876 COSMIC PJIILOSOril Y. [pt. n. 



system. So great has been the centrifugal force upon Saturn, 

 due to his rapid rotation and small specific gravity, that the 

 detachment of rings would seem to have gone on after the 

 surface of the planet had assumed the liquid state ; and 

 whether the rings thus formed be now continuous, or (as is 

 far more probable) discontinuous, they have obviously had a 

 much better chance of preserving their equilibrium than the 

 ordinary vaporous moon-forming rings. The dynamics of the 

 Saturn ian system still present many difficult questions; but 

 the fact that Saturn is the one planet which is still girdled by 

 rings that are apparently-continuous, is a very powerful 

 argument in favour of the nebular hypothesis. 



But the evidence does not end with these mechanical illus- 

 trations. In the present physical condition of the various 

 planets, so far as it can be determined, we shall find further 

 corroborative testimony. It is a corollary from the nebular 

 hypothesis that all the planets, having successively originated 

 from the same vaporous mass, must be composed in the main 

 of similar chemical elements ; and this inference has thus far 

 been uniformly corroborated by spectroscopic observation 

 wherever there has been an opportunity to employ it. Hence 

 it follows that the process through which the earth has 

 passed in contracting to its present dimensions has been, or 

 will be, repeated to a certain extent upon all the other 

 planets. Upon any planet there must eventually occur a 

 solidification of the crust, an extensive evaporation and pre- 

 cipitation of water, an upheaval of mountains, an excavation 

 of river-beds, and a deposit of alluvium, resulting in sedi- 

 mentary strata. But obviously the time at which these 

 phenomena occur must depend, not merely upon the an- 

 tiquity of the planet, but also upon the rate with which it 

 parts with the heat generated during its contraction. Since 

 the outer planets are so much older than the inner ones, it 

 might at first be supposed that they must have progressed 

 much further in consolidation. But against this must be 



