yuly 1 6, 1874I 



NA TURF. 



It may be aisumcd that the spcctrani of ihc aurora is 

 composed of /'MO different spectra, which, even although 

 appearing sometimes simultaneously, have in all proba- 

 bility 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 manifestations. The other 

 spectrum consists of extremely feeble bands of light, which 

 only in the stronger aurora; attain such an intensity as 

 enables one to fix their position, though only approxima- 

 tively. 



As to the yellow lines 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 (ComptL-s Rendus, Ixxiv. 

 S97) 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 right, but in Angstrom's opinion is found on a 

 pure misconception. The spectrum of rarefied air has 

 in the green-yellow 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. 

 Observations on the spectrum have not hitherto 

 agreed with each other ; partly, perhaps, because of the 

 weak light of the obiect, but partly also, it may be, on 

 account of the variability of the aurora. The red does 

 not always appear, and when it does is often so weak that 

 it cannot be observed in the spectroscope. If now it be 

 assumed that the aurora lias its final cause in electrical 

 discharges in the upper strata of the atmosphere, and that 

 I these discharges, whether disruptional or continuous, take 

 j place sometimes on the outer boundary of the atmosphere, 

 , and sometimes near to the surface of the earth, this varia- 

 bility will easily show in the appearance of the spectrum 

 j what the observations appear to confirm. 



If we consider the conditions under which the electric 

 light appears on the boundary of the atmosphere, mois- 

 ture in that region must be set down as nil, and conse- 

 ' quently the oxygen and hydrogen there must alone act as 

 ' conductors of electricity. Angstrom has tried to re- 

 j produce these conditions on a small scale. Into a flask, 

 .'the bottom of which is covered with a layer of phosphate, 

 :the platinum wires are introduced and tha air is pumped 

 jout to the extent of several millimetres. If the inductive 

 'current of a RuhmkorlT coil be sent through the llask, the 

 ' whole llask will be tilled, as it were, with that violet light 

 which otherwise only proceeds from the negative pole, 

 land from both electrodes a spectrum is obtained consist- 

 ' ing chiefly of shaded violet bands. 



I If this spectrum be compared with that of the aurora, 

 Angstrom thinks that the agreement between the former 

 and some of the best established bands of the latter is 

 > satisfactory. 



I Lines Wave-Iciigtlis 



'Acccrding to Barker . 431 470'5 

 „ ,, Vogel . — 469-.t 523-3 

 J Of the auror.-i I ,, ,, Angstrom — 472 521 

 spectrum. .\ ,, >, Lemstrom' 426-2 469-4 523-5 



' Mean 



Of the spectrum of the violet light 



. 4cSG 470-3 

 • 4-72 4707 



522-6 



522-7 



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 light has also two conespond- 

 ing shaded bands, 465-4 and 460-1. 



Should the aurora be flamy and shoot out like raj s, 

 \ there is good reason for assuming a disruptive discharge 



of electricity, and tiieu there ought to ajjpcar the strongest 

 line in the line-spectrum of the air, the green, whose wave- 

 length is Soo'3- Precisely this has been actually observed 

 by Vogel, and has moreover been seen by Angstrom and 

 others. 



Finally, should the aurora be observed as it appears at 

 a less height in the atmosphere, then are recognised both 

 the hydrogen lines and also the strongest of the bands of 

 the dark-banded air-spectrum, as £.0. 497-3. There are 

 found also again nearly all the lines and light bands of the 

 weak aurora spectrum, whose position has with any cer- 

 tainty been observed. 



There still remains the line in the red field, the wave- 

 length of which, according to Vogel, may be valued at 630. 

 Angstrom has chanced to see it only a single time, while 

 on various occasions, when the aurora has shown red 

 lights, he has found it impossible to distinguish any liaes 

 whatever in this part of the spectrum. The cause of this 

 may be that while the red bands in the spectrum of the 

 negative pole are broad and very feeble in light, the 

 corresponding light in the aurora may be imperceptible in 

 the spectroscope on account of the dispersion of the 

 prism, although it is strong enough to give to the aurora 

 a reddish appearance. Angstrom does not venture to 

 decide whether the red Ime observed by Vogel coincides 

 with the strongest of these bands, but so much is at least 

 certain, that it may coincide wiih more than one of the 

 bands to the red field of Plucker's air-spectrum. 



In general it may be thus assumed that the feeble 

 bands in the aurora spectrum belong to the spectrum of 

 the negative pole, and that the appearance of this spec- 

 trum may be changed more or less by additions from the 

 banded air-spectrum or the line-spectrum of the air. 



But by this is not yet explained the one-coloured spec- 

 trum or the origin of the yellow line. The only explana- 

 tion of the origin of this line which in Angstrom's opinion 

 is in any way probable, is that it owes its origin to f.iior- 

 cscciue or phosphorescence. Since fluorescence is pro- 

 duced by the ultra-violet rays, an electric discharge may 

 easily he iinaoincd, -ojhich, though in itself of feeble light, 

 may be rich in ultra-violet liglit, and there/ore in a condi- 

 tion to cause a sufficiently strong fluorescence. It is also 

 known that oxygen is phosphorescent, as also several of its 

 compounds. 



There is therefore no need, in order to account for the 

 spectrum of the aurora, to have recourse to the " very 

 great variabihty of gas spectra according to the varying 

 circumstances of pressure and temperature," a variability 

 which according to Angstrom's twenty years' observations 

 does not exist. Just as little can Angstrom admit that the 

 way in which a gas may be brought to glov/ or burn, can 

 alter the nature of the spectnmi ; since it is an established 

 fact in physics that the state of light and of heat which 

 puts a body into a glowing condition is imconnccted in 

 character with that which produces glowing. 



Angstrom does not entirely deny the possibility that a 

 simple body by glowing in a gaseous condition will offer 

 several spectra. Just as one simple body can form a 

 chemical" combination with another, and tliis_ body by 

 glowing in a gaseous condition, so long as it is not de- 

 composed, gives its own spectrum, so must it also be 

 able to form combinations with itself— thus to form 

 isomeric combinations— it being always supposed that 

 it exists in the gaseous form and can maintain itseh 

 in a glowing condition without decompcsition. In this 

 way it is indeed possible to conceive an absorption for 

 oxygen which belongs to ozone; but since ozone, as is 

 well known, cannot maintain itself in a glowing conditio!;, 

 it is in vain to look for more than one spectrum of o :y- 

 gen. There is, however, at le st a poss.bility of obtaining 

 several spectra from sulphur, while again vvilh respe t to 

 cirbon, whii-h c.mnot even be exhibited in a gasc u ■ 

 condition, a like assumption in the author's opini'-'n v; nt; 

 the suviport of experience. 



