PORTION OF THE COSMOS. THE PLANETS. 807 



of Mercury (according to the mass of that pla,net as deter- 

 mined by Encke) is only a little greater. On the other 

 hand, none of the outer planets exceed J- in density ; 

 Saturn is even only Uh, being little more than half 

 as dense as the other outer planets and the sun. The 

 outer planets present to us a phenomenon unique in the 

 entire solar system i. e., the wondrous appearance of 

 a solid ring encircling its primary planet, but detached from 

 it and suspended freely in space. They also present to us at- 

 mospheres which, by the peculiarities of the condensations 

 taking place in them, appear to our eyes as variable, and in 

 Saturn sometimes even as interrupted, streaks or belts. 



Although, in the important division of the planets into 

 two groups of interior and exterior planets, the facts of abso- 

 lute magnitude, density, compression at the poles, velocity 

 of rotation, and presence or absence of moons or satellites, 

 show a general connection with the solar distances, or with 

 the semi-major axes of the orbits, yet this connection or de- 

 pendence can by no means be asserted in respect to each 

 individual member of these groups. As I have already before 

 remarked, we as yet know of no inherent necessity, no me- 

 chanical natural law, which as the fine law which links toge- 

 ther the squares of the times of revolution and the cubes of 

 the major axes should represent the above-named elements 

 of magnitude, density, &c., for the succession of the several 

 planetary bodies in each group, in connection with or depend- 

 ence on their respective solar distances. Although it is true 

 that the planet which is nearest to the sun (Mercury) is 

 also the densest, and even six or eight times as dense as the 

 exterior planets, Jupiter, Saturn, Uranus, and Neptune; 

 yet the order of succession between Venus, Earth, and 



