HISTORY OF COLD AND THE ABSOLUTE ZERO. 233 



which encounter them to \'ibnite; yet we ure certain that an electric 

 (lischarg-e in a hio-hly rarefied mixture of g-a.ses lights one element 

 and not another, in a way wliich, to our ignorance, seems .capri- 

 cious. The Swedish North Polar Plxpedition concluded from a great 

 iuun])er oi trigonometrical measurements that the average above the 

 ground of the base of the aurora was 50 kilometers (34 miles) at Cape 

 Thorsden, Spitzbergen; at this height the pressure of the nitrogen of 

 the atmosphere would be only about one-tenth of a millimeter, and 

 Moissan and Deslandres have found that in atmospheric air at pressures 

 less than 1 mm. the rays of nitrogen and oxygen fade and are replaced 

 by those of argon and hy five new rays which Stassano identities with 

 I'ays of the more volatile gases measured by us. Also Collie and 

 Kamsay's observations on the distance to which electrical discharges 

 of equal potential traverse diil'erent gases explosively throw much 

 light on the question; for they find that, while for helium and neon 

 this distance is from :^5(i to 300 mm., for argon it is 45^ nun., for 

 hydrogen it is ol> 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 imderstand why neon and 

 argon, which exist in the atmosphere in larger proportions than helium, 

 kr3q:)ton, or xenon, should make their appearance in the spectrum 

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

 nmcli depends not onl}^ on the constitution and it may be temperature 

 of the gases, but also on the character of the electric discharge, is evi- 

 dent from the diii'erence between the spectra at the cathode and 

 anode in different gases, notably in nitrogen and argon, and not less 

 remarkabl}' in the more volatile compounds of the atmosphere. 

 Paulsen thinks the auroral spectrum wholly due to cathodic rays. 

 Without stopping to discuss that question, it is certain that changes in 

 the character of the electric discharge produce definite changes in the 

 spectra excited \)y them. It has long been known that in many spectra 

 the rays which are inconspicuous with an uncondensed electric dis- 

 charge become very pronounced when a Leyden jar is in the circuit. 

 This used to be ascribed to a higher temperature in this condensed 

 spark, though measurements of that temperature have not borne out 

 the explanation. Schuster and Hemsalech have shown tliat these 

 changes of spectra are in part due to the oscillatory cliaracter of the 

 condenser discharge which ma}" be enhanced by self-induction, and 

 the corresponding change of spectrum therel)y made more pronounced. 

 Lightning we should expect to resemble condensed discharge nuich 

 more than aurora, but this is not borne out by the spectrum. Pick- 

 ering's recent analysis of the spectrum of a flash obtained l)y photog- 

 raphy shows, out of nineteen lines measured b}" him, only two which 

 can be assigned with prol)al)ility to nitrogen and oxygen, while three 

 hydrogen rays most likely due to water are very conspicuous, and 



