42 REPORT— 1902. 



different from anything which could be produced artificially by electric 

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

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

 able to enumerate no more than nine auroral rays, of which but one could 

 with any probability be identified with rays emitted by atmospheric air 

 under an electric discharge. Vogel attributed this want of agreement 

 between nature and experiment, in a vague way, to difference of tempera- 

 ture and pressure ; and ZoUner 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 

 ultra-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 upwards of one hundred auroral rays, of which the wave-length 

 is more or less approximately 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 

 Liveing 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 amongst those most frequently observed, while the 

 red ray of hydrogen and one red ray of krypton have been noticed only 

 once. The predominance of neon is not surprising, seeing that from its 

 relatively greater proportion in air and its low density it must tend to 

 concentrate at higher elevations. So large a number of probable identifi- 

 cations 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 appearance of some, and the absence of other, rays 

 of the newly discovered gases, which in the way in which we stimu- 

 late them appear to be equally brilliant, and for the absence, with 

 one doubtful exception, of all the rays of nitrogen. If we cannot give 

 the reason of this, it is because we do not know the mechanism of lumi- 

 nescence — nor even whether the particles which carry the electricity 

 are themselves luminous, or whether they only produce stresses causing 

 other particles which encounter them to vibrate ; yet we are certain 

 that an electric discharge in a highly rarefied mixture of gases lights one 

 element and not another, in a way which, to our ignorance, seems 

 capricious. The Swedish North Polar Expedition concluded from a great 



