162 SECTION PHYSICS. 



is caused by the fact that the very source of electricity in the atmo- 

 sphere, that is to say, the evaporation, is very much reduced ; however, 

 another important cause is here active namely, the heightened con- 

 ductive power that the air possesses in consequence of its greater 

 quantity of moisture, whereby the electricity becomes unable to keep 

 itself, beyond a certain latitude, upon the clouds, until it has attained 

 a greater tension, but is conducted down to the earth in form of a slow 

 current, visible in the polar light. 



It results from experience, with a high degree of probability, that 

 the polar light is an electric phenomenon, for its effects are of the 

 same nature as those of the electric currents. Thus the polar light 

 causes disturbances in terrestrial magnetism, induces currents in the 

 telegraphic wires, and furnishes a spectrum of nine bands, which 

 coincide, except one, with the spectral lines produced if an electric 

 current goes through a rarefied space of air. Thus there is no doubt 

 that the polar light is caused by an electric current going down from 

 the upper rarefied layers of air to the earth ; this current, during its 

 passage through the rarefied air, produces light phenomena that cannot 

 arise in denser layers of air. 



The polar-light apparatus now exhibited shows that an electric 

 current flowing out from an insulated body does not produce any light 

 phenomena in air of normal pressure, but as soon as it rises to the 

 rarefied air in the Geissler's tubes, there is directly produced a light 

 phenomenon very like the real polar light. In the apparatus the 

 upper end of the tubes is in union with the earth ; this is by no means 

 necessary, for the light phenomenon is also produced if this union be 

 removed, provided that in such case the tubes be brought a little 

 nearer to the insulated sphere. For the rest, the earth represents here 

 the wide space of rarefied air that we find beyond the limits of the 

 conductor of air, and which serves here as an electric reservoir. 



Let us now consider how the polar light on a large scale is formed 

 in nature. As before said, the earth, and the conductor of air, hold 

 to each other the position above-mentioned, and the two electricities, 

 the negative electricity of the earth and the positive electricity of the 

 conductor of air, endeavour with a certain force to unite in a belt 

 around the north pole. The insulating power of the denser air 

 prevents this reunion ; but if we assume that perfect equilibrium is 



