=The nebular hypothesis.= It was long held that the earth began as a vaporous, shining sphere, formed by the gathering together of the material of a gaseous ring which had been detached from a cooling and shrinking nebula. Such a vaporous sphere would condense to a liquid fiery globe, whose surface would become cold and solid, while the interior would long remain intensely hot because of the slow conductivity of the crust. Under these conditions the primeval atmosphere of the earth must have contained in vapor the water now belonging to the earth's crust and surface. It also held all the oxygen since locked up in rocks by their oxidation, and all the carbon dioxide which has since been laid away in limestones, besides that corresponding to the carbon of carbonaceous deposits, such as peat, coal, and petroleum. On this hypothesis the original atmosphere was dense, dark, and noxious, and enormously heavier than the atmosphere at present. =The accretion hypothesis.= On the other hand, it has been recently suggested that the earth may have grown to its present size by the gradual accretion of meteoritic masses. Such cold, stony bodies might have come together at so slow a rate that the heat caused by their impact would not raise sensibly the temperature of the growing planet. Thus the surface of the earth may never have been hot and luminous; but as the loose aggregation of stony masses grew larger and was more and more compressed by its own gravitation, the heat thus generated raised the interior to high temperatures, while from time to time molten rock was intruded among the loose, cold meteoritic masses of the crust and outpoured upon the surface. Such a spiral nebula might be formed by the close approach of one star to another,--of a passing star to our own sun, for example, before the birth of the solar system. As the pull of the moon raises the tides on opposite sides of the earth, so, it is supposed, the pull of the passing star released the explosive forces of the sun, and two streams of matter were flung out from it. The knots in the arms formed the nuclei of the planets. The gaseous matter scattered outside the knots cooled into small stony masses, revolving about a central mass and hence called planetesimals (little planets). Like the meteorites which still fall upon the earth, the planetesimals were gradually gathered in by the nuclear knots, which thus grew to the present planets.