COMETS' TAILS. 271 



have I any theory to propose to account for the certain conriection between dis- 

 turbances of the solar surface and of terrestrial magnetism. 



The words we have underlined in this passage have almost a Sopho- 

 clean irony to a reader acquainted with the further developments of 

 Arrhenius' theory to which we now turn. 



The Zodiacal Light and the Gegenschein. 



Not only is the sun the source of those eruptions of ordinary matter 

 which form the prominences, but we have every reason to be believe 

 that he must emit streams of electrically charged corpuscles both 

 directly, as a hot body, and indirectly, since the electrical discharges 

 which, according to all terrestrial analogies, must accompany the violent 

 chemical actions going on near his surface, will, when they take place in 

 the higher and rarer regions of his atmosphere, give rise to kathode 

 rays, and these, in turn, to Rontgen rays. As Professor Thomson says : 

 "As a very hot metal emits these corpuscles, it does not seem an improb- 

 able hypothesis that they are emitted by that very hot body, the sun," 



Now the negatively charged corpuscles are preeminently fitted to 

 serve as nuclei for the condensation of the ordinary matter. Hence 

 those particles of the latter which, having more than the critical diam- 

 eter, fall back to the sun, will carry back a negative charge to him; 

 while those which have less than the critical diameter will carry a 

 negative charge off into space. On both counts the corona will be left 

 with a surplus of positive charge. The same arguments hold for the 

 vapors emitted by the nucleus of a comet. Thus comets' tails should 

 consist of negatively charged particles. 



Let us follow the career of the particles launched into space. They 

 proceed radially from the sun above the regions of sunspots with 

 rapidly increasing speed, which, however, may be shown to approach a 

 finite limit at a distance of about ten radii from the sun. If they en- 

 counter another body, such as the earth, they charge its outer atmos- 

 phere negatively, and when this charge reaches a certain value, 

 it will begin to repel them. The oncoming rush will be de- 

 flected, and stream past the earth on each side in hyperbolic orbits. 

 Far out in space they must sooner or later meet particles from other 

 bodies, and, if by collision or aggregation they increase beyond the crit- 

 ical diameter, they will first lose speed and then drift back with ever 

 increasing velocity past the earth, directly towards the sun. The space 

 immediately behind the earth would be screened by her, and so be void 

 of particles. Could we take our stand on the moon, we should thus see 

 the earth attended by a faint double tail with a dark dividing line Cso 

 conspicuous a feature in comets), immediately behind it, pointing from 

 the sun; and a similar, though perhaps fainter, tail, pointing towards 

 him. Not only so, but the earth helps to form her own tail. For when 



