224 FROM NEBULA TO NEBULA 



As the molecules of matter are fundamentally con- 

 strued to be perfectly elastic, Arrhenius makes no con- 

 structive use of their high velocities, but the dust par- 

 ticles he supposes to beat each other into a glow and in 

 this state to attach to themselves the various free mole- 

 cules with which they come in contact. Thus, he says, 

 are the comets and the nuclei of planets formed. 



The orbital motions of the planets he " explains " 

 thus (Ibid., pp. 203 and 204) : 



By their collisions with the masses of gases which they en- 

 counter, they (the dust particles) gradually assume a circular 

 movement about the axis of rotation of the nebula. In this ro- 

 tation they condense portions of the gases on their surface, and 

 hence acquire a high temperature which they soon lose again, 

 however, owing to the comparatively rapid radiation. 



Our considerations lead to the conclusion that there is rotat- 

 ing about the central body of the nebula an immense mass of 

 gas, and that, outside this mass, there are other centres of con- 

 densation moving about the central body together with the masses 

 of gas concentrated about them. Owing to the friction between 

 the immigrated masses and the original mass of gas which cir- 

 culated in the equatorial plane of the central body, all these 

 masses will keep near the equatorial plane, which will therefore 

 deviate little from the ecliptic. We thus obtain a proper plane- 

 tary system, in which the planets are surrounded by colossal 

 spheres of gas like the stars in the Pleiades. If now, the planets 

 have very small mass by comparison with the central body as in 

 our solar system they will be cooled at an infinitely faster rate 

 than the sun. The gaseous masses will soon shrink, and the 

 periods of rotation will be shortened; but for those planets, at 

 least, which are situated near the centre, these periods will orig- 

 inally differ little from the rotation of the central body. The 

 dimensions of the central body will always be very large, and 

 the planets circulating about it will produce very strong tidal ef- 

 fects in its mass. Its period of rotation will be shortened, while 

 the orbital rotation of the planets will tend to become lengthened. 

 Thus the equilibrium is disturbed; it is re-established again, be- 

 cause the planet is, so to say, lifted away from the sun, as G. H. 

 Darwin has so ingeniously shown with regard to the moon and 

 the earth. Similar relations will prevail in the neighborhood of 

 those planets which will thus become provided with moons. 

 Hence we understand the peculiar fact that all the planets move 

 almost in the same plane, the so-called ecliptic, and in approxi- 

 mately circular orbits; that they all move in the same direction, 



