THE WHIRLING FIRE-MIST. 217 



Thus two planetary masses come into existence. By repe- 

 titions of the same process, a complete series of planetary 

 masses becomes scattered over the distance between the 

 original periphery and the center ; and at each occasion of 

 planetary birth, all the older planets recede a certain distance 

 farther from the center, and undergo a certain retardation in 

 their orbital velocities. The different planetary masses, how- 

 ever, do not possess equal densities ; they are not composed of 

 such ingredients as to furnish, on cooling to a given temper- 

 ature, the same proportion of solid, liquid, and gaseous con- 

 stituents. Before planet making began, we may suppose the 

 heavier constituents of the general mass had gravitated to the 

 central regions ; while the lighter constituents remained 

 nearer the periphery. If so, the first planets separated would 

 contain more of the substances which, at temperatures familiar 

 to us, make gases and water. Similarly, the later planets 

 disengaged would acquire a larger proportion of the sub- 

 stances which form solid rocks. In the case of the earth we 

 may suppose the greater part was rock-making material, since 

 the earth's specific gravity is so high ; but watery stuff in 

 sufficient amount to provide oceans and rains, went off with 

 the rock material, and with these, the lighter stuff for an 

 atmosphere. But in the case of Venus, most of the stuff was 

 rock-material, if not the whole of it ; while with Mercury it 

 seems probable that little water-stuff was included. In the 

 opposite direction, Saturn, Uranus, and Neptune must have 

 received a large excess of water and atmospheric stuff. It is 

 rational to suppose that their oceans have always covered the 

 whole land, as ours does more than half. In fact, these bodies 

 must be composed chiefly of water and atmosphere ; as their 

 specific gravities are low as water and cork. 



So, in the history of our system, the work went on as 

 long as the conditions existing permitted the central mass to 

 detach rings. Meanwhile, the planetary masses entered sev- 

 erally on their separate careers. Each career was, in effect, 

 a history of cooling. They did not proceed with equal pace, 

 since some, with larger mass than others, had more heat to 



