ADDRESS. 43 



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, Spitzbergen ; 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 atmospheric air at pressures 

 less than one millimetre the rays of nitrogen and oxygen fade and are 

 replaced by those of argon and by live new rays which Stassano 

 identifies with rays of the more volatile gases measured by us. Also 

 Collie and Ilamsay's observations on the distance to which electrical 

 discharges of equal potential traverse different gases explosively throw 

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

 this distance is from 250 to .300 mm., for argon it is 45^ 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 discharge, is evident from the difierence between 

 the spectra at the C3.thode and anode in different gases, notably in 

 nitrogen and argon, and not less remarkably 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 by them. It has long 

 been known that in many spectra the rays which are inconspicuous 

 with an uncondensed electric discharge 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 Hem- 

 salech have shown that these changes of spectra are in part due to the 

 oscillatory character of the condenser discharge which may be enhanced 

 by self-induction, and the corresponding change of spectrum thereby 

 made more pronounced. Lightning we should expect to resemble 

 condensed discharge much more than aurora, but this is not borne out by 

 the spectrum. Pickering's recent analysis of the spectrum of a flash 

 obtained by photography shows, out of nineteen lines measured by him, 

 only two which can be assigned with probability to nitrogen and oxygen, 

 while three hydrogen rays most likely due to water are very conspicuous, 

 and eleven may be reasonably ascribed to argon, krypton, and xenon, one 

 to more volatile gas of the neon class, and the brightest ray of all is but a 

 very little less refrangible than the characteristic auroral ray, and coincides 

 with a strong ray of calcium, but also lies between, and close to, an argon 

 and a neon ray, neither of them weak rays. There may be .some doubt about 

 the identification of the spectral rays of auroras because of the wide limits 



