CHAPTER VI.

Analysis continued. Excessive heat of nebula involved condensation only at the surface. Proof that this was Laplace's idea 108 Noteworthy that some astronomers still believe in excessive heat 109 Interdependence of temperature and pressure in gases and vapours. Collisions of atoms the source of heat 110 Conditions on which a nebula can be incandescent. Sir Robert Ball 110 No proper explanation yet given of incandescent or glowing gas 112 How matter was thrown off, or abandoned by the Jovian nebula 115 Division into rings of matter thrown off determined during contraction 116 How direct rotary motion was determined by friction and collisions of particles 117 Saturn's rings going through the same process. Left to show process 118 Form gradually assumed by nebulæ. Cause of Saturn's square-shouldered appearance 120 A lens-shaped nebula could not be formed by surface condensation 120 Retrograde rotary motion of Neptune and Uranus, and revolution of their satellites recognised by Laplace as possible 121 Satellites of Mars. Rapid revolution of inner one may be accounted for 123 Laplace's proportion of 4000 millions not reduced but enormously increased by discoveries of this century 124