10 THE STORY OF THE EARTH AND MAN. 



diameter than that which it now occupies, and whirl- 

 ing in its annual round about the still vaporous centre 

 of our system, in which at an earlier period the earth 

 had been but an exterior layer, or ring of vapour. 

 The atoms that now constitute the most solid rocks 

 are in this state as tenuous as air, kept apart by the 

 expansive force of heat, which prevents not only their 

 mechanical union, but also their chemical combination. 

 But within the mass, slowly and silently, the force of 

 gravitation is compressing the particles in its giant 

 hand, and gathering the denser toward the centre, while 

 heat is given forth on all sides from the condensing 

 mass into the voids of space without. Little by little 

 the denser and less volatile matters collect in the 

 centre as a fluid molten globe, the nucleus of the 

 future planet ; and in this nucleus the elements, obey- 

 ing their chemical affinities hitherto latent, are arrang- 

 ing themselves in compounds which are to constitute 

 the future rocks. At the same time, in the exterior 

 of the vaporous envelope, matters cooled by radiation 

 into the space without, are combining with each other, 

 and are being precipitated in earthy rain or snow into 

 the seething mass within, where they are either again 

 vaporised and sent to the surface or absorbed in the 

 increasing nucleus. As this process advances, a new 

 brilliancy is given to the faint shining of the nebulous 

 matter by the incandescence of these solid particles in 

 the upper layers of its atmosphere, a condition which 

 at this moment, on a greater scale, is that of the sun ; 

 in the case of the earth, so much smaller in volume, 



