42 NATIONAL ACADEMY OF SCIENCES 



suppose that electrified particles, shot out from the sun with 

 great velocity, are drawn in toward the earth's magnetic poles 

 along the lines of force. Striking the rarified gases of the upper 

 atmosphere, they illuminate them, just as the electric discharge 

 lights up a vacuum tube. There is reason to believe that the 

 highest part of the earth's atmosphere consists of rarified hydro- 

 gen, while nitrogen predominates at a lower level. Some of 

 the electrons from the sun are absorbed in the hydrogen, above 

 a height of 60 miles. Others reach the lower-lying nitrogen, and 

 descend to levels from 30 to 40 miles above the earth's surface. 

 Certain still more penetrating rays sometimes reach an altitude 

 of 25 miles, the lowest hitherto found for the aurora. The pas- 

 sage through the atmosphere of the electrons which cause the 

 aurora also gives rise to the irregular disturbances of the mag- 

 netic needle observed during magnetic storms. 



The outflow of electrons from the sun never ceases, if we may 

 reason from the fact that the night sky is at all times feebly 

 illuminated by the characteristic light of the aurora. But when 

 sun-spots are numerous, the discharge of electrons is most violent, 

 thus explaining the frequency of brilliant auroras and intense 

 magnetic storms during sun-spot maxima. It should be remarked 

 that the discharge of electrons does not necessarily occur from 

 the spots themselves, but rather from the eruptive regions sur- 

 rounding them. 



Our acquaintance with vacuum tube discharges dates from an 

 early period, but accurate knowledge of these phenomena may 

 be said to begin with the work of Sir William Crookes in 1876. 

 A glass tube, fitted with electrodes, and filled with any gas, is 

 exhausted with a suitable pump until the pressure within it is 

 very low. When a high voltage discharge is passed through 

 the tube, a stream of negatively charged particles is shot out 

 from the cathode, or negative pole, with great velocity. These 

 electrons, bombarding the molecules of the gas within the tube, 

 produce a brilliant illumination, the character of which depends 

 upon the nature of the gas. The rare hydrogen gas in the upper 

 atmosphere of the earth, when bombarded by electrons from the 



