222 ESSAYS BIOGRAPHICAL AND CHEMICAL 



upward ascent at the equator, and therefore tend to con- 

 centrate in the ascending current as it reaches the con- 

 fines of the atmosphere, owing to the more rapid escape 

 of oxygen and nitrogen by diffusion. 3. The phenomena 

 are reversed in the descending currents at the poles, and 

 the argon gases tend to mix with neighbouring layers of 

 gas at high altitudes. 4. The higher strata of the atmo- 

 sphere are probably richer in the inactive gases than the 

 lower strata. 



As electric discharges producing the aurora certainly 

 occur at great altitudes, the spectra seen are those of the 

 inactive gases; and owing to the fact that the krypton 

 green line, of wave-length 5570 units,, is remarkably easily 

 visible, even in an admixture of other gases, it happens to 

 be the most conspicuous line in the auroral spectrum. 



This leads us to consider, in the last place, the cause of 

 the electric current. And here we enter on a different 

 region of thought. 



There are two theories on the subject, one due to Pro- 

 fessor Birkeland, 1 of Christiania, the other to Professor 

 Arrhenius, 2 of Stockholm. It has long been known that 

 violet light rays and the invisible rays of the spectrum 

 beyond the violet, which can be detected by photography, 

 have the property of discharging a negatively electrified 

 body. It is suggested by Professor Birkeland that the 

 spots on the sun are caused by solar eruptions, or, to use a 

 familiar word, volcanoes ; and that the sun then emits 

 negatively charged corpuscles, similar to those which are 

 believed to constitute, partly at least, the cathode rays 

 rays producing those utilised for surgical practice in taking 

 photographs of bones. Birkeland supposes that such 

 corpuscles are ' sucked in ' to the earth's magnetic poles, 

 giving rise to vortices of electric currents in the upper 



1 Archives des Sciences Phys. et Nat. tie Geneve, June 1896. 



2 Physikalische Zeitschrift, ii. Nos. 6 and 7. 



