when apparently pure, stands revealed as a dense swarming of millions of motes if a sunbeam passes through it. Even such a fog is out of the question. If we conceive of a fog so attenuated that there is only one minute liquid or solid particle in every cubic mile, we should still have matter enough, in all probability, to form a corona. That the coronal matter is of the nature of a fog is shown by the three kinds of light which the corona sends to us—reflected solar light scattered by particles of matter, solid or liquid; and, secondly, light giving a continuous spectrum, which tells us that these solid or liquid particles are incandescent; while the third form of spectrum of bright lines, fainter and varying greatly at different parts of the corona and at different eclipses, shows the presence also of light-emitting gas. This gas existing between the particles need not necessarily form a true solar atmosphere, which the considerations already mentioned make an almost impossible supposition, for we may well regard this thin gas as carried up with the particles, or even to some extent to be furnished by them under the sun's heat.
It will be better to consider first the probable origin of this coronal matter, and by what means it can find itself at such enormous heights above the sun.
There is another celestial phenomenon, very unlike the corona at first sight, which may furnish us possibly with some clew to its true nature. The head of a large comet presents us with luminous streamers and rifts and curved rays, which are not so very unlike, on a small scale, some of the appearances which are peculiarly characteristic of the corona.[1] We do not know for certain the conditions under which these cometary appearances take place, but the hypothesis which seems on the way to become generally accepted attributes them to electrical disturbances, and especially to a repulsive force acting from the sun, possibly electrical, which varies as the surface, and not, like gravity, as the mass. A force of this nature in the case of highly attenuated matter can easily master the force of gravity, and, as we see in the tails of comets, blow away this thin kind of matter to enormous distances in the very teeth of gravity.
If such a force of repulsion is experienced in comets, it may well be that it is also present in the sun's surroundings. If this force be electrical, it can only come into play when the sun and the matter subjected to it have electric potentials of the same kind, otherwise the attraction on one side of a particle would equal the repulsion on the other. On this theory the coronal matter and the sun's surface must both be in the same electrical state, the repelled matter negative if the sun is negative, positive if the sun is positive.
The grandest terrestrial displays of electrical disturbance, as seen in lightning and the aurora, must be of a small order of magnitude as compared with the electrical changes taking place in connection
- ↑ See "Comets," Royal Institution Proceedings, vol. x, p. 1.
