8 o 4 THE POPULAR SCIENCE MONTHLY. 



Colored rays shoot forth therefrom in all directions. They inter- 

 lace and intertwine until they seem to be woven into one flut- 

 tering band of color. And anon it changes to a sea of fire ! The 

 rays leap upward far above the zenith, form there a flaming crown, 

 and then sink back, to begin anew the wondrous play. The most 

 beautiful descriptions and trustworthy observations we owe to 

 Nordenskjold and Ekama. All explanations which have been 

 attempted as to the origin and the cause of northern lights are 

 based on analysis by the spectroscope, on the determination of the 

 elevation, and on observations of the peculiar behavior of the 

 magnetic needle. 



If northern lights are observed through a spectroscope, a char- 

 acteristic yellowish-green line will always be seen, no matter how 

 faint the light may be. If the phenomena are well pronounced, 

 several red lines appear in the spectrum. What inferences may 

 be drawn from these observations ? 



If a very powerful electric current is sent through what is 

 called a Geissler tube filled with dried and rarefied air, sparks will 

 flash from one platinum wire to the other, and if these are exam- 

 ined by the spectroscope they show the characteristic yellowish- 

 green line. In a tube filled with nitrogen these lines are especially 

 well pronounced. As nitrogen is the predominant constituent of 

 the air, we infer the northern lights to be a phenomenon similar to 

 the glowing of the electric spark in dry and rarefied air, in which 

 oxygen and nitrogen are the sole conductors of electricity. If 

 white sunlight is caused to shine on a solution of chlorophyl (the 

 green coloring matter of leaves), it shows a blood-red color. This 

 phenomenon, when the reflected light appears of a different color 

 from that which is transmitted, is termed fluorescence. Electric 

 light possesses to a marked degree the property of calling forth 

 fluorescence, as can easily be demonstrated by various experi- 

 ments. It may hence be assumed that the red lines in the spec- 

 trum of northern lights are due to the fluorescence of oxygen in 

 the upper layers of air, caused by powerful electric discharges. 



The determination of the elevation of northern lights serves to 

 confirm the foregoing conclusions, for it is a well-known fact that 

 in the higher regions of the atmosphere the air is dry and rarefied. 

 The height of the auroral crown has been found to be five hundred 

 kilometres, the lower boundary being about seventy kilometres. 



Of greater importance for ascertaining the true nature of north- 

 ern lights has been the demonstration of the relation these phe- 

 nomena hold to terrestrial magnetism, a mysterious power of 

 which our globe is the conductor. Powerful magnetic currents 

 appear suddenly and disappear as quickly, and we are not able to 

 name the cause of their existence. 



Like a vast ocean the magnetic force is constantly surging 



