THE AURORA BOREALIS 91 



good imitation aurora borealis on a small scale, 

 with beautiful rosy and green tints, due to the 

 gases called "neon" and "krypton." The pre- 

 sumption is therefore very strong that the aurora 

 borealis is caused by electric discharges — dis- 

 charges of electrons through the upper layers of 

 the atmosphere. But where do the electrons come 

 from ? It has been proved that all hot bodies, 

 and especially glowing carbon, give off electrons. 

 Now, the sun is a hot body, and its photosphere 

 contains enormous quantities of glowing carbon. 

 Accordingly — the hypothesis is as plausible as it 

 is audacious — the electrons probably come ninety- 

 three million miles from the sun. At the rate they 

 travel — twenty miles or so a second — it won't take 

 them long. 



But ninety-three million miles is a considerable 

 way to come, and one may well ask for some particu- 

 lars of the journey. Electrons, or minute particles 

 containing negative electricity, are certainly emitted 

 by the sun, but would they not be drawn back 

 again by gravitation and by the sun's positive 

 electricity ? Under ordinary circumstances the 

 positive electricity and gravitation would hold the 

 negative particles ; but there is another factor to 

 be reckoned with, namely, the mechanical pressure 

 of light. That light has mechanical pressure was 

 proved mathematically by Clerk-Maxwell in 1873, 



