730 THE AURORA BOREAHS, 



<* 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 lines 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 corre- 

 sponding light in the aurora may be imperceptible in the spectro- 

 scope 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 line ob- 

 served by Vogel coincides with the strongest of these bands, but 

 so much is at least certain, that it may coincide with more than 

 one of the bands in 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 spectrum 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 spectrum or 

 the origin of the yellow line. The only explanation of the origin 

 of this line which, in Angstrom's opinion, is in any way probable, 

 is that it owes its origin iQ> fluoresence or phosphorescence. Since 

 fluorescence is produced by the ultra-violet rays, an electric dis- 

 charge may easily he imagined^ which, though in itself of feeble 

 light, may be rich in ultra-violet light, and therefore in a con- 

 dition to cause a sufficiently strong fluorescence. It is also 

 known that oxygen is phosphorescent, as also several of its com- 

 pounds. 



" There is therefore 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,' a variability which, according to Angstrom's 

 20 years' observations, does not exist. Just as little can Angstrom 

 admit that the way in which a gas may be brought to glow or 

 burn, can alter the nature of the spectrum ; since it is an esta- 

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

 puts a body into a glowing condition is unconnected 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 ofier several spectra. 

 Just as one simple body can form a chemical combination with 

 another, and this body by glowing in a gaseous conditions, so long 

 as it is not decomposed, 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 itself in a glowing condition with- 

 out decomposition. 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 con- 

 dition, it is in vain to look for more than one spectrum of oxygen. 



