﻿124 M. F. Zollne" on the Spectrum of the Aurora borealis. 



it is one of another order, and one which we cannot yet produce 

 artificially. 



The considerations in question result from the theorem deve- 

 loped in the preceding memoir*, of the equivalence of thickness 

 and density of the radiant layers in the conditions of density 

 prevailing in the higher regions of our atmosphere. Let us de- 

 note by A x and E A the value of the absorptive and the emissive 

 power, referred to unity of thickness and density at a determined 

 temperature, for the wave-length X ; further, let m and a be the 

 thickness and density of the luminous layer of gas ; we thus have 

 for the brightness E of the part of the spectrum belonging to \ 

 according to the theorem mentioned, the expression 



E=[l-(l-A,)-]^- 



It will be seen that, for a given gas and a given temperature, 

 this expression depends only on the value of the product of ma. 



Let us conceive, then, for example, a Geissler's tube filled with 

 rarefied atmospheric air ; and let us take the diameter of the 

 central channel at 1 millim. as unit for the thickness m, and as 

 unit for a that density of the included air which corresponds to 

 a temperature of 0° C. and a pressure of 1 millim. of mercury. 

 If now by means of an induction-apparatus the air in the tube is 

 rendered incandescent, at constant temperature the spectrum of 

 the incandescent layer of gas would remain qualitatively and 

 quantitatively unaltered, if the thickness of the luminous layer 

 (consequently, in the case considered, the diameter of the channel) 

 were increased from 1 millim. to 1000 millims., but to compensate 

 for this the pressure of the included gas were diminished from 

 1 millim. to T qVo °^ a m il nm - Wullner, in his investigations 

 " On the Spectra of Gases in Geissler's Tubes," found that the 

 current of a smaller RuhmkorfFs apparatus, in the cases of ni- 

 trogen and oxygen, could not overcome the resistance until the 

 pressure in the tubes had been reduced by exhausting to 94 and 

 64 millims. respectively. But although at these pressures the 

 gases became permanently luminous, yet the intensity of the 

 light was still too small for spectroscopic investigation. With 

 nitrogen the requisite brightness was first obtained when the 

 pressure was reduced to 46 millims. ; with oxygen, to 28-30 

 millimsf. 



I will hence assume that, with the apparatus used by Wiillner, 

 at about 50 millims. pressure the induction current passes through 



* " Ueber den Einfluss der Dichtigkeit und Temperatur auf die Spectra 

 gluhender Gase " (Ber. Kdn. Sachs. Ges. d. Wissensch. Oct. 31, 18/0, 

 p. 233), a translation of which will appear in the Phil. Mag. 



t Pogg. Ann. vol. cxxxv. pp. 516 & 524. 



