That unification of knowledge which is the business of Philosophy, isbut little furthered by the establishment of this truth under its generalform. We must trace it out under its leading special forms. Changes, andthe accompanying transformations of forces, are everywhere in progress, fromthe movements of stars to the currents of commodities; and to comprehendthe great fact that forces, unceasingly metamorphosed, are nowhere increasedor decreased, it is requisite to contemplate the changes of all kinds goingon that we may learn whence arise the forces they and what becomes of theseforces. Of course if answerable at all, these questions can be answered onlyin the establish rudest way. The most we can hope is to establish a qualitativecorrelation that is indefinitely quantitative -- quantitative to the extentof implying something like a due proportion between causes and effects.
Let us, then, consider the several classes of phenomena which the severalconcrete sciences deal with. §68. The antecedents of those forces which our Solar System displays,belong to a past of which we can never have anything but inferential knowledge.
Many and strong as are the reasons for believing the Nebular Hypothesis,we cannot yet regard it as more than an hypothesis. If, however, we assumethat the matter of our Solar System was once diffused and had irregularitiesof shape and density such as existing nebulae display, or resulted from thecoalescences of moving nebulous masses, we have, in the momenta of its parts,original and acquired forces adequate to produce the motions now going on.
Various stages in the formation of spiral nebulae imply that rotationin many cases results from concentration: whether always, there is no proof;for large nebulae are too diffused, small ones too dense, and others areseen too much edgeways, to yield evidence. But in the absence of adversepre-arrangement some rotation may safely be inferred. So far as the evidencecarries us, we perceive some quantitative relation between the motions generatedand the gravitative forces expended in generating them. In the Solar Systemthe outermost planets, formed from that matter which has travelled the shortestdistance towards the common centre of gravity, have the smallest velocities.
Doubtless this is explicable on the teleological hypothesis, since it isa condition to equilibrium. But without insisting that this is beside thequestion, it will suffice to point out that the like cannot be said of theplanetary rotations. No such final cause can be assigned for the rapid axialmovement of Jupiter and Saturn, or the slow axial movement of Mars. If, however,we look for the natural antecedents of these gyrations which all planetsexhibit, the nebular hypothesis furnishes them; and they bear manifest quantitativerelations to the rates of motions. For the planets that turn on their axeswith extreme rapidity are those having large orbits -- those of which theonce-diffused components, probably formed into broad rings, moved to theircentres of aggregation in immense spaces, and so acquired high velocities.
While conversely; the planets which rotate with relatively small velocities,are those formed out of small nebulous rings.
"But what," it may be asked, "has in such case become ofall that motion which ended in the aggregation of this diffused matter intosolid bodies?" The answer is that it has been radiated in the form ofheat and light; and this answer the evidence, so far as it goes, confirms.
Geologists and physicists agree in concluding that the heat of the Earth'sinterior is but a remnant of the heat which once made molten the whole mass.
The mountainous surfaces of the Moon and of Venus, indicating, as they do,crusts which have, like our own, been corrugated by contraction, imply thatthese bodies, too, have undergone refrigeration. Lastly we have in the Suna still-continued production of the heat and light which result from thearrest of diffused matter moving towards a common centre of gravity . Herealso, as before, a quantitative relation is traceable. Mars, the Earth, Venus,and Mercury, which severally contain comparatively small amounts of matterwhose centripetal motion has been destroyed, have already lost nearly allthe produced heat; while the great planets, Jupiter and Saturn, imply bytheir low specific gravity, as well as by the perturbations of their surfaces,that they still retain much heat. And then the Sun, a thousand times as greatin mass as the largest planet, and having to give off an enormously greaterquantity of heat and light due to that loss of molar motion which concentrationentails, is still radiating with great intensity. §69. Those forces which have wrought the surface of our planet intoits present shape, are traceable to the primordial source just assigned.
Geologic changes are either direct or indirect results of the unexpendedheat caused by nebular condensation. They are commonly divided into igneousand aqueous -- heads under which we may most conveniently consider them.
