We may now say that we have taken our nebula to pieces, with the exception of the parts belonging to the satellites of those planets which have them; which would only be a tiresome repetition of what we have done for each principal member of the system, provided we had the necessary data, which we have not; and have thus acquired a certain amount of knowledge of the primitive conditions of each one of them. But we have still to examine into and draw conclusions from what we have seen and learned during the operation; which in some points, differ very much from our notions, formed from what we had previously read on the subject.
PAGE 108 Analysis continued. Excessive heat of nebula involved condensation only at the surface. Proof that this was Laplace's idea. 109 Noteworthy that some astronomers still believe in excessive heat. 110 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. 112 No proper explanation yet given of incandescent or glowing gas. 115 How matter was thrown off, or abandoned by the Jovian nebula. 116 Division into rings of matter thrown off determined during contraction. 117 How direct rotary motion was determined by friction and collisions of particles. 118 Saturn's rings going through the same process. Left to show process. 120 Form gradually assumed by nebulæ. Cause of Saturn's square-shouldered appearance. 120 A lens-shaped nebula could not be formed by surface condensation. 121 Retrograde rotary motion of Neptune and Uranus, and revolution of their satellites recognised by Laplace as possible. 123 Satellites of Mars. Rapid revolution of inner one may be accounted for. 124 Laplace's proportion of 4000 millions not reduced but enormously increased by discoveries of this century.
ANALYSIS OF THE NEBULAR HYPOTHESIS--_continued_.
When Laplace elaborated his hypothesis, heat was considered to be an imponderable material substance, and continued to be thought of as such--though perhaps not altogether believed to be so--for somewhere about half a century afterwards; so that it cannot be wondered at that he thought the nebula could have been endowed with excessive heat, more especially as it was looked upon as imponderable, and could in no way have any effect on the mass of the nebula. He only accepted the idea that was common to almost all astronomers of his time, that nebulæ were masses of cosmic matter of extreme tenuity but self-luminous, and consequently possessed of intense heat; they saw the sun gave light and felt its heat, and very naturally thought the nebula must be hot also. Without this idea he could not have formed the hypothesis at all, because he could not have conceived that the condensation of the nebula could only take place at its surface, or, as he terms it, "in the atmosphere of the sun," as most assuredly would be the case with an excessively hot body. And in order that there may be no doubt about this being his idea, we quote his own words as guaranteed by M. Faye in "L'Origine du Monde": "La considération des mouvements planétaires nous conduit donc à penser qu'en vertu d'une chaleur excessive l'atmosphère du soleil s'est primitivement étendu au delà des orbes de toutes les planètes, et qu'elle s'est reserrée successivement jusqu'à ses limites actuelles." And again: "Mais comment l'atmosphère solaire a-t-elle déterminé les mouvements de rotation et de révolution des planètes et des satellites? Si ces corps avaient pénétré profondément dans cette atmosphère, sa résistance les aurait fait tomber sur le soleil. On peut donc conjecturer que les planètes ont été formées à ses limites successives par la condensation des zones de vapeurs qu'elle à dû, en se refroidissant, abandonner dans le plan de son équateur." Proceeding on these ideas Laplace was quite in order and logical in conceiving that successive rings could be abandoned by the hot nebula, through the centrifugal force of rotation, for the formation of planets, more or less just in the way we have separated them. Having obtained his end quite legitimately, as he thought, in this way, he had no occasion to look any deeper into the affair, and consequently was not under the necessity of taking any thought of what the interior construction of the nebula might be, any more than so many others have not done since his day.