232 HISTORY OF COLD AND THE ABSOLUTE ZERO, 



difficult to undcrstuiid why its spectrum should be so entirely difi'er- 

 ent from anything which could l)e prodviced artificially by electric dis- 

 charges through rarefied air at the surface of the earth. Writing in 

 1879, Kand Capron, after collecting all the recorded observations, was 

 able to enumerate no more than nine auroral ra3^s, of which but one 

 could with anj^ probabilit}^ be identified with rays emitted by atmos- 

 pheric air under an electric discharge. Vogel attributed this want of 

 agreement between nature and experiment, in a vague way, to differ- 

 ence of temperature and pressure; and Zollner thought the auroral 

 spectrum to be one of a different order, in the sense in which the line 

 and band spectra of nitrogen are said to be of different orders. Such 

 statements were merely confessions of ignorance. But since that time 

 observations of the spectra of auroras have been greatly multiplied, 

 chiefly through the Swedish and Danish polar expeditions, and the 

 length of spectrum recorded on the ulti-a-violet side has been greatly 

 extended by the use of photography, so that, in a recent discussion of 

 the results, M. Henri Stassano is able to enumerate upward of one 

 hundred auroral rajs, of which the wave length is more or less approxi- 

 mately known, some of them far in the ultra-violet. Of this large 

 number of rays he is able to identify, within the probable limits of 

 errors of observation, about two-thirds as rays, which Professor Live- 

 ing and myself have observed to be emitted by the most volatile gases 

 of atmospheric air unliquefiable at the temperature of liquid hydrogen. 

 Most of the remainder he ascribes to argon, and some he might, with 

 more probability, have identified with krypton or xenon rays, if he had 

 been aware of the publication of wave lengths of the spectra of those 

 gases, and the identification of one of the highest rays of krypton 

 with that most characteristic of auroras. The rosy tint often seen in 

 auroras, particularly in the streamers, appears to be due mainly to 

 neon, of which the spectrum is remarkably rich in red and orange 

 rays. One or two neon rays are among those most f requenth' observed, 

 while the red ray of hydrogen and one red ray of krypton have been 

 noticed onl}'^ once. The predominance of neon is not surprising, 

 seeing that from its relatively greater proportion in air and its low 

 density it nuist tend to concentrate at higher elevations. So large a 

 number of proba))le identiffcations warrants the belief that we may 

 yet be able to reproduce in our laboratories the auroral spectrum in 

 its entirety. It is true that we have still to account for the appear- 

 ance of some, and the absence of other, rays of the newly discovered 

 gases, which in the way in which we stimulate them appear to be 

 equally brilliant, and for the absence, with one doubtful exception, of 

 all the rays of nitrogen. If we can not give the reason of this, it is 

 because we do not know the mechanism of luminescence — nor even 

 whether the particles which carry the electricity are themselves lumi- 

 nous, or whether they only produce stresses causing other particles 



