300 ORIGIN OF THE EARTH 



assumed by Laplace. The leading hypotheses of the earth's origin 

 fall into three groups: 



i. The gaseous hypotheses, in which the parent nebula is assumed 

 to have been formed of gas collected into a spheroid by gravity, and 

 to have been evolved into the present solar system by loss of heat, 

 and the separation of the outer parts into planets. The type of the 

 class is the Laplacian hypothesis. 



2. The meteoritic hypotheses, in which the parent nebula is 

 assumed to have been a swarm of meteorites, the members of which 

 moved in diverse directions. Frequent collisions gave rise to heat, 

 light, and vaporization. The swarm of meteorites is thought to 

 have behaved essentially as a coarse gas, and the evolution of the 

 system to have been dynamically like the preceding. 



3. The planetesimal hypothesis, in which the original constitu- 

 ents of the nebula are assumed to have been small bodies, molecules 

 or aggregates, moving in orbits about a common center and form- 

 ing a disk-like system. The evolution consisted in the gathering of 

 these small bodies (planetesimals) into planets and satellites. Dy- 

 namically, this hypothesis differs more from the other two than 

 they do from each other. 



i. The Laplacian hypothesis. During the last century the 

 Laplacian hypothesis was generally accepted, and geological theories 

 as to the early states of the earth, and as to many later events in its 

 history, were built upon it. The hypothesis is so well known that 

 a few sentences will recall its essential features. It holds that the 

 sun, the planets, and the satellites were once parts of a glowing, 

 rotating, spheroidal, gaseous nebula, which was expanded enough 

 to occupy the whole space of the solar system. The nebula was' 

 assumed to have cooled by radiation of heat, and in cooling to 

 have shrunk. The shrinkage accelerated the rate of rotation, and 

 this increased the equatorial bulge which rotation developed. The 

 progressive increase of cooling, rotation, and bulging finally led 

 to the separation of an equatorial ring. As this ring cooled and 

 contracted, it was disrupted and its substance gathered into a 

 planet whose orbit lay in the plane the ring had occupied. A series 

 of rings, separated in this way, gave rise to the several planets in 

 turn, while the central mass formed the sun. The orbit of any planet 

 bounds approximately the space assigned to the nebula at the birth 

 of that planet. At the time of origin, the several planets were 

 thought to be hot, gaseous, and rotating. Cooling and shrinkage 



