The History of Things 53 



"The history of a star," Professor R. K. Dun- 

 can writes, "begins with a nebula. A nebula 

 is a vast swarm of meteorites colliding together. 

 The meteorites are cold lumps of matter contain- 

 ing the chemical elements as we know them on 

 earth. These meteorites in accordance with their 

 gravitational attraction seek the centre of the 

 swarm, collisions result, heat is evolved, and the 

 temperature gradually rises." 



Owing to the meteoric bombardment, the con- 

 densing and colliding mass becomes converted 

 into incandescent gas, probably much simpler 

 chemically than the original swarm. As the bom- 

 bardment of meteorites ceases, the gaseous star 

 begins to cool. Chemically, it retraces its steps, 

 becoming more complex and heterogeneous again. 

 It passes through the condition now illustrated 

 by our sun or by Arcturus, and may eventually 

 become in itself extinct, like "yon dead world, 

 the moon." 



One of the most attractive forms of the Nebular 

 Hypothesis is that suggested by Professor Cham- 

 berlin. Laplace started with a gaseous nebula, 

 Lockyer and G. H. Darwin start with a swarm of 

 meteorites, Chamberlin starts with innumerable 

 small bodies (planetesimals) revolving about a 

 central gaseous mass. The central mass became 

 the sun; knots or partial concentrations in the 

 nebula became the nuclei of the planets; the res- 



