8 THE AGE OF THE EARTH 



and possesses a densit} 7 ^ of 3'2. It was from this outer 

 envelope when molten that the moon was trundled off, 

 twenty-seven miles in depth going to its formation. The 

 density of this material, as we have just seen, is supposed 

 to be 3'2 ; the density of the moon is 3*39, a close 

 approximation, such difference as exists being completely 

 explicable by the comparatively low temperature of the 

 moon. 



The outer envelope of the earth which was drawn off 

 to form the moon was, as we have seen, charged with 

 steam and other gases under a pressure of 5,000 Ib. to the 

 square inch ; but as the satellite wandered away from the 

 parent planet this pressure continuously diminished. 

 Under these circumstances the moon would become as 

 explosive as a charged bomb, steam would burst forth 

 from numberless volcanos, and while the face of the 

 moon might thus have acquired its existing features the 

 ejected material might possibly have been shot so far 

 away from its origin as to have acquired an independent 

 orbit. If so we may ask whether it may not be possible 

 that the meteorites, which sometimes descend upon our 

 planet, are but portions of its own envelope returning to 

 it. The facts that the average specific gravity of those 

 meteorites which have been seen to fall is not much above 

 3'2,* and that they have passed through a state of fusion, 

 are consistent with this suggestion. 



The solidification of the earth probably became com- 



* In a collection of meteorites the metallic kinds (irons) will be 

 found to predominate over the rocky (stones), and the average density 

 as deduced from a collection will consequently be higher than that 

 given here. In the case of meteorites accidentally encountered on the 

 ground the irons by their unusual characters may easily attract the 

 attention of an observer while the stones may escape notice : but 

 the phenomena produced by a falling meteorite are the same for both 

 irons and stones, hence we must restrict our attention to meteorites 

 of which the fall is recorded in order to obtain a fair result. 



