1 58 Transactions. — Miscellaneous. 



2nd. Loss of energy by selective escape. 3rd. The formation of compound 

 molecules, and 4tla. The aggregation of molecules into liquid and solid par- 

 ticles. The extreme density of the smaller planets gives some probability to 

 this theory of their origin. But even had they existed as independent 

 bodies previously to the impact, there is but little doubt that the tendency 

 to exchange of molecules, which I discussed in my last paxoer, would cause 

 the nearer planets to consist almost entirely of the very heavy molecules. 

 It is extremely probable that the atmosphere, and not unlikely the water, 

 was picked up from the contracting nebular sun, as it gradually shrank 

 within the orbit of the planet. 



The slowness of the rotation of the inner planets as well as their den- 

 sity, compared with the outer planets, may also be accounted for by the 

 much greater resistance that the inner part of the nebula would offer to 

 their rotation, as well as to the much greater time that must elapse before 

 the nebula shrunk within their orbit. 



Mr. Cherrill's suggestion — that each of the two sets of planets may have 

 been parts of either original body — is worth remembering. Another very 

 serious objection to this theory of the origin of the planets is the order of 

 the distances of the planets (Bode's Law) ; but this has been shown to be a 

 very empirical and imperfect law. There are some agencies which have 

 suggested themselves to accovmt for it as it now stands, — one is the rotation 

 of apsides. If we accept what is probably the case, — that, after the first 

 revolution of the planets, their orbits had become so circular that the 

 aphelion of the inferior did not extend much beyond the perihelion of the 

 next superior ; it appears reasonable to suppose that if the planet's apsides 

 rotated, it would gradually tend to pick up any bodies which were revolving 

 within those limits. This action would tend to cause the various planes of 

 the orbits to coalesce ; to render the orbits more circular ; and to lessen the 

 inclination of the axes. It might also tend to increase the rate of rotation, 

 consequently we may imagine, as suggested in last paper, that Jupiter has 

 been very active in this collecting action. It is certain that the apsides 

 would rotate, for all the resistance of the nebula except that exactly at peri- 

 helion would have this effect, in addition to bringing the body nearer the 

 sun. This rotation of apsides of more elliptical orbits therefore appears to 

 offer a hint of the order of distance of the planets. 



There are other points which have been suggested before, but which 

 have not become more mature than when first suggested. The origin of 

 the moons is another serious difiiculty. It is possible that when the planets 

 were in a half nebulous condition, these may have had their nuclei 

 formed by bodies which the planet picked up in its collecting action during 

 apsidal rotation. 



