THE STORY OF NINETEENTH-CENTURY SCIENCE 



Since then a multitude of observers have studied the 

 aurora, but the scientific grasp has found it as elusive in 

 fact as it seems to casual observation, and its exact nat- 

 ure is as undetermined to-day as it was a hundred years 

 ago. There has been no dearth of theories concerning 

 it, however. Biot, who studied it in the Shetland Isl- 

 ands in 1817, thought it due to electrified ferruginous 

 dust, the origin of which he ascribed to Icelandic vol- 

 canoes. Much more recently the idea of ferruginous 

 particles has been revived, their presence being ascribed 

 not to volcanoes, but to the meteorites constantly being 

 dissipated in the upper atmosphere. Ferruginous dust, 

 presumably of such origin, has been found on the polar 

 snows, as well as on the snows of mountain-tops, but 

 whether it could produce the phenomena of auroras is 

 at least an open question. 



Other theorists have explained the aurora as due to 

 the accumulation of electricity on clouds or on spicules 

 of ice in the upper air. Yet others think it due merely 

 to the passage of electricity through rarefied air itself. 

 Humboldt considered the matter settled in yet another 

 way when Faraday showed, in 1831, that magnetism 

 may produce luminous effects. But perhaps the pre- 

 vailing theory of to-day assumes that the aurora is due 

 to a current of electricity generated at the equator, and 

 passing through upper regions of space, to enter the 

 earth at the magnetic poles simply reversing the course 

 which Franklin assumed. 



The similarity of the auroral light to that generated 

 in a vacuum bulb by the passage of electricity lends 

 support to the long-standing supposition that the aurora 

 is of electrical origin, but the subject still awaits com- 

 plete elucidation. For once even that mystery-solver 



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