

AND THEORY OF THE HEAVENS. IOI 



It is also credible that some comets are larger than Saturn 

 and Jupiter; although it cannot be believed that the 

 magnitude of their masses always goes on thus increasing. 

 The dispersion of the primitive matter and the specific 

 lightness of its particles will make the formation of bodies 

 slow in the most remote regions of space ; and the in- 

 definite diffusion of it in the 'immeasurable range of this 

 distance, without any determination to accumulate towards 

 a certain plane, allows the formation of many smaller 

 masses instead of one single huge formation, while the 

 weakness of the central force will let the greatest portion 

 of the particles be drawn to the sun before they have 

 collected into masses. 



The specific density of the matter out of which the 

 comets arise is more remarkable than the magnitude of their 

 masses. As they are formed in the furthest region of 

 the universe, it is probable that the particles of which 

 they are composed are of the lightest kind; and it can- 

 not be doubted that this is the chief cause of the vapour 

 heads and tails by which they are distinguished from other 

 heavenly bodies. This dispersion of the matter of the 

 comets into vapour cannot be attributed mainly to the 

 action of the heat of the sun ; for some comets scarcely 

 reach as near the sun as the distance of the earth's orbit ; 

 and many stop between the orbits of the earth and Venus, 

 and then turn back. If such a moderate degree of heat 

 resolves and attenuates the matter on the surface of these 

 bodies to this degree, must they not consist of the very 

 lightest stuff which can suffer more attenuation by heat 

 than any other matter in the whole of nature ? 



Nor can these vapours so frequently rising from comets 

 be attributed to the heat which the body of the comet 



