THE AURORA. 27 



mistaken. "When it shows us such and such lines 

 bright or dark, we may conclude, without fear of being 

 misled, that such and such substances are emitting or 

 absorbing light. "What we learn certainly, therefore, 

 from the facts above stated, is this, that substances of 

 the same sort emit the light of the aurora, of the zodia- 

 cal gleam, of the sun's corona, and of the phosphores- 

 cence which illuminates at times the nocturnal skies. 

 We may conclude, but not so certainly, that the manner 

 in which the light is emitted is also the same in each case. 

 We know certainly that the auroral light is excited by 

 the solar action. We know certainly that it is associ- 

 ated with the earth's magnetism. The opinion, then, 

 which we should form of the source to which the other 

 lights are due is tolerably obvious. So long as elec- 

 tricity was merely used as a convenient way of account- 

 ing for any perplexing phenomenon, it was impossible 

 to accept explanations of cosmical peculiarities as due 

 to electrical action. But when once we have reason 

 as in the case of the aurora we undoubtedly have to 

 associate electricity with any particular form of lumi- 

 nosity, we seem clearly justified in extending the ex- 

 planation to the same form of luminosity wherever 

 it may appear. 



I believe that the key to the whole series of phe- 

 nomena dealt with above lies in the existence of myriads 

 of meteoric bodies travelling separately or in systems 

 round the sun. They are consumed in thousands 



