Chapter XX: Some Concluding Remarks

It is usual, in concluding a work on a special subject, to sum up its contents, and to examine the general results arrived at. This, however, it is not easy to do in the present case. The contents of our volume comprise a short history of the Aurora, its qualities and spectrum; and a statement has been given of the several conclusions at which various observers have arrived as to its character and causes. In the present state of our knowledge of the subject, to add an opinion to these might seem to savour of presumption; and the questions involved may perhaps be better treated as still _sub judice_, and as requiring further and fuller evidence before arriving at a verdict. The following observations must therefore be taken rather as further notes and memoranda, than as conclusions. Apart from the spectroscopic questions involved, the oldest and most received theory of the Aurora—that of its being some form of electric discharge in the more rarefied regions of the atmosphere,—seems to hold its own: and if, as is probable, some form of phosphorescence is involved in the discharge, M. Lecoq de Boisbaudran’s observations on the brightening of the red line under the influence of cold, and the falling of the yellow-green line within a band of phosphoretted hydrogen, come into play; and a connexion, though slight and imperfect, may be in this respect traced between the discharge and its spectrum. The experiments detailed in Part II. seem to have an important bearing, as showing the very marked effect of the magnet on the rarefied glow, as well as on the spark in air at ordinary pressure. The well-defined arc formed by the aura of the spark, the flickering jets which replace the even edge of the arc when partially withdrawn from the magnetic influence, and the streamers formed when the aura is blown away from the spark (Plate XVII. figs. 6, 7, and 8), are certainly highly suggestive of frequent forms of Auroral discharge; and, but for trial and failure, might lead one to expect results from a comparison of the line air-spectrum with that of the Aurora. The experiments with a wire attached to one electrode only, show how the glow may be affected and varied in colour and character when the discharge is interrupted and incomplete. Differences in electric tension may also considerably vary the character of the discharge.

The influence of the magnet in exciting and brightening the glow and spectrum of one gas, while it depresses and extinguishes the glow and spectrum of another gas in the same tube, suggests an explanation of the observed variation in intensity, and difference in number, of the Aurora-lines. Intensity of lines depending on temperature, and this again on resistance, and it appearing that resistance is influenced by the magnetic action, the same effects of brightening or depressing of the spectrum are probably produced in the Aurora, as in the vacuum-tubes placed between the magnet-poles.

In the Marquis of Salisbury’s observations, paraffin-vapour gave C and H lines when connected with both poles of the battery, but C lines only when connected with one pole; and in that case the lines were equally sharp on both sides. These observations (repeated in our experiments) may afford an explanation why the hydrogen-lines are not seen in the Aurora-spectrum; although there can be hardly any doubt that the phenomenon usually takes place in air more or less moist. Professor Ångström’s researches on the violet-pole glow are not entirely corroborated by our experiments; and it seems doubtful whether his results in the exhausted flask were not obtained from the negative pole only. One great difficulty in the comparison of the Aurora-spectrum with the violet pole of air-tubes and some other spectra (including oxygen), arises from the presence in the latter of broad bands; and it is difficult to understand how these bands can be aptly compared with the definite, though faint, lines observed by Dr. Vogel and others in the Aurora-spectrum. It must, too, be borne in mind that the conditions under which we may consider the Aurora to obtain, are such as can be only very imperfectly imitated in the laboratory. Auroræ also no doubt differ in density and thickness of layer; and Kirchhoff’s observation must be remembered:—“That if thickness of a film of vapour be increased, the lines are increased in intensity, the bright lines more slowly than the fainter; and it may happen that the spectrum appears to be totally changed when the mass of the vapour is altered.” Were it possible to test with the spectroscope a cloud or film of gaseous vapour corresponding in some degree in density and thickness with an Auroral discharge, we might perhaps get nearer the truth. Mr. Procter also remarks (as we proved in our magnet experiments):—“That frequently very small traces appropriate to themselves the whole of electrical discharges at low pressures, and completely mask the spectra of any other gases present.” The oxygen-spectrum, with its possible variation by the conversion of that gas into the allotropic condition termed ozone, seemed at first to afford a prospect of close relation to the Aurora-spectrum; which, however, disappeared on closer examination. If nitrogen could be modified in some such way as oxygen is converted into ozone, it might perhaps afford another opportunity for investigation; but we have no evidence at present of such a change. The spectrum of nitrogen is usually found singularly distinct and persistent; and, except as varied from band to line by intensity of the discharge, not liable to alteration[16].

Colours of lines are functions of wave-length, subject, however, to the observation that in a weak spectrum the colours lose their intensity. The red line in the Aurora has sometimes been found brighter than the green. It has been suggested that the red and green may be independent spectra; but the variations of tint observed in the capillary of hydrogen and other tubes according to resistance of the current, demonstrate that the varying colours of the Aurora may be connected with the lighting-up of particular parts of the spectrum, and do not necessarily indicate that different gases and spectra are excited.

Absorption may also play an important part in the nature of the Aurora-spectrum (Zöllner’s theory that the lines are really spaces between absorption bands). Most gases will give a continuous spectrum under certain circumstances, even at a low pressure.

The question of cosmic dust is inviting, but the facts collated hardly warrant at present its probable connexion with the Aurora.

If Auroræ were composed of incandescent glowing meteors, it would be reasonable to expect to find in the spectrum the lines of iron, a metal constituting so prominently the composition of meteorites. No connexion between the iron and the Aurora-spectra is, however, proved; though it may be suspected. The iron-spectrum, as remarked elsewhere, contains so many lines that some may, as a mere accidental circumstance, closely agree with the Aurora-lines.

The iron-lines are, it may be remarked, as a rule, sharper and finer than the Auroral lines, though it is possible that these characteristics might vary if the spectrum were obtained in a rarefied medium. Tubes with iron terminals are said to evolve a compound gas of H and Fe. I have not had an opportunity to verify this.

It may be added that the comparative faintness of the more refrangible lines of the Aurora-spectrum suggests a feeble resistance to the exciting current, and a low temperature inconsistent with a meteoric theory; and this is not contradicted by the brightness of the red and green lines, if these are due to a phosphorescent origin. Expansion of a line is recognized to be dependent on pressure, and consequently the breadth of the green or red lines might indicate the height of the Aurora; while their brightness or otherwise might also give some idea as to its density. No observations in this direction have, as far as I am aware, been recorded.

As the general result of spectrum work on the Aurora up to the present time, we seem to have quite failed in finding any spectrum which, as to position, intensity, and general character of lines, well coincides with that of the Aurora. Indeed, we may say we do not find any spectrum so nearly allied to portions even of the Aurora-spectrum, as to lead us to conclude that we have discovered the true nature of one spectrum of the Aurora (supposing it to comprise, as some consider, two or more). The whole subject may be characterized as still a scientific mystery—which, however, we may hope some future observers, armed with spectroscopes of large aperture and low dispersion, but with sufficient means of measurement of line positions, and possibly aided by photography, may help to solve. The singular absence of Auroræ has, for some time past, given no opportunity in that direction. May some of my readers be more fortunate in obtaining opportunities of viewing the glorious sky-fires, and assist to unravel so interesting a paradox!

APPENDICES.