160. Let us now turn to globe development. We have alluded to this already while discussing the energy of the universe. In doing so we came to the conclusion that the original state of the visible universe was a diffused or chaotic state, in which the various particles were widely separated from one another, but exerting on one another gravitating force, and therefore possessed of potential energy. As these particles came together, impinged on one another, or gathered into groups, this potential energy was gradually transformed into the energy of heat and into that of visible motion. We may thus imagine the cooling and (except under very strict conditions of original distribution) _necessarily_ revolving matter in course of time to have thrown off certain parts of itself which would thereafter form satellites or planetary attendants, while the central mass would form the sun. We have here, in fact, the development hypothesis of Kant and Laplace, and it is greatly in favour of the truth of this hypothesis that all the planetary motions of the solar system are nearly in one plane, and also that, looking down on the system from above that plane, all these motions are seen to be in one and the same direction. 161. Assuming, therefore, that the solar system and, _pari passu_, the other sidereal systems have been formed in this way, it is very easy to see why the central mass should be so much hotter than its attendants. Two causes would conduce to this. In the first place, assuming that the heat of a mass is due to the rushing together of its particles under the force of gravitation, the velocities would be much greater for the central mass, and hence the amount of heat (per unit of mass, _i.e._ the temperature) developed would be greater also. In the next place, the body being a large one would cool less rapidly than its attendant planets. These two causes thus combine to render the largest bodies of the universe ever since their aggregation (and still more now) the hottest, so that the same body which forms the gravitating centre of the system becomes, when required, also the dispenser of light and heat.