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 the various periodic 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 currents 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 corpuscles 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 electrification 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 nebulæ 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 nebulæ being quite cold. This view seems in some respects to have advantages over that which supposes the nebulæ 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 play an important part in cosmical as well as in terrestrial physics.