CHAPTER XII.

SOME NOTES ON PROFESSOR ÅNGSTRÖM’S THEORY OF THE AURORA-SPECTRUM.

[The substance of these appeared in the ‘Philosophical Magazine’ for April 1875, in conjunction with the “Comparison of the Tube and other Spectra” (Chapter XI.), but they are now, for the sake of convenience, made a separate article.]

[Sidenote: Professor Ångström’s propositions.]

In a contribution by the late Professor Ångström to a solution of the problem of the Aurora-spectrum (an abstract of which appeared in ‘Nature’ of July 16, 1874), the Professor is stated, amongst other things, to have laid down certain propositions in substance as follows:—

[Sidenote: That the Aurora has two spectra.]

1st. That the Aurora has two different spectra—the one comprising the one bright line in the yellow-green only, and the other the remaining fainter lines.

[Sidenote: That bright line does not coincide with HC₂.]

2ndly. That the bright line falls within a group of hydrocarbon lines, but does not actually coincide with any prominent line of such group, and that Dr. Vogel’s finding this line to coincide with a not well-marked band in the air-spectrum must be regarded as a case of accidental coincidence.

[Sidenote: That moisture is _nil_ in Aurora region.]

3rdly. That moisture in the region of the Aurora must be regarded as _nil_, and that oxygen and hydrogen must alone there act as conductors of electricity.

[Sidenote: Ångström’s flask-experiment described.]

Professor Ångström then details the examination of an exhausted dry air-flask filled with a discharge analogous to the glow of the negative pole of a vacuum air-tube.

[Sidenote: Flask-spectrum compared with Aurora-spectrum.]

The experiment is described as follows:—“Into a flask, the bottom of which is covered with a layer of phosphoric anhydride, the platinum wires are introduced, and the air is pumped out to a tension of only a few millimetres. If the inductive current of a Ruhmkorff coil be sent through the flask, the whole flask will be filled, as it were, with a violet light, which otherwise only proceeds from the negative pole, and from both electrodes a spectrum is obtained composed chiefly of shaded violet bands.” The comparison of the spectrum of this violet glow with that of the Aurora gives, according to Ångström, the following results:—

Aurora-lines, wave-lengths 4286 4703 5226 Violet light, wave-lengths 4272 4707 5227

Two weak light bands, found by Dr. Vogel at 4663 and 4629, are also compared with other lines in the violet light 4654 and 4601; and the Professor then concludes that it may be in general assumed that the feeble bands of the Aurora-spectrum belong to the spectrum of the negative pole, possibly changed more or less by additions from the banded or the line air-spectrum.

[Sidenote: Bright line is due to fluorescence or phosphorescence.]

4thly. That the only probable explanation of the bright line is, that it owes its origin to fluorescence or phosphorescence. The Professor remarks on this point that “an electric discharge may easily be imagined which, though in itself of feeble light, may be rich in ultra-violet light, and therefore in a condition to cause a sufficiently strong fluorescence.” He notes also that oxygen and some of its compounds are fluorescent.

[Sidenote: No need of Dr. Vogel’s theory of variability.]

5thly. That there is no need, in order to account for the spectrum of the Aurora, to have recourse to the “very great variability of gas-spectra according to the varying circumstances of pressure and temperature” (Dr. Vogel’s theory). Professor Ångström does not admit such variability, and does not admit that the way a gas may be brought to glow or burn can alter the nature of the spectrum.

[Sidenote: Professor Ångström’s conclusions tested.]

In order to test some of the Professor’s conclusions in an experimental way, I examined some tube and other spectra not only for line-positions, but also for general resemblance to an Aurora-spectrum.

These experiments are detailed in the last Chapter, and the results are comprised in Plates XIV., XV., and XVI., in which the spectra obtained are represented in black for white.

[Sidenote: Result of examination of the Professor’s propositions.]

The result of the examination of Professor Ångström’s principal propositions seems to be this:—

1st. Two Auroral spectra. I agree in this, but question whether the fainter lines may not possibly comprise more than one spectrum.

2nd. I agree also that the bright yellow-green line falls, as Professor Ångström describes, just behind the second line in the hydrocarbon yellow group (see Plate V. fig. 7). And I find, in common with the Professor, no well-marked or prominent line in the air-spectrum with which it accords.

3rd. This may be conveniently divided into two parts, viz.:—

A. The proposition that “moisture in the region of the Aurora must be regarded as _nil_.”

[Sidenote: Moisture probably not _nil_ in the Aurora region. Reasons for this given. Aurora in vapour or mist. Frequently near to earth’s surface.]

Here I see reason to differ, since (to quote a letter of Mr. Procter’s) “the vapour-density of OH₂ is only 9 against 14 for N and 16 for O;” and again, “electrical or heat-repulsion may carry water-dust up to enormous heights.” There are, too, I think, circumstances connected with the Aurora itself which make the assumption of moisture being _nil_ in the Auroral regions untenable. The first of these is the fact that the white arc, streamers, and floating patches of light, found in some Auroræ, have frequently the peculiarly dense and solid look of vapour-clouds—a circumstance with which I have been frequently struck. Mr. Procter and others have also remarked that the Aurora is generally formed in a sort of “mist or imperfect vapour.” The second, that Auroræ, or portions of them, are frequently near to the earth’s surface. Instances of this are given in the section on the Height of the Aurora, notably the experiences of Sir W. Grove and Mr. W. Ladd.

[Sidenote: Coincidence of Auroral lines with telluric solar lines.]

On this point, too, note the peculiarities of the red line, which (and, as I find, the green line also) are coincident with, or very close to, telluric bands or groups of lines in the solar spectrum usually attributed to moisture. (See Plate XIII. fig. 2.)

[Sidenote: Continuous spectrum.]

I think we may also claim the continuous spectrum in the Aurora in further proof of water-vapour (see Plate XV. spectrum 7). The continuous spectrum of the Aurora is also, to my observation, more local and dense in the spectroscope than the glow generally seen between the lines or bands in gas-spectra.

[Sidenote: Violet-pole spectrum discussed. Most spectra have a general as well as special character.]

B. The question of the violet-pole spectrum. Here I make the remark that in comparing other spectra with that of the Aurora, it is, I think, too much the practice to trust to the coincidence (more or less perfect) of one or perhaps two lines out of many; whereas we know by experience that most spectra have so well-marked a general as well as special character that, when once seen, they are recognized afterwards with the greatest ease and without measurements. An experience and proof of this is found in a set of “Photographed Spectra” which the Autotype Company have reproduced for me.

[Sidenote: Coincidence of one or two lines not sufficient to establish identity.]

Of course no two given spectra can be considered identical unless their principal lines coincide; but, on the other hand, the coincidence of one or two lines out of many, without other features, cannot be satisfactorily or conclusively held to establish identity.

[Sidenote: Ångström’s compared spectra.]

In Professor Herschel’s letter (Phil. Mag. ser. 4, vol. xlix. p. 71), Professor Ångström’s representation of the “spectrum of the glow discharge round the negative pole of air-vacuum tubes” is given, in comparison with the Aurora-lines and those of olefiant gas. This illustration is here introduced.

Ångström’s representation of the Spectrum of the glow discharge round the negative pole of Air-vacuum tubes, and its comparison with the Spectrum of the Aurora.

It is unfortunate that in this illustration and in Professor Herschel’s paper the wave-lengths of the Aurora-lines are not given in figures, but must be roughly calculated from the scale. Professor Herschel speaks of Ångström’s drawing as representing a _normal_ spectrum, and as derived from authentic sources, such as Vogel, Barker, and others; but beyond this we are not certain as to its origin.

In illustration of the difficulty of constructing any thing like a general typical Aurora-spectrum I append a Table of eight Auroral spectra taken at hazard:—

[Sidenote: Table of compared Aurora.]

Auroral lines and bands.

---------------------+----+---------+------+-----+-----------+----------- Observers. |Red.| Yellow. |Green.|Blue.| Indigo. | Violet. ---------------------+----+----+----+------+-----+-----+-----+------+---- | | | | | | | |{4694}| Vogel, April 9, 1871 |6297|5569|5390| 5233 | 5189| 5004| — |{ to }| — | | | | | | | |{4629}| | | | | | | | | | Barker, Nov. 9, 1871 |6230|5620| — | — | 5170| 5020| 4820| — | — | | | | | | | | | | | | |{5330}| |{5050| 4930| 4740}| Barker, Oct. 14, 1873|6300|5550| — |{ to }| — |{ to | to | to }|4310 | | | |{5200}| |{4990| 4850| 4670}| A. Clarke, junr., | | | | | | | | | Oct. 24, 1870 | |5690| — | 5320 | — | — | 4850| — |4350 | | | | | | | | | Backhouse, 1873 |6060|5660| — | — | 5165| 5015| — | 4625 |4305 | | | | | | | | | Backhouse, Feb. 4, | | | | | | | | | 1874 | * |5570| — | — | 5180| 4980| 4830| 4640 |4320 | | | | | | | | | H. R. Procter, 1870 | * | * | — | * | — | — | — | * | * | | | | | | | | | Lord Lindsay, 1870 | — | * | * | * | * | — | — | — | * ---------------------+----+----+----+------+-----+-----+-----+------+----

* Mr. Procter’s and Lord Lindsay’s lines had no wave-lengths.

[Sidenote: Ångström’s drawing discussed.]

On examining Ångström’s diagram it certainly seems to me that, upon the showing of the drawing itself, the coincidences are not very exact. All three of the violet-pole bands appear to be less refrangible than the Aurora-lines with which they are compared-the middle one (at 47) considerably so, the one near E (at about 52·30) appreciably so, and the third (at 43) slightly so.

[Sidenote: Diagram of Vogel’s Aurora and violet-pole spectrum.]

As it seemed desirable to adopt a specific Aurora-spectrum for comparison, and to show such comparison on a somewhat larger scale than Ångström’s drawing, I prepared the diagram shown on Plate XI. fig. 1. The upper spectrum is Vogel’s, already described and figured on Plate XIII. The lower spectrum is that of “Air, violet pole,” Plate XV. spectrum 2, graphically shown. I can only find one absolute coincidence in the two compared spectra in this diagram.

It should, too, I think, be borne in mind that there is a great difference in the character of the compared spectra, whether as shown in Ångström’s drawing or mine—the bands of the violet-pole spectrum mostly degrading towards the violet, while the lines or bands of the Aurora in no way possess that character[14].

[Sidenote: Dr. Vogel’s violet-pole and Aurora-lines.]

To assist in the foregoing violet-pole comparison I add the following Table derived from Dr. Vogel’s memoir:—

Violet-pole lines. Aurora-lines. W.L. W.L. 6100,} broad, moderately 5945,} bright stripe 6297, very bright stripe.

5459,} broad, moderately {5569, brightest line of spectrum. 5289,} bright stripe {5390, extremely faint line.

5224, very bright line 5233, moderately bright. 5147, faint line 5189, moderately bright. 5004, bright line 5004, very bright line. 4912, fainter than last.

4808, very faint line. {6694,} 4704, very intense line. {4663,} band less brilliant in 4646, very faint line. {4629,} the middle.

4569, moderately bright. 4486, moderately bright. 4417, quite faint line. 4346, moderately bright line. 4275, very bright line.

On examination of these figures it will be seen that 5224 and 5233 are fairly close, and that 5004 is coincident. Beyond these there is little to identify the spectra.

[Sidenote: Conclusions arrived at adverse to the violet-pole theory.]

As the general result of my observations and a comparison of the foregoing spectra and tables, I see no reason for giving to the violet-pole glow any special or distinguished place in a comparison with the Aurora, and certainly not for assigning to it the nearly absolute monopoly of the spectrum. It is true that the line γ in the violet-pole glow (Plate XV. spectrum 2), which, by the way, degrades towards the red, is in close coincidence with one of the Aurora-lines; but how are the brighter bands α and β accounted for? These, as I have before pointed out, alone survive when the tube is placed at a distance from the slit. It is true they are thus reduced to shaded-off lines in lieu of bands; but the difficulty still remains, that they are conspicuous for their absence in the Aurora-spectrum. On the whole, I cannot but conclude that Professor Ångström’s theory fails. At all events, if the violet-pole glow-spectrum is to represent the Aurora-spectrum, it must be under conditions different from those by which it obtains in dry-air vacuum-tubes or flasks at ordinary temperature.

[Sidenote: Phosphorescence or fluorescence of the yellow-green line.]

4th. I feel more in accord with Professor Ångström’s memoir upon the subject of the phosphorescence or fluorescence of the bright yellow-green Aurora-line.

[Sidenote: External features of Auroræ confirmatory of this.]

I do not notice that the Professor touches upon the external features of the Aurora in respect of this question.

October 20, 1870.—I noted the grand Auroral display of that evening, including “streamers of opaque-white _phosphorescent_ cloud very different from the more common transparent Auroral diverging streams of light.”

February 4, 1872.—A fine display. The first signs were (in dull daylight) “a lurid tinge upon the clouds, which suggested the reflection of a distant fire, while, scattered among these, torn and broken masses of white vapour, _having a phosphorescent appearance_, reminded me of a similar appearance in October 1870.” (Other instances of this effect will be found in the section Aurora and Phosphorescence.) Day Auroræ, too, we might suppose could hardly be seen without the presence of some phosphorescent glow.

[Sidenote: Other confirmatory circumstances. Conclusion in favour of the theory.]

Having regard to the near proximity of the phosphoretted-hydrogen band to the bright Aurora-line, to the circumstance of this band brightening by reduction of temperature (a phenomenon probably connected with ozone), to the peculiar brightening of one line in the green in the “Aurora” and “phosphorescent” tubes (the phosphorescent tubes probably containing O), and to the observed circumstance that the electric discharge has a phosphorescent or fluorescent after-glow (isolated, I believe, by Faraday), I feel there is strong evidence in favour of such an origin to the principal Aurora-line, if not to the red line as well.

[Sidenote: Invariability of gas-spectra questioned.]

5th. Professor Ångström opens a wide door to discussion in his proposition of the invariability of gas-spectra, and I do not now attempt to follow in detail this interesting part of the present subject. Suffice it to say, that if the Professor lays down this proposition in its strictest sense (I can hardly suppose he so meant it), there is, so far as I am aware, no one spectrum that can at all claim comparison with the Aurora-spectrum. Giving greater latitude to the Professor’s words, I reply, upon competent authority, that lines vary in number and brilliancy with temperature, and in breadth with pressure. Kirchhoff, too, in speaking of vapour-films as increasing the intensity of lines, states “it may happen that the spectrum appears to be totally changed when the mass of vapour is altered.” We may, too, now add magnetism as capable of effecting a change in certain spectra, not only as to brilliancy, but even as to position of lines. (Chautard’s Researches, ‘Philosophical Magazine,’ 4th series, vol. 1. p. 77, and experiments detailed in Chap. III. of this work.)