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.
As you will see, LaPlace's nebular hypothesis assumes that both sun and moon were in a highly heated condition when they were separated from the nebulous sun, so that we can understand that the former molten condition of their interiors was due to the heat they originally possessed.
[CHAPTER XIX]
THE EARTH'S HEATED INTERIOR, THE CAUSE OF VOLCANOES
As we have already seen, the nebular hypothesis of LaPlace would seem to make it more than probable that the earth was originally in a highly heated condition, and only reached its present state after long cooling. While this cooling has gone on for probably millions upon millions of years both before and during the geological past, yet in the opinion of perhaps the best geologists the interior of the earth is still very hot, only the outer portions or crust having hardened by loss of heat.