Page:Encyclopædia Britannica, Ninth Edition, v. 10.djvu/228

Rh 214 the disappearance of the crude and unscientiﬁc cosmologies by which the writings of the earlier geologists were distin- guished. But there can now be little doubt that in the reaction against those visionary and often grotesque speculations, geologists were carried too farin an opposite direction. In allowing themselves to believe that geology had nothing to do with questions of cosmogouy, they gradually grew up in the conviction that such questions could never be other than mere speculation, interesting or amusing as a theme for the employment of the fancy, but hardly coming within the domain of sober and inductive science. Nor would they soon have been awakened out of this belief by anything in their own science. It is still true that in the data with which they are accustomed to deal, as comprising the sum of geological evidence, there can be found no trace of a beginning. The oldest rocks which have been discovered on any part of the globe have probably been derived from other rocks older than themselves. Geology by itself has not yet revealed, and is little likely ever to reveal, a trace of the ﬁrst "solid crust of our globe. If then geological history is to be compiled from direct evidence furnished by the rocks of the earth, it cannot begin at the beginning of things, but must be content to date its ﬁrst chapter from the earliest period of which any record has been preserved among the rocks. Nevertheless, though geology in its usual restricted sense has been, and must ever be, unable to reveal the earliest history of our planet, it no longer ignores, as mere specula- tion, what is attempted in this subject by its sister sciences. Astronomy, physics, and chemistry have in late years all contributed to cast much light on the earlier stages of the earth’s existence, previous to the beginning of what is commonly regarded as geological history. But whatever extends our knowledge of the former conditions of our globe may be legitimately claimed as part of the domain of geology. If this branch of inquiry therefore is to continue worthy of its name as the science of the earth, it must take cognizance of these recent contributions from other sciences. It must no longer be content to begin its annals with the records of the oldest rocks, but must endeavour to grope its way through the ages which preceded the formation of any rocks. Thanks to the results achieved with the tele- scope, the spectroscope, and the chemical laboratory, the story of these earliest ages of our earth is every year becoming more deﬁnite and intelligible. RELATIONS or THE EARTH IN THE SOLAR SYSTEM. Before entering upon the study of the structure and history of the earth, we may with advantage consider the general relations of our planet to the solar system, especially in view of its origin and history. It is now regarded as in the highest degree probable that all the members of that system have had a common origin. The investigations of recent years have revived and given a new form and meaning to the well-known nebular hypothesis, in which Laplace sketched the progress of the system from the state of an original nebula to its existing condition of a central incandescent sun with surrounding cool planetary bodies. He supposed that the nebula, originally diffused at least as far as the furthest member of the system, began to condense towards the centre, and that in so doing it threw off or left behind successive rings which on disruption and further condensation assumed the form of planets, some- times with a further formation of rings, which in the case of Saturn remain, though in other planets they have broken up and united into satellites. According to this view we should expect that the matter composing the various members of the solar system should (‘r E 0 L 0 G Y [L cosmos r.. be everywhere nearly the same. The fact of condensation round centres, however, indicates at least differences of density throughout the nebula. Mr Loekyer has, indeed, suggested that the materials composing the nebula arranged themselves according to their respective densities, the lightest occupying the exterior and the heaviest the ll1tL‘l'l-vl‘ of the mass. And if we compare the densities of the various planets, they certainly seem to support this sugges- tion. These densities are shown in the following table, that of the earth being taken as the unit :— Density of the Sun ................................. .. 0'23 ,, Mercury ..................... .. . 1'12 ,, Venus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 1'03 ,, Earth ...................................... .. 1'00 ,, Mars ....................................... .. 0'70 ,, Jupiter .................................... .. 0'24 ,, Saturn .................................... .. 0'13 ,, Uranus .................................... .. 0'17 ,, Neptune .................................. .. 0'16 There is not indeed a strict progression in the diminu- tion of density, but the fact remains that, while the planets near the sun are about twice as heavy as they would be if they consisted of such a substance as granite, towards the outer limits of the system they are composed of matter as light as cork. Again, in some cases, a similar relation has been observed between the densities of the satellites and their primaries. The moon, for example, has a density little more than l1alf that of the eartl1. The first satellite of Jupiter is less dense, though the other three are found to be more dense than the planet. Further, in the condition of the earth itself, a very light gaseous atmosphere forms the outer portion, beneath which lies a heavier layer of water, while within these two envelopes the materials form- ing the solid substance of the planet are so arranged that the outer layer or crust has only about half the density of the whole globe. Mr Loekyer ﬁnds in the sun itself evidence of the same tendency towards a stratified arrange- ment in accordance with relative densities, as will be im- mediately further alluded to. There. seems therefore to be much probability in the hypothesis that, in the gradual condensation of the original nebula, each successive mass left behind represented the density of its parent layer, and consisted of progressively heavier matter. The remoter planets, with their low density and vast absorbing atmospheres, may be supposed to consist of metalloids like the outer parts of the snn’s atmosphere, while the interior planets are no doubt mainly metallic. The rupture of each planetary ring would, it is conceived, raise the temperature of the resultant llel)11l011s planet to such a height as to allow the vapours to rearrange themselves by degrees in successive layers, or rather shells, according to density. And when the planet gave off a satellite, that body would, it might be expected, have the composition and density of the outer layers of its primary.‘ For many years the only evidence available as to the actual composition of other heavenly bodies than our own earth was furnished by the aerol-ites, meteorites, or falling stars, which from time to time have entered our atmo- sphere from planetary space, and have descended upon the surface of the globe. Subjected to chemical analysis these foreign bodies show considerable (liversities of composit ion ; but in no case have they yet yielded a trace of any element not already recognized among terrestrial materials. Up- wards of twenty of our elements have been detected in aerolites, sometimes in the free state, sometimes combined with each other. More than half of them are metals, in- cluding iron, nickel, manganese, calcium, sodium, and potas- 1 Mr Loekyer communicated some of his views to Professor Prest- wich, who gave them in his interesting Inaugural Lecture at Oxford, in 1875. He has further stated them in his IIa.nchester Lectures, lI'h_r/ the 1'.'urth’s C'hcm1'stry is as it is.