 Returning to the vortex theory, Descartes allots a considerable portion of the Principles to explicating various celestial phenomena, all the while adopting and adapting numerous sub-hypotheses that apply his overall mechanical system to specific celestial events. One of the more famous of these explanations is the Cartesian theory of vortex collapse, which also provides an hypothesis on the origins of comets (Pr III 115–120). Briefly, Descartes reckons that a significant amount of first element matter constantly flows between adjacent vortices: as the matter travels out of the equator of one vortex, it passes into the poles of its neighbor. Under normal conditions, primary matter flows from the poles of a vortex into its center, i.e., the sun, which is itself comprised of primary matter. Due to centrifugal force, these particles press out against the surrounding secondary globules as they begin their advance towards the equator (Pr III 120–121); the pressure exerted by the primary and secondary elements (on a person’s optic nerve) also serving as the cause of light (Pr III 55–64, IV 195). Since the adjacent vortices also possess the same tendency to swell in size, a balance of expansion forces prevents the encroachment of neighboring vortices. On occasion, however, a buildup of larger elements on the sun’s surface, identified as sunspots, may conspire to prevent the incoming flow of first element matter from the poles. If the sunspots ultimately cover the entire surface of the sun, the vortex’s remaining primary matter will be expelled at the equator, and thus it no longer has a source of outward pressure to prevent the encroachment of neighboring vortices. Once the vortex is engulfed by its expanding neighbors, the encrusted sun may become either a planet in a new vortex, or end up as a comet passing through many vortices.