o 



400 REPORT — 1873. 



motions of tlie planets and of the comets in their paths. It is also important 

 to observe that among the spectra of several telescopic comets which have 

 been examined, there is a typical resemblance which leads us to infer that the 

 coma or envelope of such comets is at least in great measure composed of 

 ases shining, for some reason, with self-resplendent light. A state of 

 liquid or solid aggregation of vaporizable materials by extreme cold cannot 

 on this account be regarded as a complete explanation of the original con- 

 dition of their nuclei, unless, with Zollner, we admit that a feeble electrical 

 excitation accompanies the development of the vapours from them that pro- 

 duce the envelope and tail ; and that a restoration of the disturbed electrical 

 equilibrium among these vapours produces in them (as in the extensive tracts 

 of auroral clouds) a sufficiently strong illumination to be visible on account 

 of their great depth ; as even bright auroral beams may be produced by weak 

 electrical discharges lighting up vast volumes of air through which they pass. 

 The free electricity with which the vapours are charged would be suffi- 

 cient, as shown by Dr. Zollner, to account for the rapid projection of the 

 extremely rarefied materials of the tail in an outward direction from the 

 sun, if its tension, and that of free electricity similarly present in the sun 

 itself, is supposed not to exceed the amount assigned by Hankel as the ordi- 

 nary tension of free electricity in the earth's atmosphere. On account of 

 their larger masses (compared to the surfaces, upon which electricity resides) 

 no sensible effect of repulsion is produced by solar electricity on the nucleus, 

 and on the larger fragments separated from the comet's mass, that appre- 

 ciably diminishes the force of universal gravitation upon them, to which, in 

 common with all other bodies coming within the sphere of the sun's attrac- 

 tion, the separate particles of the cometary cloud are principally subject. 

 Similar -views to those of Dr. ZoUner on the electrical origin of the sun's 

 repulsive force on the tails and envelopes of comets (a force whose intensity 

 was first mathematically investigated by Bessel) were previously entertained 

 by Olbers, and discussions of some of their principal consequences, with 

 excellent illustrations derived from cometary observations by M. Faye, will 

 be found in the ' Comptes Rendus ' (vol. xlviii. p. 421) for 1870, and in a con- 

 temporary number of the French ' Eevue des Courses Scientifiques.' The 

 theory of a self-luminosity in comets, and perhaps in the vaporous nebulae, 

 resembling the glow-discharge in the vacuum of a barometer-tube when the 

 mercury is shaken, suggests, as shown by Dr. ZoUuer, no insuperable diffi- 

 culties, when the enormous thickness of the vapour-tracts is considered, in 

 which a very feeble illumination of this description would be sufficient to 

 render them very discernibly self-luminous, with all the visible characters 

 of a glowing gas. 



Dui'ing the last two or three years the discovery of energetic forces of 

 eruption on the sun, and therefore also probably on the surfaces of the stars, 

 has demonstrated the occasional occurrence of some convulsions so extremely 

 violent that they would suffice (at least, if they were but Httle stronger, or 

 equally energetic at an earlier period of the sun's history, when its diameter 

 was somewhat larger) to project molten and gaseous matters from its mass 

 to distances beyond the sphere of its own attraction. One of the most violent 

 eruptions of this description was observed by Prof. Young in America on the 

 7th of September, 1871, when masses of glowing hydrogen left the sun's 

 surface with a velocity of projection which cannot have been less than 200 

 miles per second ; had it started with this velocity from an elevation but 

 little more than twice its actual distance from the sun's centre, it would have 

 been projected beyond the orbit of the planet Neptune, and a velocity of 



