Mr. A. S. Davis on a Theory of Nebula and Comets. 407 



the unheated invisible gases, will have a tendency to escape out 

 of the solar envelope in addition to that which they possess 

 from their momentum in common with the rest of the comet. 

 Hence the comet will depart with its tail directed away from 

 the sun. 



The hollow appearance of many comets, and the isolation of 

 the nucleus from the vertex of the coma, are in accordance with 

 this theory. 



On a comet's approach to the sun, it often happens that a tail 

 of immense length is formed in a very few days. It is usually 

 supposed that the matter forming the tail has all been projected 

 from the head w T ithin the time of its first becoming visible, and 

 consequently that it has moved with enormous velocity in a di- 

 rection opposed to the sun's attraction. Hence it has been con- 

 jectured that the matter forming the tail is not subject to the same 

 mechanical laws as those which govern all other known matter. 



On the present hypothesis there is no need to assume this 

 enormous velocity. A comet, in fact, enters the solar envelope 

 with a tail of invisible gas. It may be that chemical union can- 

 not take place between this gas and the sun's envelope until 

 the heat of the sun, acting on the head of the tail, has set up 

 chemical action or combustion — until, in short, the comet has 

 been lit by the sun's heat. When once combustion has com- 

 menced, it would spread into the tail with prodigious velocity. 



Tails of comets have been observed to form with enormous 

 speed only on their approach towards the sun. The tails which 

 form when a comet is receding from the sun are produced with 

 comparative slowness : this we should expect ; for in this case 

 there is not already in existence a tail needing simply to be lit 

 to become visible. 



The more any portion of the gas of a comet becomes removed 

 from the nucleus, the greater will be the volume it occupies, 

 because of the diminution in the pressure due to a diminution 

 in the attractive force of the nucleus. This will account for the 

 spreading shape of the tail of a comet. 



We may explain in a similar way the increase in the size of a 

 comet as it recedes from the sun ; for the pressure of the solar 

 envelope upon it will become less as its distance from the sun 

 becomes greater. M. Valz has attempted an explanation of this 

 fact in a somewhat similar manner. He conceives that the in- 

 crease in the size is due to a diminution in the pressure of the 

 aetherial medium, which he supposes to be denser in the neigh- 

 bourhood of the sun than elsewhere. Sir J. Herschel objects 

 to this explanation, on the ground that we must suppose that the 

 aether does not pervade the matter composing the comet. 



This objection does not apply to the present explanation. 



