10 Dr. Hiiggins [Jan. 20, 



mously great that the original motion of the nucleus tells for very 

 little, and hence the secondary tail is but slightly curved, or even 

 is sensibly straight. Again, if we take the hypothesis that this 

 repulsive force, of whatever character it may be, varies as the surface, 

 and not, like gravity, as the mass, substances of different specific 

 gravity would be differently affected and separated from each other, 

 and these secondary straight, or nearly straight tails would, on this 

 view, consist of the lightest matter. 



On this hypothesis a comet would suffer of course a large waste 

 of material at each return to perihelion, as the nucleus would be 

 unable to gather up again to itself the scattered matter of the tail ; 

 and this view is in accordance with the fact that no comet of short 

 period has a tail of any considerable magnitude. 



A theory, based on chemical decomposition, has been proposed by 

 Professor Tyndall,* but as this view has been illustrated here 

 by the eloquent author himself, I will not now enter upon it. 



A different view of the whole matter has been suggested by 

 Professor Tait.j He supposes, not the nucleus only, but the whole 

 comet, to consist of a swarm, of enormous dimensions, of minute 

 meteoroids, which become self-luminous at and about the nucleus, in 

 consequence of the impacts of the various meteoric masses against 

 each other, giving rise to incandescence, melting, the development of 

 glowing gas, and the crushing and breaking up of the bodies into frag- 

 ments of different sizes, and endowed with a great variety of 

 velocities. The tail he conceives to be a portion of the less dense 

 part of the train illuminated by sunlight, and visible or invisible to 

 us, according not only to circumstances of density, illumination, and 

 nearness, but also of tactic arrangement, as of a flock of birds 

 under different conditions of perspective, or the edge of a cloud of 

 tobacco smoke. 



On this hypothesis we should expect to find a more comj)licated 

 spectrum, and the spectra of comets to differ greatly from each other. 



There seems to be a rapidly-growing feeling among physicists that 

 both the self-light of comets and the phenomena of their tails belong 

 to the order of electrical phenomena. One of the most distinguished of 

 the American astronomers wrote to me recently : " As to the American 

 views of the self-light of comets I cannot speak with authority for 

 any one but myself, still I think the prevailing impression amongst 

 us is that the light is due to an electric, or, if I may coin the word 

 electric-oid action of some kind." Here I confess I tread most 

 cautiously, for we have no longer any stepping-stones of fact on 

 which to place our feet. I am ready to admit that the spectroscopic 

 evidence, especially that furnished by the photographs of last year, 

 favours, though it does not necessarily demand, the view that the 

 self-light of comets is due to electric discharges. I do not attach 



* Phil. Soc. Cambridge, and ' Phil. Mag.' April 1869. 

 t ' Proceedings K. Society Edinburgh,' vol. vi. p. 553. 



