COMETS, THE SOLAR CORONA, AND THE AURORA, 51 



not the tails of comets and the corona also be electric 

 phenomena ? Their appearance and behaviour correspond 

 exactly with those of the aurora; and there is surely nothing 

 very difficult in imagining the sun, which is the source of 

 so much heat, being also the source of some electricity. 

 Neither will there appear any thing wonderful in the elec- 

 tricity of comets when we consider that of the earth. "We 

 must not look on our inability to explain the cause of such 

 an electric discharge as fatal to its existence ; for we cannot 

 any more explain the existence of the electricity which 

 causes the aurora. If we cannot explain whence these 

 electricities come, we can at least show that the con- 

 ditions which are most favourable to the development 

 of the aurora exist in much greater force on the comets 

 than they do on the earth. The greatest development of 

 the aurora borealis takes place at the equinoxes. There 

 is a cessation in summer, and another in winter. Now 

 the equinoxes are the times when the action of the sun on 

 our northern hemisphere is changing most rapidly. Hence 

 the condition favourable for the aurora is change in the 

 action of the sun. The same thing is pointed out by the 

 diurnal variation in the electricity of the atmosphere; 

 for, as has been already shown, the change in tempera- 

 ture on the comets is incomparably greater than it is on 

 the earth, and its variation corresponds with the variation 

 in the atmosphere of the comet. 



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Angstrom has also shown that the light from the aurora, 

 the corona, and the zodiacal light are all of the same cha- 

 racter, or all give the same bright lines when viewed through 

 the spectroscope, and that these lines correspond to the 

 light from no known substance. This indicates that, what- 

 ever this light may be, the incandescent material is the 

 same in all cases ; or may we not assume that it is the 

 medium which fills space that is illuminated by the electric 

 discharges ? This would be supported by the fact that the 



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