In order to show the extent to which LaPlace's nebular hypothesis explains the peculiarities of the solar system, we must inquire what are the most important of these peculiarities. We will take these from Young's general book on Astronomy, from which most of the facts in this chapter have been condensed. They are as follows: The orbits of nearly all the planets and their satellites are nearly circular; they are all in the same plane; and all revolve in the same direction. They are, moreover, with the single exception of Neptune, arranged at distances from the sun in accordance with Bode's Law. All the planets increase in both directions, towards and from the sun, in density from Saturn, the least dense. All the planets, with the exception probably of Uranus, rotate in a plane that is nearly the same as the plane of the orbit in which they revolve. Moreover, with the exception of probably both Uranus and Neptune, all the planets rotate in the same direction as that in which they revolve. The satellites revolve in orbits whose planes nearly coincide with the plane of the planets' rotation, while the direction of the revolution of the satellites is the same as that in which their planets revolve. Finally, the largest planets rotate most swiftly. Now, LaPlace's nebular hypothesis explains nearly all of the above facts. The following modifications of the hypothesis, however, are necessary. Let us briefly examine some of these modifications. In the first place it can be shown that the original nebulous mass instead of being at a higher temperature than that of the sun was probably at a much lower temperature, since the condensation of the gaseous matter must have increased the temperature. Instead, therefore, of the original nebulous mass being purely gaseous it was, as Young expressed it: "Rather a cloud of ice cold meteoric dust than an incandescent gas or a fire mist." Or in other words, the original nebulous mass from which the solar system was evolved, consisted of finely divided particles of solid or liquid matter surrounded by an envelope of permanent gaseous matter. A doubt, too, has been raised as regards the manner in which the planets were liberated from the central sun. Instead of separating in the form of a regular ring, it has been thought that probably in most cases this separation assumed the shape of a lump. It might, however, have occurred at times in the ring-like form as may be seen in the case of the planet Saturn. Again, instead of the outer rings being separated first, and the others in regular order, so that the outer planets are much the older, it would seem possible, or, as Young states, even probable, that several of the planets may be of the same or nearly the same age, as they would be if more than one ring had been separated at one time, or, indeed, several planets may have been formed from different zones of a single ring.