ON BODIES SMALLER THAN ATOMS. 335 



such highly rarefied gas that they produce but little luminosity, while 

 at the pole, where the magnetic force would pull them straight down 

 into the denser air, there are not nearly so many corpuscles; the 

 maximum luminosity will therefore be somewhere between these places. 

 Arrhenius has worked out this theory of the Aurora very completely 

 and has shown that it affords a very satisfactory explanation of tlie 

 various j^eriodic variations to which it is subject. 



As a gas becomes a conductor of electricity when corpuscles pass 

 through it, the upper regions of the air will conduct, and when air 

 currents occur in these regions, conducting matter will be driven 

 across the lines of force due to the earth's magnetic field, electric cur- 

 rents will be induced in the air, and the magnetic force due to these 

 currents will produce variations in the earth's magnetic field. Balfour 

 Stewart suggested long ago that the variation on the earth's magnetic 

 field was caused b}^ currents in the upper regions of the atmosphere, 

 and Schuster has shown, by the application of Gauss' method, that the 

 seat of these variations is above the surface of the earth. 



The negative charge in the earth's atmosphere will not increase 

 indefinitely in consequence of the stream of negatively electrified cor- 

 puscles coming into it from the sun, for as soon as it gets negatively 

 electrified it begins to repel negatively electrified corpuscles from the 

 ionized gas in the upper regions of the air, and a state of equilibrium 

 will be reached when the earth has such a negative charge that the 

 corpuscles driven by it from the upper regions of the atmosphere are 

 equal in number to those reaching the earth from the sun. Thus, on 

 this view, interplanetary space is thronged with corpuscular traffic, 

 rapidly moving corpuscles coming out from the sun while more slowly 

 moving ones stream into it. 



In the case of a planet which, like the moon, has no atmosphere 

 there will be no gas for the corpuscles to ionize, and the negative elec- 

 trification will increase until it is so intense that the repulsion exerted 

 by it on the corpuscles is great enough to prevent them from reaching 

 the surface of the planet. 



Arrhenius has suggested that the luminosity of nebulte may not be 

 due to high temperature, but may be produced by the passage 

 through their outer regions of the corpuscles wandering about in 

 space, the gas in the nebulae being quite cold. This view seems in 

 some respects to have advantages over that which supposes the nebulge 

 to be at very high temperatures. These and other illustrations, which 

 might be given did space permit, seem to render it probable that these 

 corpuscles may pla)- an important part in cosmical as well as in 

 terrestrial physics. 



