150 ANNUAL REPOET SMITHSONIAN INSTITUTION, 1913. 



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 6,000° and 7,000° C, twice 

 as high as we can obtain by artificial means. Under solar condi- 

 tions, the velocity of the electrons emitted in regions where the pres- 

 sure 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 condi- 

 tions in the sun and earth. Much alike in chemical composition, these 

 bodies differ principally in size, in density, and in temperature. The 

 diameter of the sun is more than 100 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 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 magnet 

 to a red heat it loses its magnetism and drops the iron bar which it 

 previously supported. Hence, while some theories attribute ter- 

 restrial magnetism -to the presence within the earth of permanent 

 magnets, no such theory can apply to the sun. If magnetic phe- 

 nomena are to be found there they must result from other causes. 



The familiar case of the helix illustrates how a magnetic field is 

 produced by an electric current flowing through a coil of wire. But 

 according to the modern theory, an electric current is a stream of 

 electrons. Thus a stream of electrons in the sun should give rise to a 

 magnetic field. If the electrons were whirled in a powerful vortex, 

 resembling our tornadoes or waterspouts, the analogy with the wire 

 helix would be exact, and the magnetic field might be sufficiently 

 intense to be detected by spectroscopic observations. 



A sun spot, as seen with a telescope or photogTaphed in the ordi- 

 nary way, does not appear to be a vortex. If we examine the solar 

 atmosphere above and about the spots, we find extensive clouds of 

 luminous calcium vapor, invisible to the eye, but easily photographed 

 with the spectroheliograph by admitting no light to the sensitive 

 plate except that radiated by calcium vapor. These calcium flocculi 

 (fig. 4), like the cumulus clouds of the earth's atmosphere, exhibit no 

 well-defined linear structure. But if we photograph the sun with the 

 i-ed light of hydrogen, we find a very different condition of affairs 

 (fig. 5). In this higher region of the solar atmosphere, first photo- 

 graphed on Mount Wilson in 1908, cyclonic whirls, centering in sun 

 spots, are clearly shown. 



