8o6 THE POPULAR SCIENCE MONTHLY. 



of air as the separating medium. An actual proof that the north- 

 ern lights are caused by electric currents in the atmosphere was 

 attempted by Lemstrom in 1883. He covered the plateaus of two 

 mountains in northern Finland with a network of copper wires 

 raised several metres above the ground and provided it with hun- 

 dreds of metal points. The whole was insulated and connected 

 with a zinc plate buried in damp ground in the plain below. A 

 continuous electric current from the air to the ground was notice- 

 able, and a light which appeared hovering over the metal points 

 showed, when examined by the spectroscope, the characteristic 

 line of the auroral spectrum. 



The theories, however, according to which the northern lights 

 are a flowing together of terrestrial and atmospheric electricity 

 of opposite kinds, leave unanswered the question as to the origin 

 of these electric fluids. As no adequate cause could be found on 

 the globe for such a tremendous evolution of electricity, attention 

 was directed to the sun as the source of it all. Why should not 

 Helios, the giver of all light and life of our world, be as well the 

 creator of that inexhaustible force of nature that is revealed in 

 the splendors of the northern lights ? 



As the endless supply of light and heat which is radiated into 

 space by the sun, is accounted for by the contraction of that body, 

 this may also be assigned as the cause of the stupendous genera- 

 tion of electricity. According to the theory of Kant and Laplace, 

 the sun and other heavenly bodies are assumed to have been 

 formed by the condensation of vapors which originally filled all 

 space. This condensation is still going on in the sun in conse- 

 quence of the enormous radiation of heat into space, and with it 

 the consequent contraction. 



Possibly also there might be suggested as a cause the cooling 

 process which the sun is undergoing. It may be assumed, too, 

 that vast amounts of electricity are hurled into space with the 

 ignited masses of gas, whose eruption from the sun may be con- 

 stantly observed. But it is more probable that the sun acts upon 

 the earth by induction. Try the following experiment : Two insu- 

 lated spheres are placed near one another, but without being in 

 contact. On one of these spheres a bar of metal is placed, to which 

 there is fastened a screen made of some good conducting material. 

 If one of the spheres is charged with a certain kind of electricity, 

 say, for instance, negative electricity, the opposite kind in this 

 case positive electricity will, by induction, be generated on the 

 other sphere. A corresponding amount of negative electricity 

 will in the mean time be discharged on to the screen. An action 

 similar to this may be assumed to be going on between the sun 

 and the earth. 



The sun's electricity, which may be assumed to be negative on 



