676 TRAIVSACTIONS OF THE AMERICAN INSTITDTE. 



now the actual constitution of the comets. The nebula became a 

 vast Avilderness of comets. As the resisting medium affected the 

 lighter and more expanded matter more than it did the denser, it 

 follows that there was a tendency of the atmospheric envelope, or 

 outer portion of each mass, to become separated from the denser 

 nucleus, and to move to the sun — leaving the denser portions be- 

 hind to constitute planets and planetettes. The greater orbital 

 velocities of the nebulous matter near the primary, cause(] all but 

 the very densest portion of it to be swept into the sun. As the 

 distance from the sun increased, the influence of orbital velocity 

 was lessened, and the densities tended to decrease in the same ratio. 

 Let us examine a table (1) of the densities (according to Hum- 

 boldt), and compare them with the velocities of the same planets, 

 to see how far they confirm this theory. 



TABLE 1. 



Mercury — 



Venus - 



Earth 



Mars 



Jupiter 



Saturn _ _ — 



Uranus - — 



Neptune _. 



It will be seen that from Mercuryto Saturn the actual densities 

 are generally accordant with our vicAvs; beyond this point they 

 increase in density, and thus appear at first thought to contradict 

 theory. But, upon a more careful examination, we find that this 

 apparent discrepanc}^ is in reality a confirmation. The matter 

 swept into the sun from the interior parts of the system was, in 

 some degree, replaced hy that swept inward from the middle 

 regions; and that swept inward from the middle regions was re- 

 placed b}^ that from the outer regions; but the light matter swept 

 inward from the outer regions could not be replaced; and, there- 

 fore, the outer planets would be more dense than those in the 

 middle, while those in the interior would be the densest of all. 



The internal heat of celestial bodies must make some diflerence 

 in their densities. The density of the sun is about the same as 

 that of Jupiter; and this is readily accounted for by the amazing 



