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SCIENCE. 



[N. S. Vol. XVI. No. 407. 



has shown no diifer.ence in composition 

 from that at the ground, whereas, accord- 

 ing 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 dif- 

 ferent layers of the atmosphere is still ta- 

 king place at this elevation. This is eon- 

 firmed by a study of the motions of clouds 

 about six miles high, which reveals an aver- 

 age velocity of the air currents of some 

 seventy miles an hour; such violent winds 

 must be the means of causing the inter- 

 mingling 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 low- 

 er layers of atmosphere are carried to a 

 great elevation. The existence of an atmos- 

 phere at more than a hundred miles above 

 the surface of the earth is revealed to us 

 by the appearance of meteors and fireballs, 

 and when we can take photographs of the 

 spectrum of such apparitions we shall learn 

 a great deal about the composition of the 

 upper air. In the meantime Pickering's 

 solitary spectriun of a meteor reveals an 

 atmosphere of hydrogen and helium, and 

 so far this is corroborative of the doctrine 

 we have been discussing. It has long been 

 recognized that the aurora is the result of 

 electric discharges within the limits of the 

 earth's atmosphere, but it was difficult to 

 imderstand why its spectrum should be so 

 entirely different from anything which 

 could be produced artificially by electric 

 discharges through rarefied air at the sur- 

 face of the earth. Writing in 1879, Rand 

 Capron, 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 un- 

 der an electric discharge. Vogel attributed 



this want of agreement between nature and 

 experiment, in a vague way, to difference 

 of temperature and pressure; and ZoUner 

 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 state- 

 ments were merely confessions of igno- 

 rance. But since that time observations of 

 the spectra of auroras have been greatly 

 multiplied, chiefly through the Swedish 

 and Danish Polar Expeditions, and the 

 length of 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 Stas- 

 sano is able to enumerate upwards of one 

 hundred auroral rays, of which the wave- 

 length is more or less approximately 

 known, some of them far in the ultra- 

 violet. 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 Liveing and my- 

 self have observed to be emitted by the 

 most volatile gases of atmospheric air un- 

 liquefiable at the temperature of liquid 

 hydrogen. Most of the remainder he 

 ascribes to argon, and some he might, with 

 more probability, have identified with 

 krypton or xenon rays, if he had been 

 aware of the publication of wave-lengths 

 of the spectra of those gases, and the iden- 

 tification of one of the highest rays of 

 krypton with that most characteristic of 

 auroras. The rosy tint often seen in au- 

 roras, particularly in the streamers, ap- 

 pears to be due mainly to neon, of which 

 the spectrum is remarliably 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 predominance of neon is not 

 surprising, seeing that from its relatively 

 greater proportion in air and its low den- 



