AND ITS INHABITANTS n 



heat it contracted and, with the same energy of rotation that 

 it possessed before, necessarily revolved on its axis in a shorter 

 time. At last a stage was reached where, in the equatorial 

 belt, centrifugal force balanced gravitation and the matter 

 subjected to this balance of forces could sink in no further. 

 It is thought to have existed as a ring, left behind by the 

 condensing mass. The ring, however, was unstable; it broke 

 up and gathered into one body. During the further shrinking 

 of the main mass other rings were in turn abandoned. Each 

 gathered into a subordinate nebula, passed through an inde- 

 pendent evolution, and the whole gave rise to the system of 

 planets and their satellites. 



Modifications of the nebular hypothesis. During the first 

 half of the nineteenth century the nebular hypothesis was 

 accepted by astronomers almost without question, but during 

 the second half many serious dynamical objections were de- 

 veloped and a process of modification began, until now not 

 much remains of the original conception of Laplace. A 

 rather full statement of the hypothesis and the objections to 

 it has been given recently by Campbell. 2 A briefer summary 

 and a citation of but a few of the modifications in the general 

 concept must here suffice. 



George Darwin, Lockyer, Faye, Fouche, Poincare, and 

 others have taken part in this work, and in the opinion of these 

 mathematicians and astronomers the framework of the result- 

 ing structure is still sound, though subject of course to further 

 modification as knowledge increases. It was shown that the 

 original nebula need not have been hot, but, as perceived by 

 Kant, would develop heat from its self-condensation. A loose 

 swarm of cold meteorites would suffice as well as an original 

 gas for the initial state. The mass could never have revolved 

 as a unit body, as if it were a solid. On the contrary, the inner 



2 Campbell, W. W., "The Evolution of the Stars and the Formation of the 

 Earth." Scientific Monthly, vol. 1, 1915, pp. 189-194. 



