Page:Encyclopædia Britannica, Ninth Edition, v. 3.djvu/106

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Vogel remarks that the line at 5569, which is often the only one visible, as well as the faint band at 4667, become noticeably fainter when the red line is visible, while under the same circumstances that near 5189, as well as the red line, is very brilliant. This fact, which has also been noted by other observers, makes it almost certain that the auroral spectrum is not a simple one, but is derived either from two or more sources, or from the same source under very varying conditions. Angström says (Nature, x. 211)

&ldquo; It may be assumed that the spectrum of the aurora is composed of two different spectra, which, even although appearing sometimes simultaneously, have in all probability different origins. The one spectrum consists of the homogeneous yellow light which is so characteristic of the aurora, and which is found even in its weakest manifestation. The other spectrum consists of extremely feeble bands of light, which only in the stronger aurora attain such intensity as enables one to fix their position even approximately. As to the yellow line in the aurora, or the one-coloured spectrum, we are as little able now as when it was first observed to point out a corresponding line in any known spectrum. True, Piazzi Smyth (Comptes Rendus, lxxiv. 597) has asserted that it corresponds to one of the bands in the spectrum of hydrocarbons; but a more exact observation shows that the line falls into a group of shaded bands, which belong to the spectrum, but almost midway between the second and third. Herr Vogel has observed that this line corresponds to a band in the spectrum of rarefied air (Pogg. Ann., cxlvi. 582). This is quite true, but in Angström’s opinion is founded on a pure misconception. The spectrum of rarefied air has in the yellow-green part seven bands of nearly equal strength, and that the auroral line corresponds with the margin of one of these bands, which is not even the strongest, cannot be anything else than merely accidental.&rdquo;

Angström’s own view is that this line is due to fluorescence or phosphorescence, and he remarks that &quot;since fluorescence is produced by the ultra-violet rays, an electric discharge may easily be imagined, which though in itself of feeble light, may be rich in ultra-violet rays, and therefore in a condition to cause a sufficiently strong fluorescence. It is also known that oxygen is phosphorescent, as also several of its compounds.&ldquo; We are, however, just as ignorant of any body which would give such a light by phosphorescence or fluorescence as by ignition, and it seems more probable that the light may be due to chemical action. It is assumed by Angström that water vapour is necessarily absent in the higher atmosphere on account of the cold, but when we remember that its molecular weight is lighter than that of oxygen in the, proportion of 9 to 16, it is not unlikely that it may attain great elevations under the very low tensions that prevail at such heights, and it is possible also that both it and other bodies may, by electric repulsion in the auroral beams, be carried up much above the level which they would attain by gravity. If, then, electric discharges take place between the small sensible particles of water or ice in the form of mist or cirrus, as Silbermann has shown to be likely, surface decomposition would ensue, and it is highly probable that the nascent gases would combine with emission of light. It has been almost proved in the case of hydrogen phosphide that the very characteristic spectrum produced by its combustion is due neither to the elements nor to the products of combustion, but to some peculiar action at the instant of combination, and it is quite possible that, under such circumstances as above described, water might also give an entirely fresh spectrum.

It is, perhaps, proper to mention that H. R. Procter found an apparent coincidence by often repeated direct comparison with a band frequently seen both in air and oxygen tubes, which he eventually succeeded in tracing with tolerable certainty to some form of hydrocarbon. The comparison spectroscopes were only of low dispersion, but on more accurate measurement of the carbon band it was found that, though more refrangible than the first band of citron acetylene (candle-flame), it was still less so than careful measurement assigns to the aurora. In addition, the band was shaded towards the violet, which is not the case with that of the aurora, though with feeble light it seemed like a line.

If, leaving the citron line, we pass on to the feeble spectrum towards the violet, we shall obtain more hopeful coincidences. Angström thinks that three of the bands correspond with the three brightest bands of the violet aurora of the negative pole in rarefied air, and has tried to reproduce the conditions of the aurora on a small scale. He says&mdash;

&ldquo;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 then sent through the flask, the whole flask will be filled, as it were, with the violet light, which otherwise proceeds only from the negative pole, and from both electrodes a spectrum is obtained consisting chiefly of shaded violet bands. If this spectrum be compared with that of the aurora, Angström thinks the agreement between the former and some of the best established bands of the latter is satisfactory.

In the neighbourhood of the line 469.4 Herr Vogel has, moreover, observed two weak light-bands, 466.3 and 462.9(?). The spectrum of the violet has also two corresponding shaded bands, 465.4 and 460.1. &ldquo;Should the aurora be flamy, and shoot out like rays, there is good reason for assuming a disruptive discharge of electricity, and then there ought to appear the strongest line in the spectrum of the air, the green, whose wave-length is 500.3. Precisely this has actually been observed by Vogel, and has, moreover, been seen 