THE AUROEA BOREALIS. 731 



There is, however, at least a possibility of obtaining several 

 spectra from sulphur, while again with respect to carbon, which 

 cannot even be exhibited in a gaseous condition, a like assumption, 

 in the author's opinion, wants the support of experience." 



17. Prof. A. S. Herschel on the Spectrum of the Aurora. 



The spectrum of the Aurora is no doubt in the main the same as 

 that of the pale blue light round the negative pole in an air or 

 nitrogen vacuum-tube, with the induction-spark passing through 

 it. There are so many well marked lines in this spectrum that, 

 looking at Angstrom's representation of them, it is probably 

 owing to the insignificant appearance of that part of the vacuum- 

 spark that its proper spectrum has not been more frequently 

 studied with reference to the Aurora, as Angstrom seems to have 

 done by an experiment specially adapted for the purpose. 



There are several forms or modes (apparently four or five) of 

 electrical discharge through rarefied gases.* When veiy much 

 rarefied, air transmits the electricity so as to discharge the 

 Ruhmkorff poles without a spark. In that state there is still a 

 glow of heated air round both poles, which increases in size and 

 • length along the tube as the air pressure is increased, faster 

 round the positive than round the negative pole. This has been 

 accounted for by showing that the air oflfers far greater resistance 

 to the passage of electricity when it surrounds a cathode or 

 negative, than when it surrounds an anode or positively electrified 

 pole. The difference becomes more obvious as the pressure and 

 density of the gas are increased. The negative glow shrinks into 

 a very small space, while the positive brush extends through 

 nearly the whole length o£ the tube, abolishing at last the dark 

 space that at very low tensions separates the two lights from each 

 other. At pressures not exceeding one or two millimetres the 

 positive glow is stratified ; but if the pressure is increased it 

 becomes continuous ; and if the air-pressure amounts to that of 

 J or -^ an inch of mercury, or upwards, it again gathers into 

 somewhat larger light clouds ; and at about 1 or 2 inches of 

 barometric pressure a spark passes between the poles. This 

 spark is red; it scarcely diminishes the strength of the con- 

 comitant glow discharge; and it is far less luminous than the 

 white spark which begins to appear at 5 or 6 inches of pressure, 

 and may often be seen at first broken up along its length into 

 parts which are alternately white and red. The spectra, like the 

 general appearances, of these two forms of the spark are quite 

 distinct. 



* An examination of these with revolving mirrors, by A. Wiillmer, at Aix 

 la Chapelle, appeared in the " Jubelband " of Poggendorff's Annalen this year 

 at the same time as Angstrom's paper in that volume, which also contains 

 some other tracts (by A. de la Rive and others) tracing the effects of magnets 

 and of metallic vapours in augmenting the discharge through air. 



