28 THE NEBULAE HYPOTHESIS. 



about the weight of so much cork ; and Herschel manifests a 

 corresponding decrease of density. This regular gradation in 

 the specific densities of the planets, in the order of their 

 occurrence, from innermost to outermost, is precisely what it 

 should be, supposing that they were all formed by the oper- 

 ations of a common law, from an original sphere of fluid 

 matter, which must have been most dense near the center, 

 and most rare on its outer extremities. 



There is a similar relation between the distances of the 

 different planets ; for, proceeding outward from Mercury, each 

 successive planet (including the asteroids as equivalent to one 

 planet) is about double the distance of the previous planet 

 from the sun. This curious relation of distances seems, in 

 like manner, to argue their production by a common cause, 

 and by the operations of a common law, of which the only 

 explanation yet found seems to be given in the nebular 

 theory. 



3. If the theory in question is admitted as the true one, it 

 inight accordingly be supposed, that after the evolution of 

 Mercury, which is the planet nearest the sun, there would still 

 be a residuum of nebulous or planetary matter in an unformed 

 state, surrounding the more dense mass of the sun. Accord- 

 ingly there actually appears to be an extensive mass of attenu- 

 ated matter surrounding the sun, and is sometimes visible 

 immediately after sunset, or before sunrise, as a conic, lumin- 

 ous streak, projected from the horizon in the direction of the 

 path of the sun, and which is called the " Zodiacal light." 



4. There are still many planets, or wandering celestial 

 bodies, in a nebulous state, in which state they are called 

 "comets." These appear to have been formed from a re- 

 siduum of attenuated matter, after the agglomeration of the 

 denser materials took place. 



