no atmosphere, so that the particles which reach her give up their negative charge to her directly, and it spreads equally all over her surface. When in turn she herself discharges the particles, it will be uniformly 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 exposed 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 themselves 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 auroræ. 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 auroræ observed. Again, since a negative charge