ON THE SOLAR CORONA. 765 



when apparently pure, stands revealed as a dense swarming of mill- 

 ions 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 stream- 

 ers 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.* 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 sub- 

 jected 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," Koyal Institution Proceedings, vol. x, p. 1. 



