OCTOBEE 17, 1902.] 



SCIENCE. 



625 



sity it must tend to concentrate at higher 

 elevations. So large a number of probable 

 identifications warrants the belief that we 

 may j^et be able to reproduce in our labo- 

 ratories 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 excep- 

 tion, 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 en- 

 counter them to vibrate ; yet we are certain 

 that an electric discharge in a highly rare- 

 fied 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 measure- 

 ments that the average above the ground 

 of the base of the aurora was fifty kilo- 

 meters (thirty-four miles) at Cape Thors- 

 den, Spitzbergen; at this height the pres- 

 sure of the nitrogen of the atmosphere 

 would be only about one tenth of a milli- 

 meter, and Moissan and Deslandres have 

 found that in atmospheric air at pressures 

 less than one millimeter the rays of nitro- 

 gen 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 Ramsay's observations on the distance 

 to which electrical discharges of equal po- 

 tential 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 45J mm., for hydrogen it is 39 mm., 

 and for air and oxygen still less. This in- 



dicates 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 atmos- 

 phere in larger proportions than helium, 

 krypton or xenon, should make their ap- 

 pearance in the spectrum of auroras almost 

 to the exclusion of nitrogen and oxygen. 

 How much depends not only on the consti- 

 tution and it may be temperature of the 

 gases, but also on the character of the 

 electric discharge, is evident from the dif- 

 ference between the spectra at the cathode 

 and anode in different gases, notably in 

 nitrogen and argon, and not less remark- 

 ably 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 tem- 

 perature have not borne out the explana- 

 tion. Schuster and Hemsalech have shown 

 that these changes of spectra are in part 

 due to the oscillatory character of the con- 

 denser 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 an- 

 alysis 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 nitro- 

 gen and oxygen, while three hydrogen rays 

 most likely due to water are very conspicu- 



