14 GEOLOGY. 



of light. Lockyer's treatise does not enter into a definite discussion 

 of the mechanics involved, in the original gathering together of the 

 nebular swarm, nor does it attempt to trace the course of evolution 

 down to the origin of the planets and the specific genesis of the earth. 



In a very notable paper/ George H. Darwin, starting with such a 

 swarm already aggregated and already possessed of a mass and a dis- 

 tribution suited to the formation of the solar system, has elaborately 

 discussed its mechanical characters. The essential result of his labori- 

 ous inquiry is the conclusion that such a swarm is dynamically analo- 

 gous to a gas, and that the laws governing gases may be applied to the 

 discussion of its mechanical properties. He does not attempt to carry 

 the history forward into the development of the planets, but apparently 

 believes that they arose from equatorial detachment, as in the Lapla- 

 cian hypothesis. In its immediate application to the origin of the 

 earth, this special form of the meteoritic hypothesis seems therefore 

 to become practically identical with the gaseous hypothesis, and hence 

 to be subject to the criticisms urged against that hypothesis, notably 

 those arising from the relations of mass and momenta, which seem to be 

 the most grave. 



There arises also the question whether such a swarm of meteorites 

 would not actually pass into the gaseous condition, as the result of the 

 heat developed by the frequent and violent collisions of the meteorites. 

 To the swarm under investigation, Darwin assigned a radius 44J times 

 the radius of the earth's orbit, which involves a quite attenuated dis- 

 persion. At even this dispersed stage, he computed that collisions, in 

 the region of the present earth's orbit, would occur at a rate of ten 

 or more per day, and at an average velocity of three or more miles per 

 second. With this frequency and velocity, it would seem that the 

 meteorites could not long escape comminution and vaporization. As 

 the comminution and vaporization progressed, and as the nebula con- 

 densed, the frequency of the collisions must have increased, and the 

 escape of the augmented heat, so caused, must have been more and 

 more obstructed. The logical conclusion seems to be that an actual 

 passage into the gaseous condition would ensue. 



As this form of the meteoritic hypothesis thus appears to merge 

 into the gaseous one, dynamically, if not literally, before the birth- 



1 On the Mechanical Conditions of Swarms of Meteorites and on Theories of Cos- 

 mogony, Phil. Trans. Roy. Soc, 1888. 



