Page:Popular Science Monthly Volume 60.djvu/279

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COMETS' TAILS.
have I any theory to propose to account for the certain connection between disturbances of the solar surface and of terrestrial magnetism.

The words we have underlined in this passage have almost a Sophoclean 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 Röntgen rays. As Professor Thomson says: "As a very hot metal emits these corpuscles, it does not seem an improbable 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 diameter, 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 encounter another body, such as the earth, they charge its outer atmosphere negatively, and when this charge reaches a certain value, it will begin to repel them. The oncoming rush will be deflected, 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 critical 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 (so 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