In the preceding Chapter we endeavoured to show how the action of gravitation upon the particles of diffused primordial matter would result in the formation, by condensation and aggregation, of a spherical planetary body. We have now to consider another result of the gravitating action, and for this we must call to our aid a branch of scientific enquiry and investigation unrecognized as such at the period of Laplace’s speculations, and which has been developed almost entirely within the past quarter of a century. The “great philosophical doctrine of the present era of science,” as the subject about to engage our attention has been justly termed, bears the title of the “Conservation of Force,” or—as some ambiguity is likely to attend the definition of the term “Force”—the “Conservation of Energy.” The basis of the doctrine is the broad and comprehensive natural law which teaches us that the quantity of force comprised by the universe, like the quantity of matter contained in it, is a fixed and invariable amount, which can be neither added to nor taken from, but which is for ever undergoing change and transformation from one form to another. That we cannot create force ought to be as obvious a fact as that we cannot create matter; and what we cannot create we cannot destroy. As in the universe we see no new matter created, but the same matter constantly disappearing from one form and reappearing in another, so we can find no new force ever coming into action—no description of force that is not to be referred to some previous manner of existence. Without entering upon a metaphysical discussion of the term “force,” it will be sufficient for our purpose to consider it as something which produces or resists motion, and hence we may argue that the ultimate effect of force is motion. The force of gravity on the earth results in the motion or tendency of all bodies towards its centre, and, similarly, the action of gravitation upon the atoms or particles of a primeval planet resulted in the motion of those particles towards each other. We cannot conceive force otherwise than by its effects, or the motion it produces.