1902.] on Problems uf the Atmosphtre. 229 



The rosy tint often seen in auroras, particularly in the streamers, 

 appears to be due mainly to neon, of which the spectrum is remark- 

 ably 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 predomin- 

 ance 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 identifications 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 stimulate 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 luminescence 

 — nor even when the particles which carry the electricity are them- 

 selves luminous, or whether they only produce stresses causing other 

 particles which encounter them to vibrate ; yet we are certain that an 

 electric discbarge 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 

 number of trigonometrical measurements that the average above the 

 ground of the base of the aurora was fifty kilometres (thirty-four 

 miles) at Cape Thorsden, Spitsbergen ; at this height the pressure 

 of the nitrogen of the atmosphere would be only about one-tenth of a 

 millimetre, and Moissan and Deslandres have found that in atmo- 

 spheric air at pressures less than one millimetre the rays of nitrogen 

 and oxygen fade and are replaced by those of argon and by five new 

 rays which Stassano identifies with rays of the more volatile gases 

 measured by us. Also Collie and Earn say's observations on the 

 distance to which electrical discharges of equal potential traverse 

 diflferent gases throw much light on the question. They find that, 

 while for helium and neon this distance is from 250 to 300 mm., for 

 argon it is 45 J mm., for hydrogen it is 39 mm., and for air and oxygen 

 still less. 



This indicates that a good deal depends on the very constitution 

 of the gases themselves, and certainly helps us to understand why 

 neon and argon, which exist in the atmosphere in larger proportions 

 than helium, krypton, or xenon, should make their appearance in the 

 spectrum of auroras almost to the exclusion of nitrogen and oxygen. 



How much depends not only on the constitution, and it may be 

 temperature, of the gases, but also on the character of the electric 

 discbarge, is evident from the difference between the spectra at 

 the cathode and anode in different gases, notably in nitrogen and 

 argon, and not less remarkably in the more volatile compounds of 

 the atmosphere. 



