THE STORY OF THE EARTH 29 



In the hottest stars, as we saw, the heat is so great 

 that the chemical elements are dissociated. Modern 

 physics has taught us to regard each atom of matter as 

 a complex system of still smaller particles. Take the 

 smallest and lightest of them all, the atom of hydrogen 

 of which there are 36,000 billions in a cubic millimetre 

 of that gas (the size of a small pin-head) and imagine it 

 magnified to the size of a large ball. We should find it 

 to be composed of about a thousand tiny particles, at enor- 

 mous distances from each other relatively to their size, 

 circling round within the limits of the atom at a speed of 

 at least 100,000 miles a second. An atom of oxygen would 

 contain about 16,000 of these revolving particles; an 

 atom of mercury would be an intricate system of 200,000; 

 an atom of radium would be larger still. These 

 "electrons" are generally regarded as strain-centres, 

 possibly of a whirlpool character, in the mysterious ether 

 that fills the whole of space, and is thus gathered up into 

 the more ponderable masses. What forces cause the 

 strain-centres, and what forces link them together into 

 atoms, we do not know. But the evidence of astronomy 

 seems to point to a dissociation or loosening of these 

 atomic systems in condensing nebula?. Up to a certain 

 point the condensation generates heat more rapidly than 

 it can be radiated away. After that point the production 

 of heat decreases, and the condensing mass slowly cools. 

 As it cools, the electrons draw closer together once more, 

 and the atoms of the different chemical elements make 

 their appearance first the lighter gases, with simple 

 systems of electrons, and on through the scale to the 

 heaviest elements. 



In the story of our earth, therefore, one of the first 

 phases would be the evolution of our familiar chemical 

 elements. The heavier metals would sink toward the 

 centre, the lightest gases hover about the fringe of the 



