It was noticed at an early date, that a gap existed between Mars and Jupiter, so that astronomers began to believe that there was probably a missing planet in that space, and this belief was greatly strengthened when Neptune was discovered in 1781. Without going any further into this story in this book, it may be said that it is the general opinion of astronomers that the planetoids or asteroids were formed possibly from the fragments of the missing planet, or, more probably, from the breaking up of some of the outer rings on the planet Mars. The distances of the planets from the central sun vary from the nearest planet, Mercury, which is about 36,000,000 miles from the sun, to the furthest, or Neptune, which is 2,766,000,000 miles from the sun. All the major planets have a single moon, or more, revolving around them. For example, Jupiter has four moons; Uranus, six; Saturn, eight; Neptune, one. As to the minor planets, Mars has two moons; and, as far as is known, neither Mercury or Venus has a moon. Our earth has one moon, but, as we shall afterwards see, this is not to be regarded as a moon or satellite of the earth, but rather as a twin planet to the earth. LaPlace's nebular hypothesis was proposed by LaPlace during the year 1796. While there are many objections that can be brought against it, since it fails to account for all of the phenomena of the solar system, yet it is a significant fact now, in the year 1907, nearly a century and a quarter after the hypothesis was first announced, that although modified in many respects, there has not been any hypothesis proposed to entirely replace it. While the nebular hypothesis of LaPlace is necessarily a matter that belongs to astronomy, yet it will be advisable to consider it here, since it explains the source of the original heat of both the earth and the moon, which we believe is the true cause of volcanoes. In his nebular hypothesis, LaPlace assumes that all the materials of which the solar system is formed, were originally scattered throughout space in the shape of an exceedingly rare form of matter known as nebulous matter. He points out that if it be granted that this medium began to accumulate around a common centre, so as to form a huge globe or sphere, and if a motion of rotation on its axis from west to east were given to this sphere that, on strictly mechanical principles, a system of heavenly bodies corresponding to the solar system might have been evolved. Let us, therefore, try to understand how this might have been brought about. The nebulous matter that LaPlace assumed originally constituted all the matter in the solar system, was highly heated gaseous matter. In other words, it consisted of ordinary matter raised to a very high temperature; LaPlace thought at a temperature very much hotter than that of the sun.