The great French mathematician Laplace worked out the theory mathematically, so that at the time it appeared to be dynamically possible ; but the knowledge we now have of gases makes it improbable that such a gaseous ball could have existed and have revolved at such a terrific rate, for as we now know the gaseous matter would have diffused through inter-stellar space. From the geological side, also, the theory has been attacked, especially from the evidence afforded by earthquakes; in these the tremors pass through the centre of the earth and can be recorded on the other side, where they show that they have passed the whole distance through solid substances of high rigidity. Volcanoes and folded mountain ranges can also be quite well explained, supposing the earth to be solid. A new theory, making no use of a central molten nucleus, became necessary, and this was proposed by T. C. Chamberlin in 1905: it is called the Planetismal Hypothesis, because the earth is supposed to have been formed by the infalling of fragments of solid matter floating in space like the actual planets, and hence called planetismals, or little planets. A great deal will have to be done before the Planetismal Hypothesis can be finally accepted as proved; but as it accounts for more facts than the Nebular Hypothesis, and, so far, no fatal error has been discovered in it, as in the Nebular Hypothesis, we must accept it as our basis. The theory is that as meteorites or shooting stars fall upon the earth, and many of them arrive solid on the surface, it is quite possible that in past ages they not only fell as now, but fell in greater quantity. Some 20,000 tons of meteoritic matter fall upon the earth annually, and there is no doubt that the earth is growing, although very slowlv. If now we conceive a swarm of meteorites,
