposition
is offered that its spectrum was similar to the spectrum
named; as to Encke's Comet III. 1871, it remains un-
certain in which class it is to be reckoned (Huggins'
observations being at variance with those of Young and
myself). There remains only the Comet II. i868, for
which Huggins' and Secchi's observations assert a proba-
bility 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 material
 of which that part of the comet consists, the spectrum of
 which has been already observed, must be acknowledged
 as being still sub judice: 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 either so small or so
 distant that the record of aigrettes or envelopes on the
 spectrum has not been determined ; 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 localising6
 the spectrunm-giving region or regions.
   What a glorious harvest will be reaped should the jets
 appear as decided as in the comet of iS6i, or in Dalley's
 comet at its return in iS35; "jets, as itwere, of Ilame, or
 rather of luminous smoke, like a gas fan-light," whllich, as
 described by Sir John I lerschel, " varied from day to day
 as if waving backwvards and forwards, as if they were
 thrown out of particular parts of the internal nucleus or
 kernel, which shitted rouind, or to and fro, by their recoil,
 like a squib not held fast.'
    Or again, suppose the sx stem of concentric envelopes is
  developed to the same axtent as in D)onati's comet, in
  which the action at all points of the nucleus, to followv Sir
  John I lerschcl's rease~ninr, weas probably more general, a
  result due to a more uniform chemical constitution.
    I lence  tile  cornit  i  iv  IL ave  us a  rich  inlheritance  in  the
  sh apc Of " sped I u1n of ljets," or " spectiun of cnvelopes;
  and  froiil  what I lhav C l, (.:   \ seeln d iiN  fSoil such  obscrva-
  tions are I y cond  itll) iistri iuiental pow  *i 1, the  foi  ilt r is
tlhe
  miorc  p) rol ;b bl, a11id  iil II I I I( le'CLS  W e 1113lo  IL hVe  tl
  Ltquiva-
  IC lt (of tlC 1 iin, or1 the  { abin ,  le of an  .lectric lamp. with 
I1
  Colntiiiu0oiis  sfwct i X 1,  atid  ill  tlte  jt I s  plcnio io iLla 
i(dlCitic  l
  %  'ithi l t   p)1 r{ eot-,   ( y  oli ,,1.,,stolt lot   Or  the  cecti
it:  arc
  thait  is, hillues  of  XII i ..  this in dtlicatll2  %a . l S  , apours
  shtooLIt, I  out  ()I   tl idii  to vatiouis  distances  accordiln
  to their' VolatilitiCs, 01 vapl)outr densities.



[7ulY 9, 1874



  We seem, in(lCed, to have got a true physical approxima.
tion to this state of things in thc comet of ' 868, 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 (didl not so thin out, but retaincd their
breadth till they disappeared altogethlcr.
  The extcnt to wvlich this action wvill go on wvill obviously
depend tipOfl two things, first the tcmpcrature and secondly
the materials of the comet ; and this raiscs ain important
question, wvhich lerhaps is casier of solution than the
determination of the materials ejected, should that phe-
nomenon be spectroscopically recognisable.
   I have already communicated to NATURE1 the fact that
 to me the contilln.ous spectrum of the nucleus appears
 deficient in blue rays. The effect 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 question 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 aqueous, I
 \voulld remark; that wve must take the proper light of the
 comet, which appears from spectral analytic observations
 to be generally more intense than the reflected light, as
 determining its colour. According to the observations
 made, -e 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 lig'ht,
 of which one is in the yellow, the second and brightest in
 the green, and the weakest in the beg1inning, 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 S chmidt ; and the head of the Comet i S Ii,
 e.,. had, according to Herschel. a  -reenish or bluish
 colour; the nucleus was sllightly red. The colour of
 Halley's comet, at its return in i823, was a bluish-green
 (Struve). \V-innecke says of the comet of i8S62 ' The
 colour of the neck appears to me yellowish ; the coma
 has bluish light.
    It wvill 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 deficint in blue rays. then a great point will be
  gailled, /< ' its.         1a:. ;c m Cs't  l. ,,,
      Antstiliiln whose death the world of science is now de-
   plori'½  lived to say that he conceded that different mole-
   cullar arrangements of the same element might give us
   ditffrent spectra ; and Roscoc and Schuster have recently
- placed beyond all doubt that, besides the well-knoe
c high temperature spectra of sodium and potassium, there
  are other spectra appertaining to the vapour of these
clcmeilits at a lower temperature.
I- ow these spectra are o/nu! 'cd-.4   c,4c/nz, thatiS
   similar in character to the spectrum which has alreadY
   ; 1een obscr\ ed in the case of comets; and if such spectra
   be oubtiedL for all elements (and I have already added to
   the list), if a comet be a body at a lowv temperature, it is