August 2Ci, 1889] 



NATURE 



431 



Chemical Sigtiijicance of the Lines. 



Until I can obtain more photographs taken on different parts 

 of the nebula, I wish to be understood to speak on this point 

 with much hesitation, and provisionally only. We know cer- 

 tainly that two of the lines are produced by hydrogen. The 

 fineness of these lines points to a high temperature and condition 

 of great tenuity of the hydrogen from which the light was 

 emitted. This condition of the hydrogen may give us a clue 

 as to the probable interpretation of the other lines. These may 

 come from substances of very low vapour-density, and under 

 molecular conditions which are consistent with a high temper- 

 ature. It is in accjrdance with this view that the recent mea- 

 sures of Dr. Copeland, since confirmed by Mr. Taylor {loc. cit.), 

 show with great probability that the line known as I '3, which 

 has been supposed to indicate some substance of low vapour- 

 density, which shows itself only at the hottest region of the sun, 

 is present in the nebular spectrum. The great simplicity of the 

 three pairs of lines seen in the photograph of 1889 suggests a 

 substance of a similar chemical nature. 



If hydrogen can exist at half its usual vapour-density, with a 

 molecule of one atom only, we might possibly expect to find it 

 in some of these bodies, but at present we do not know what its 

 spectrum would be in such a condition. It may be possibly that 

 it is in molecular states of our elements other than those we are 

 acquainted with that we may have to look for an interpretation 

 of some of the lines of these bodies. 



[With respect to the groups of lines which cross the star 

 spectra, any statements must also be provisional only. 



These lines are distinct and fairly strong in the star spectra, 

 and do extend, some farther than others, into the adjoining 

 nebular matter. Whether they are peculiar to these particular 

 stars and the matter close about them, or whether they will be 

 found everywhere in the nebula, or in certain parts of greater 

 condensation only, can be known only from future photographs. 



The first group shows some general agreements with a strong 

 iron group, but there are also formidable discrepancies. 



The position of the third group suggested the well-known 

 cyanogen group, especially as this group, beginning at A 3883, 

 is the first to appear under the chemical conditions which might 

 have been conceived to exist under circumstances of condensa- 

 tion (see Liveing and Dev\ar, Roy. Soc. Proc. , vol. xxxiv., 

 1883, p. 128). Under these conditions this group appears alone 

 in a photograph, without the less refrangible group, as was 

 probably the case in the photograph I took of Comet II., 1881. 

 I therefore took a photograph of an oxy-coal-gas flame, the coal- 

 gas having passed through ammonia, and a magnesium-flame 

 spectrum on the same plate for comparison. 



On comparing this photograph with that of the nebula it was 

 seen by eye, and afterwards confirmed by measurement, that the 

 nebula group begins soooner by one strong line than the cyanogen 

 group, and presents besides in the relative strength and grouping 

 of the lines a distinctly different character. The evidence ap- 

 pears to me to be against attributing these lines to cyanogen. 



I took great pains to ascertain if the group of lines which ac- 

 companies the triplet of the magnesium-flame spectrum could be 

 made to agree with the much longer group of lines in the nebula 

 at this part of the spectrum. Again, as in the case of the 

 cyanogen group, the whole aspect of the grouping of lines is quite 

 different. The groups begin and end difierently, and the rela- 

 tive strength of different parts of the group is not the same. The 

 great increase of strength which is seen in the middle of the 

 magnesium group is not present at the corresponding part of the 

 nebula group. 1 do not think therefore there should be much 

 weight given to the near positions of several individual lines of 

 the two groups, whicli in the case of so close a grouping might 

 well be accidental, especially as the wave-lengths can be but 

 approximate only, (The strongest lines of the magnesium-flame 

 group are those forming the triplet which appears also in the 

 spark and the arc. A nebular line is near the middle line of the 

 triplet, but there are no lines corresponding to the other lines of 

 the triplet. The other lines of the flame group are too faint to 

 be expected to appear, unless the triplet at 3720 — 3730 were 

 strong upon the plate. — Afay 18.) 



The three pairs of lines in the photograph of 1889, which are 



that the spark of magnesium in hydrogen does not give the bands, and that 

 the oxyhydrcgen fl^me hardly produces them fron iliagnesia when the 

 hydngen is in excess " (Roy S-c. Proc, vol. xliv. p. 245). Mr. Taylor 

 records a brightening of the continuous spectrum of the nebula at A 5200, 

 which he suggests may be magnesium. But this position is twenty-five 

 units from that cf the middle of the magnesium triplet at b (^Monthly 

 Notices R.A.S., vol. x'ix p 125). 



doubtless rhythmically connected, appear to me to possess great 

 interest, especially if it should come to be found from future 

 photographs that these groups are characteristic of the most 

 tenuous part of the nebula. At prescn*, I am not able to make 

 any suggestion as to their chemical origin, but the suggestion 

 presents itself that we may have to do with some molecule of 

 very low vapour density. 



The pair of lines on the more refrangible side of the line at 

 A 3724, may possibly be connected with the state of the nebula 

 as it exists in the neighbourhood of the stars. — April 26.] 



General Conclusions. 



It seems to me premature until we can learn more of the 

 significance of the new groups of lines, and especially of their 

 connection with the nebular matter generally, or with certain 

 condensed parts only, to express more than provisional sugges- 

 tions as to the nature of these nebulae. It may be that they re- 

 present an early stage in the evolutionary changes of the heavenly 

 bodies. 



As some physical importance, in the relation of these nebula? 

 to each other, has been given to my inability, in consequence of 

 insufficient optical means in my original observations in 1864, to 

 see all three of the bright lines in some faint nebulcc, I may men- 

 tion that in the case of one object, the Ring Nebula in Lyra, in 

 which at that time the light appeared monochromatic, as only 

 the brightest line could be certainly seen, as soon as larger means 

 were placed at my disposal by the loan of the Royal Society 

 telescope in 1870, I had no difficulty in seeing all three lines on 

 any night of sufficient clearness. There is little doubt that the 

 came cause prevented me from seeing more than the brightest 

 line in Nebula 4572 of Herschel's *' General Catalogue." Vogel 

 saw two lines (" Beobachtungen zn BOThkamp," 1872, p. 59). 



These bodies may stand at or near the beginning of the evolu- 

 tionary cycle, so far as we can know it. They consist probably 

 of gas at a high temperature and very tenuous, where chemical 

 dissociation exists, and the constituents of the mass, doubtless, 

 are arranged in the order of vapour-density. As to the condi- 

 tions which may have been anterior to this state of things, the 

 spectroscope is silent. We are free, £0 far as the spectroscope 

 can inform us, to adopt the hypothesis which other considerations 

 may make most probable. On Dr. CroU's form of the impact 

 theory of stellar evolution, which begins by assuming the exist- 

 ence of stellar masses in motion, and considers all subsequent 

 evolutional stages to be due to the energy of this motion con- 

 verted into heat by the collision of two such bodies, these 

 nebulae would represent the second stage in which these exist- 

 ing solid bodies had been converted into a gas of a very high 

 temperature. They would take ihe same place, if we assume 

 with Sir William Thomson (Roy. Instit. Proc, vol. xii. p. 15) 

 the coming together of two or more cool solid masses by the 

 velocity due to their mutual gravitation alone. 



I pointed out in 1864 (Phil. Trans., 1864) that the gaseous 

 nature of these bodies would afford an explanation of the 

 appearance of flat disks without condensation which many of 

 them present. The light emitted by the portions of the gas 

 further from us would be in part or wholly absorbed by the gas 

 through which it would have to pass, in this way giving to us 

 the appearance of a luminous surface only. 



In some of these bodies there is ah o a very faint continuous 

 spectrum, which if we had more light might be found to consist, 

 in great part at least, of closely adjacent bright lines. Such is 

 probably the nature, in part, of the apparently continuous spec- 

 trum of the nebula with which this paper deals chiefly, the 

 Great Nebula in Orion, 



In other gaseous nebulae strong condensations are seen, and a 

 stronger "continuous" spectrum. When we come to nebulae 

 of which the nebula in Andromeda may be taken as representa- 

 tive, the strong bright-line spectrum is ab.-.ent, and we have 

 what for convenience I called, in my original observations of 

 these bodies, a "continuous" spectrum, though I was careful to 

 point out that it wa- probably " crossed by bright or dark lines." 



Out of :il out sixty nebulae and close clusters observed by me 

 up to 1866, I found a proportion' of about one-third — namely, 

 nineteen— to present the spectrum of bright lines (Phil. Trans., 

 1866, p. 383). 



The stage of evolution which the nebula in Andromeda repre- 

 sents is no longer a matter of hypothis's. The splendid photo- 

 graph recently taken by Mr. Roberts of this nebula shows a 

 planetary s)stem at a somewhat advanced stage of evolution ; 

 already several planets have been thrown off, and the central 



