THE SEMI-CENTENNIAL ANNIVERSARY 43 



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 emis- 

 sion 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 electro- 

 scope with which it is connected. Everyone who 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, emit electrons in the same way. Hence we may infer that 

 electrons are abundant in the solar atmosphere. 



The temperature of the sun is between 6000 and 7000 C, 

 twice as high as we can obtain by artificial means. Under solar 

 conditions, the velocity of the electrons emitted in regions where 

 the pressure is not too great may be sufficient to carry them to the 

 earth. Arrhenius holds that the electrons attach themselves to 

 molecules or groups of molecules, and are then driven to the 

 earth by light-pressure. 



In certain regions of the sun, we have strong evidence of the 

 existence of free electrons. This leads us to the question of solar 

 magnetism and suggests a comparison of the very different con- 

 ditions in the sun and earth. Much alike in chemical composi- 

 tion, these bodies differ principally in size, in density and in 

 temperature. The diameter of the sun is more than one hundred 

 times that of the earth, while its density is only one quarter as 

 great. But the most striking point of difference is the high 

 temperature of the sun, which is much more than sufficient to 

 vaporize all known substances. This presumably means that no 

 permanent magnetism, such as is exhibited by a steel magnet or 

 a lodestone, can exist in the sun. For if we bring this steel mag- 



