Chacornac on Comets. 209 



CHACORNAC ON COMETS. 



The following is a translation of a paper for which we are 

 indebted to M. Chacornac. It forms part of the Recherches 

 d'Astronomie Physique, which he issues from time to time : — 



"Since the great comet of 1811, it has been known that 

 the nuclei of comets as they approach the sun dilate into 

 vaporous atmospheres, to a certain limit, uniformly round the 

 nuclei, since if that limit is passed, these atmospheres are 

 compelled, by the action of an unknown force, to now back on 

 a level surface, in the prolongation of the radius vector, with a 

 velocity nearly equal to that of light. 



" In seeking to interpret this grand phenomenon of nature 

 with the aid of simple physical laws, we are led to the fol- 

 lowing conclusions. If no repulsive force emanated from 

 the sun, and opposed their dilatation, these cometary atmo- 

 spheres would extend in all directions, at least as far from the 

 nucleus as the extreme length of the tails, since the dilatation 

 of a gas in a vacuum appears to be indefinite. The rapidity 

 with which these atmospheres are developed is such, that even 

 at a temperature such as that of the earth's surface, it would 

 be considerable. 



" The aigrette of the comet of 1862 extended through a 

 length equal to four times the diameter of the earth in an in- 

 appreciable time, since the first trace of this jet, though weak, 

 was observed throughout the whole extent the moment it was 

 perceived. This fact shows that the expansive force of the 

 gaseous matter is great enough to produce effects like those 

 repulsions which arise from elevation of temperature, and we 

 are naturally led to conceive of similar actions in the solar 

 photosphere. 



" If we consider what must occur at the limits of the pressure 

 of the exterior atmosphere, where the force of gaseous ex- 

 pansion exceeds the attraction of the solar mass, it would seem 

 that the gaseous matter heated by the photosphere would 

 overflow into the vacuum of space. The behaviour of the 

 sun's external atmosphere appears in perfect accordance with 

 this consequence of the physical causes in operation. Thus 

 above the purple zone {zone pourj>ree) , which appears conti- 

 guous to the photosphere during total eclipses, a zone of dense 

 atmosphere is constantly noticed which reflects a brilliant light, 

 and on this account has frequently been confounded with the 

 reappearance of the disk on its emersion. It is from the 

 external surface of this atmosphere that those rays of the solar 

 aureole spring, the configuration of which indicates the ex- 

 pansive force of gases projected violently into planetary space. 

 vol. x. — NO. III. p 



