COMETS' TAILS. 273 



no atmosphere, so that the particles which reach her give up their 

 negative charge to her directly, and it spreads equally all oyer her sur- 

 face. When in turn she herself discharges the particles, it will be uni- 

 formly in all directions, and she should appear surrounded with a 

 uniform sheath. Possibly this sheath of cosmical dust affords the 

 reason that in a lunar eclipse the shadow of the earth can be traced a 

 short distance beyond the limb of the moon on each side. 



The Aurora Borealis. 



Perhaps the most interesting application of Arrhenius' theory is his 

 explanation of the Aurora. In a well-known experiment the streams 

 of negative particles forming kathode rays in a Crookes tube are ex- 

 posed to a magnetic field, when they are seen to describe helices round 

 the lines of force. If the field is powerful enough, they may thus be 

 bent into a complete circle inside a moderately large tube. 



Now the negative particles discharged from the sun arrive most 

 thickly over the equatorial regions of the earth, which are most directly 

 exposed to him. Long before they reach any atmosphere dense enough 

 to excite luminescence, they are caught by the lines of force of the 

 earth's magnetic field, which are horizontal over the equator, and have 

 to follow them, winding round them in helices whose radii are so much 

 less than their height above us that the effect to a beholder on the earth 

 is as if they moved along the lines of force. Over the equator there is 

 little luminescence, for lack of atmosphere. But as the lines of force 

 travel north and south, they dip downwards making for the magnetic 

 poles, over which they stand vertical. Soon the particles find them- 

 selves in lower layers of the atmosphere, comparable in density with 

 our highest artificial vacua, and begin to give out the darting and 

 shifting lights of the kathode ray. But this can only be at the cost of 

 absorption, and by the time the denser layers of air are reached, their 

 energy is exhausted. Hence the dark circles round the magnetic poles 

 from which, as from behind a curtain, the leaping pillars of the Aurora 

 rise. From this point of view it is significant that Dr. Adam Paulsen, 

 who has made a special study of the northern lights, found so many 

 points of correspondence between them and kathode rays that in 1894 

 he was led to regard the aurora as a special case of the latter, though 

 unable to give any account of their origin in the upper atmosphere, 

 such as is supplied by Arrhenius' theory. 



The most obvious test to which we can subject such a theory is to 

 ask from it some explanation of the very remarkable periodic variations 

 in the frequency of aurorge. If they are caused by streams of particles 

 ejected from the sun, there should be some connection between the 

 changes in the sun's activity, as indicated by the number of sunspots, 

 and the number of aurorae observed. Again, since a negative charge 



VOL. LX. — 18. 



