210 Chacornac on Comets. 



Ci If we calculate the velocity of outflow with which a gas 

 precipitates itself into a vacuum, we find that at the pressure 

 of our atmosphere, and at a temperature of zero, it is. greater 

 than that of a cannon-ball; and, in fine, it is demonstrated 

 that the pressure is insignificant in determining the velocity of 

 the flow of different gases, which depends on their density 

 only, and is greater as the density is less.* 



1 ' If these considerations are applied to the question of the 

 limitation of planetary atmospheres, it will be seen that it is 

 possible that at the temperature of planetary space there may 

 be an equilibrium between the weight of the external atmo- 

 spheric layers and the elasticity of those below them, but this 

 cannot be the case with a vaporous atmosphere exposed to a 

 temperature of many thousand degrees. Moreover, for an 

 external layer to press inwards from its weight, we must con- 

 ceive that it can no longer dilate itself in the vacuum of 

 planetary space, that is to say, it must be more dense than 

 what is beyond it. This last consideration affords mathematical 

 ground for regarding the external layer as crystalline,^ in order 

 to comprehend how any atmosphere — like that of the earth — 

 can be limited. 



"At the surface of the sun such an hypothesis is not 

 admissible, and, moreover, in the phenomenon of re-incandes- 

 cence, seen in the photosphere, there must evidently be a 

 repulsive force acting upon gases, and producing violent dila- 

 tation according to laws which are not known. We do not 

 know what forces of dilatation would be developed at such 

 enormous temperatures, but it is incontestable that the radiating 

 aureoles of the sun point to a projective force acting outwards, 

 and shooting forth perpendicularly to the solar periphery masses 

 like innumerable and very dilated aigrettes of comets.'" 



M. Chacornac appends sketches of aigrettes formed by the 

 comet of 1802. 



* "Et enfin ou demontre que la pression est insignifiante pour la vitesse 

 d'ecoulement des differents gaz, que celle-ci est dependaute de le densite des gaz 

 seulement, puiB-ce-qu'elle est autant plus grande que la densite est moindre." 



t We have before had occasion to notice the peculiar sense in which 3M. 

 "Chacornac uses the words crystal and crystalline. We must either suppose the 

 elasticity of the atmosphere to be limited, or we must imagine such an arrange- 

 ment of its particles as to give rise to, or be acted upon by, opposing forces. Why 

 this should be called a crystalline state we do not see. 



