He assumes that our globe was once very much smaller than it is at present, and that independent masses of matter, perhaps other planets, have fallen upon it, the shock of collision generating an enormous degree of heat. This is an effect of the "crush of worlds" not commonly apprehended. If two bodies, conjointly equal to the bulk of the earth, were rapidly traveling through space, and should violently come together, their collision would evolve enough heat to convert the united mass into lava, or heated vapor. An asteroid falling upon the sun would generate from 4,600 to 9,200 times as much heat as would come from the combustion of an equal mass of bituminous coal, the force of velocity changing into caloric. This view, though adequate to explain the origin of the nebula, does not account for the existence of the colliding bodies. It might be consistent with the doctrine of the eternity of matter, in which case the colliding planets may have been coursing about the sun for myriads of ages as aggregations of matter corresponding to the last term of the great cosmic cycle—inorganic sterility. Could we understand how all the planets might eventually fall into the sun, we might suppose the present series of changes is only one of several cycles, in agreement with the speculations of certain writers.
Dr. Mayer carries his theory much farther. He does not confine these cataclysmic unions to the ante-nebulous periods. It is suggested that there may have been similar accretions to the surface of our planet after the introduction of life. A luxuriant vegetation, or a thickly-peopled continent, may have been often buried beneath the fiery débris resulting from the conflict. There are frequent occurrences of a similar character at the present day, but of trifling influence upon the general temperature. Every solid meteor that falls from the sky develops heat; and it cannot be denied that, were these bodies of large size, the calamitous occurrences depicted by Dr. Mayer would be experienced. Each one of these cataclysms would interrupt the cycle of progress as set forth above, and carry the order of the mutations back to the beginning.
When we study the scheme of worlds revolving around the sun, we discover that they all rotate on their axes in the same direction; that they all proceed from west to east, their orbits being nearly circular, and in almost the same plane, which is nearly coincident with that of the sun; that the sun moves on his axis in less time than any of the planets, and each planet rotates more quickly than its satellite. These and other facts point out a community of origin and development inexplicable by chance or the law of gravitation. We suppose, then, that the sun and all the planets and their satellites composed originally a single mass of luminous fog, with a diameter exceeding that of the orbit of Neptune, the remotest planet, or not less than three thousand million miles. This would correspond well with the supposed dimensions of the smaller nebulæ now seen in the skies. The