PRESENT PROBLEMS 91 



a system of planetary bodies may originate in this way, although he 

 is very cautious and conservative in stating conclusions. The great 

 importance of this theory of planetary origin from the standpoint of 

 planetary geology and the evolution theory seems to demand that 

 it should receive more attention than it has yet received. The tem- 

 perature of the great mass of such a swarm will be very much lower 

 than in the case of the gaseous nebula. The larger part of such a mass 

 will approach absolute zero in temperature. According to this hypo- 

 thesis, even Mercury may have been solid when it separated from the 

 parent mass, although in its later stages a large mass might become 

 a gaseous nebula, as the sun now is. But in case of a body like our 

 earth, of such relatively small size, and so far removed from the 

 heated core, there does not seem to be any necessity for the assump- 

 tion that it was ever in a fused condition. 



In view of these new developments, it seems peculiarly important 

 that a discussion of the limits of maximum temperature which the 

 mass of our earth has reached in the past should now be taken in 

 hand again. Suppose a swarm of meteorites to fill the space internal 

 to the moon's orbit, having a total mass equal to that of our earth. 

 Assume that the mass is in rotation, so that the moon is about to 

 separate from the parent mass. It would probably be too radical 

 to assume that each element of mass has either the same actual 

 velocity or the same angular velocity. Various hypotheses, more or 

 less probable, are possible. Assume an initial temperature approach- 

 ing zero absolute. It seems clear that the highest temperature 

 reached in passing to the present condition of things may be far below 

 the temperature of fusion. 



A body falling directly from the moon's distance to the earth will 

 develop 59/60 of the kinetic energy it would acquire in falling from 

 an infinite distance. The earth is yet being bombarded by meteoric 

 matter having such velocities. But the operation is taking place 

 so slowly that the heat has time to become dissipated by radiation, so 

 that no appreciable rise in temperature of the earth results. To what 

 extent may this condition have held in the past? Darwin discussed 

 the tendency of the larger masses in such a swarm to accumulate 

 towards the centre. It is a kind of sorting process. These larger 

 masses would be in general of a metallic character. The more brittle 

 rocks of smaller density would therefore form the outer layers of our 

 earth. May not the heterogeneous character of our so-called igneous 

 rocks be explained in this way? And the shrinking of the earth would 

 then perhaps be in part the flowing of this porous mass into con- 

 tinuity. And it may incidentally be pointed out that the existence 

 of the belt of meteorites known as the asteroids is a most significant 

 indication of the conditions which must have existed at a certain 

 stage in the history of our solar system. 



