238 THE NEW KNOWLEDGE. 



of the atmosphere so that it will stream past the earth on 

 each side in a hyperbolic orbit. Far out in space on the 

 side away from the sun they will meet with other particles. 

 We have shown, page 63, that corpuscles are remarkable 

 in being able to act as nuclei about which other corpuscles 

 and atoms and molecules will collect. When they meet, 

 therefore, with other particles back of the earth, this con- 

 densation will take place and if the particles formed are 

 greater than the critical diameter which the pressure of 

 light can control they will drift back with increasing velocity 

 past the earth and towards the sun. If we could take our 

 stand on the moon we should probably see the earth at- 

 tended by a faint double tail the more conspicuous one 

 pointing away from the sun and a fainter one pointing 

 toward him. It is this sheaf of light on each side of the 

 earth, due to a corpuscular bombardment from the sun, 

 which, in the opinion of Arrhenius, is the cause of the zodi- 

 acal light. It should be stated here that Arrhenius is the 

 father of nearly all this theoretical interpretation of the ob- 

 served mechanical pressure of light. 



THE AURORA BOREALIS. 



The Aurora Borealis has been still another mystery. 

 We shall see that the new knowledge has something to say 

 as to its elucidation. In Part III it was shown that the 

 corpuscles from red-hot carbon, Crookes' tubes and the 

 beta-rays of radium are deflected by a magnet in such a 

 way that they describe helices around the lines of magnetic 

 force. If the magnetic field is strong enough they may be 

 bent into a complete circle inside a moderately large tube, 

 page 57. If there is such a thing as the continuity of 

 natural law, the earth should be bombarded by these cor- 

 puscles projected from the sun. But the earth is a magnet, 



