ii2 THE POPULAR SCIENCE MONTHLY 



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. Every 

 one 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 6,000° and 7,000° 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 exist- 

 ence of free electrons. This leads us to the question of solar magnetism 

 and suggests a comparison of the very different conditions 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 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 means that no perma- 

 nent 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 sup- 

 ported. Hence, while some theories attribute terrestrial magnetism to 

 the presence within the earth of permanent magnets, no such theory can 

 apply to the sun. If magnetic phenomena 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 elec- 

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

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

 resembling our tornadoes or water-spouts, 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 photographed in the ordinary 

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

 phere above and about the spots, we find extensive clouds of luminous 

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



