228 Professor Dewar [April 11, 



elevation of 9 miles has shown no difference in composition from 

 that at the ground, whereas, according to our hypothesis, the oxygen 

 ought to have been diminished to 17 per cent., and the carbonic acid 

 should also have become much less. This can only be explained by 

 assuming that a large intermixture of different layers of the atmo- 

 sphere is still taking place at this elevation. This is confirmed by a 

 study of the motions of clouds about six miles high, which reveals an 

 average velocity of the air currents of some seventy miles an hour ; 

 such violent winds must be the means of causing the intermingling 

 of different atmospheric strata. Some clouds, however, during hot and 

 thundery weather, have been seen to reach an elevation of seventeen 

 miles, so that we have direct proof that on occasion the lower layers 

 of atmosphere are carried to a great elevation. 



The existence of an atmosphere at more than a hundred miles 

 above the surface of the earth is revealed to us by the phenomenon of 

 twilight and the luminosity of meteors and fireballs. When we can 

 take photographs of meteoric spectra, a great deal may be learnt 

 about the composition of the upper air. In the meantime Pickering's 

 solitary spectrum of a meteor reveals an atmosphere of hydrogen and 

 helium, and so far this is a corroboration of the doctrine we have been 

 discussing. It has long been recognised that the aurora is the result 

 of electric discharges within the limits of the earth's atmosphere, 

 but it was difficult to understand why its spectrum should be so 

 entirely different from anything which could be produced artificially 

 by electric discharges through rarefied air at the surface of the 

 earth. Eand Capron, in 1879, 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 electric discharge. Vogel attri- 

 buted this want of agreement between nature and experiment, in a 

 vague way, to difference of temperature and pressure ; and Zollner 

 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. 

 The 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. 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 Liveingand myself have observed 

 to bu emitted by the most volatile gases of atmospheric air unlique- 

 fiablc at the temperature of liquid hydrogen. Most of the remainder 

 he ascribes to argon, and some might, with more probability, have 

 been identified with krypton or xenon. 



