Kepler was no cosmogonist, but he aspired to found a 'physical astronomy,' and in his gropings for a mechanical power that might suffice to regulate the movements of the heavenly bodies, he stumbled upon a mode of action highly appropriate for the explanation of their growth. His ignorance of the laws of motion precluded him from the conception of velocities persistent in themselves, and merely deflected from straight into curved paths by a constant central pull. Hence he was driven to the twofold expedient of creating a whirling medium for maintaining the revolutions of the planets, and of supposing the sun to exercise a 'magnetic influence,' by which they were drawn into closed orbits. Here, then, central forces made a definitive entry on the astronomical stage, although with scarcely a discernible promise of their brilliant future. But it was otherwise with the clumsy machinery they helped to animate. Kepler's simple _modus operandi_, adopted, or more probably re-invented by Descartes, was published as an epoch-making discovery in his _Principia Philosophica_ (1644), and sprang under its new aspect into swift notoriety. The wide acceptance of the theory of vortices was at least in part due to the impressive largeness of its framework. Descartes left nothing out. The spacious scope of his speculations embraced all that was knowable--nature, animate and inanimate, life and time: 'Planets and the pale populace of heaven, The mind of man, and all that's made to soar.' A philosophy, a metaphysic, and a cosmogony were linked together in a single plan. Its author distinguished in matter three gradations of fineness. The coarsest kind was that composing the earth and other opaque bodies; the more sublimated materials of the sun and stars came next; finally, there was the ethereal substance of the skies, so delicately constituted as to be luminous or luminiferous. This last variety was regarded as of subordinate origin. It represented, in fact, a kind of celestial detritus. Interstellar space had gradually become filled with intangible dust, the product of molecular attrition among originally angular solar and stellar particles. Ether was thus supposed to bear to the subtlest description of ordinary matter very much the same sort of relationship that ions presumably do to atoms.