Mr Milne's Prize Essay on Comets. ^49 



the progress of consolidation^ they have arrived. Observation has, in 

 fact, already furnished us with an extensive scale of comets, which are 

 distinguishable by means of this important criterion. Several have been 

 seen which had no nucleus at all, presenting only a gradual thickening 

 towards the middle parts, which were nearly translucent; while, on the 

 other hand, there are many whose condensation has proceeded so far, 

 by having been more subjected to the action of the solar heat, as to 

 have a nucleus 100, 1000, or even 2000 miles in diameter. Those of 

 the latter description approach, in all the circumstances of their physical 

 character, to the nature of planetary bodies ; and particularly, like them, 

 are less exposed to those sudden changes from the violent action of the 

 sun s heat near their perihelion, which comets of a smaller size and a 

 looser texture ai*e observed to undergo. 



" III. From these observations, we shall be the better able to esti- 

 mate the probability of a supposition, perhaps it may be said more spe- 

 culative than useful, but nevertheless founded on philosophical princi- 

 ples, whether or not comets be habitable bodies ? It is very evident 

 that such a supposition can never apply to the generality of comets ; 

 for, with regard to those whose consolidation is still only partial, the 

 violent changes which take place in their constitution and structure, 

 both at the perihelion and at the aphelion , are totally incompatible with 

 all our ideas of either animal or vegetable existence. But with respect 

 to those comets, whose advanced state of maturity renders the sun's 

 influence incapable of materially affecting the surface of the nucleus, 

 there seems to be no physical impossibility why many of them may not 

 be the abode of living creatures, as, well as the Earth and the other pla- 

 nets of the system. 



" Yet considering the extremes of distance from the sun, at which 

 the comets are placed in different parts of their eccentric orbits, it has 

 been conceived, that the prodigious variations of heat and cold to which 

 the inhabitants of a comet must be exposed, render the above supposi- 

 tion quite untenable. This, however, is an objection, which, though 

 applicable to all comets, whatever be their state of consolidation, is truly 

 more specious than substantial. Newton, indeed, calculated that the 

 great Comet of 1680, which passed within 150,000 miles of the sun's 

 surface, must have been heated to a temperature 2000 times greater 

 than red hot iron. But the simple fact, that the comet, even if its den- 

 sity had exceeded that of iron itself, was not instantly dissipated by the 

 violence of such a combustion, indicates some error in the data on which 

 this calculation is founded. Still, though it should be allowed that the 



