EARTH AND SUN AS MAGNETS HALE. 149 



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 hydrogen, 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 passage 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 tlie night sl^ is at all times feebly illumi- 

 nated by the characteristic light of the aurora. But when sun spots 

 are numerous, the discharge of electrons is most violent, thus ex- 

 plaining 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 surrounding 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 1870. 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. \Vlien 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 brillant 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 sun, glows like the hydrogen in this tube. Nitrogen, which 

 is characteristic of a lower level, shines with the light which can be 

 duplicated here. 



But it may be remarked that this explanation of the aurora is 

 only hypothetical, in the absence of direct evidence of the emission 

 of electrons by the sun. However, we do know that hot bodies emit 

 electrons. Here is a carbon filament in an exhausted bulb. When 

 heated white hot a stream of electrons passes off. Falling upon this 

 electrode the electrons discharge the electroscope with which it is 

 connected. Everyone Avho has to discard old incandescent lamps 

 is familiar with the result of this outflow. The blackening of the 

 bulbs is due to finely divided carbon carried away by the electrons 

 and deposited upon the glass. 



Now, we know that great quantities of carbon in a vaporous state 

 exist in the sun and that many other substances also present there 



