1885.] on the Solar Corona. 213 



rotational velocity of the photosphere, and lagging behind would give 

 rise to curved forms ; besides, the forces of irruption and subsequent 

 electrical repulsion might well vary in direction and not be always 

 strictly radial, and under such circumstances a structure of the 

 character which the corona presents might well result. The sub- 

 permanency of any great characteristic coronal forms, as, for ex- 

 ample, the great rilt seen in the photographs of the Caroline Island 

 eclipse and also in those taken in England a month before the eclipse 

 and about a month afterwards, must probably be explained by the 

 maintenance for some time of the conditions upon which the forms 

 depend, and not to an unaltered identity of the coronal matter ; the j)er- 

 manency belonging to the form only, and not to the matter, as in the 

 case of a cloud over a mountain top, or of a flame over the mouth of 

 a volcano. If the f(n'ces to which the corona is due have their seat 

 in the sun, the corona would probably rotate with it ; but if the corona 

 is produced by conditions external to the sun, then the corona might 

 not be carried round with the sun. 



We have seen that the corona consists probably of a sort of incan- 

 descent fog, which at the same time scatters to us the photospheric 

 light. Now we must bear in mind the very different behaviour of a 

 gas, and of liquid or solid particles in the near neighbourhood of the 

 sun. A gas need not be greatly heated, even when near tlie sun, by 

 the radiated energy ; heated gas from the photosphere would rapidly 

 lose heat ; but on the other hand liquid or solid particles, whether 

 originally carried uj) as such, or subsequently formed by condensation, 

 would absorb the sun's heat, and at coronal distances would soon rise 

 to a temperature not very greatly inferior to that of the photosphere. 

 The gas which the spectroscope shows to exist along with the incan- 

 descent particles of the coronal stuff, may therefore have been carried 

 up as gas, or have been in part distilled from the coronal particles 

 under the enormous radiation to which they are exposed. Such a view 

 would not be out of harmony with the very different heights to which 

 different bright lines may be traced at different parts of the corona 

 and at different eclipses. For obvious reasons, gases of different vapour 

 density would be differently acted upon by a repulsive force which 

 varies as the surface and would to some extent be winnowed from 

 each otber; the lighter the gas the more completely would it come 

 under the sway of repulsion, and so would be carried to a greater 

 height than the gas more strongly held down by gravity. The 

 relative proportions, at different heights of the corona, of the gases 

 which the spectroscope shows to exist there (and recently Captain 

 Abney and Professor Schuster have shown that in addition to the 

 bright lines already known, the spectrum of the corona of 1882 gave 

 the rhythmical group of the ultra-violet lines of hydrogen which are 

 characteristic of the photographic sj^ectra of the white stars, and some 

 other lines also) would vary from time to time, and depend in ynvt 

 upon the varying state of activity of the photosphere, and so probably 

 establish a connection with the spectra of the prominences. This 



