Page:The Music of the Spheres.djvu/301

 the other is still fresh with sparkling waters, green verdure, and is teeming with life.

According to the later and more generally accepted Planetesimal Theory of Moulton and Chamberlain, these globes were cold in the beginning but became hot through compression as the material of which they were composed was drawn toward their centers by the pull of gravitation.

The four outer planets are mostly gaseous; the four inner ones have hardened crusts. Let us scan for a moment the various surface features of the third one in order from the sun,—the Earth, our own globe, and consider the unique and marvelous manner in which its various features have come to be.

In the course of millions of years, the earth, through the pull of gravitation, became gradually more and more dense. Different elements in its gases (which according to the later theory were squeezed out by pressure), combined to form rock, water and atmosphere. Hydrogen united with oxygen formed water; nitrogen, oxygen and small quantities of carbonic acid gas formed the atmosphere; oxygen mixed with silicon made quartz, the earth's first rock. This rock, called granite, rose to the top of the molten liquid material and formed a thin film of crust, for it was lighter than the metallic substance which sank to form the earth's core.

This thin, rocky film of crust which covered the hot liquid matter was at first so hot that the gases which had formed the water could not stay on its surface but rose in the air as steam. After the temperature became lower and the rocks cooled, rain poured down in sheets of water, remained on the crust and formed a warm sea.

For a long time this film was but a slight obstruction to the raging fiery forces confined beneath it and again and again these forces tore the crust asunder and made the surface of the earth a scene of wild commotion. But each time the cooling gained a