A HISTORY OF SCIENCE 



years ago. There has been no dearth of theories con- 

 cerning it, however. Biot, who studied it in the Shet- 

 land Islands in 1817, thought it due to electrified 

 ferruginous dust, the origin of which he ascribed to 

 Icelandic volcanoes. 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 phe- 

 nomena 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 au- 

 rora is of electrical origin, but the subject still awaits 

 complete elucidation. For once even that mystery- 

 solver the spectroscope has been baffled, for the line it 

 sifts from the aurora is not matched by that of any 

 recognized substance. A like line is found in the zo- 



174 



