the earth which we happen to have adopted. Until recently the earth was - supposed to have cooled from a state of incandescent liquid. The commonest form of this theory is the Nebular Hypothesis. On this theory the solar system was conceived to have been one originally in a state of gas, which on cooling began to revolve. The particles of matter fell in towards the centre owing to contraction and gravitation, but were also thrown outwards by the centrifugal force, on the principle of a stone whirled at the end of a string. The particles at the equator of the revolving sphere, where the motion was most rapid, became subjected to two opposing forces, one causing them to fall in, the other causing them to fly out, and it is supposed that at a certain stage the two forces balanced, and the equatorial belt was left self-sustaining in space. The rest of the gaseous matter continued to contract, and at various stages further rings of matter were left balanced, until the central mass was enabled to contract as a whole. The central mass became the sun, and the rings of gaseous matter after a while cooled sufficiently for the gas to become a liquid. At this stage the rings snapped, and the matter became aggregated in liquid balls, which followed the paths of the parent gaseous rings; thus were born the planets, of which the earth is one. Finally, the liquid balls cooled sufficiently for the surface to solidify, and a crust or outer shell of solid rock was formed over a molten interior. This is the state in which the earth is supposed, on this theory, to be in at the present time. The theory explains so simply the phenomena of volcanoes and the ridging of the crust into mountain ranges due to the crust having to follow the shrinking nucleus, that it has been very generally accepted in the past.
