THE BIRTH OP THE SUN AND PLANETS 19 



now. Nebulae, no doubt, in some cases (compare dumb- 

 bell nebulae), split in two from the speed of rotation. In 

 whichever way it was, the original nebula of our system 

 went on contracting, spinning round on its axis, and 

 casting detached masses off to form planets. We cannot 

 enter here into the dynamical considerations by which 

 experts follow the phases of contraction. They show 

 that the matter would in time arrange itself in the form 

 of a disc, thus the detached planetary masses would lie 

 in or about the same plane. They show that the very 

 fact of condensation would bring about that turning on 

 the central axis, which accounts for the revolution of the 

 planets round the sun. They show that the spiral form 

 is a natural consequence of the rotation, as the inner 

 particles will revolve more rapidly than the outer ones. 

 But for these dynamical arguments the interested reader 

 must consult specific works on nebular evolution, like Sir 

 Robert Ball's Earth's Beginning. 



In outline, the first chapter of our story is now fairly 

 clear. It opens with a great nebula, luminous or dark, 

 with a girth of at the least 20,000 million miles. Under 

 the forces of gravitation, or the pressure of the ether of 

 outer space, its particles push inward toward the centre. 

 The irregular mass rounds itself more or less, turns with 

 increasing speed on its axis, and thins out into a plate or 

 disc. Its even texture breaks up, and the matter gathers 

 into vast arms flung out spirally from the main centre- 

 mass. Thousands of small bodies may crystallise out of 

 the mass, but the greater number will be absorbed or 

 thrust into the larger fragments. In the end nine 

 smaller masses (assuming that the planetoids are frag- 

 ments of a disrupted body) will circle round the pre- 

 dominant mass that has gathered together at the centre 

 to form the sun. Whatever the temperature at first, the 

 collisions of the swiftly moving particles will, as they 



