A HISTORY OF SCIENCE 



In the closing years of the eighteenth century La- 

 place took up the nebular hypothesis of cosmogony, to 

 which we have just referred, and gave it definite pro- 

 portions; in fact, made it so thoroughly his own 

 that posterity will always link it with his name. 

 Discarding the crude notions of cometary impact 

 and volcanic eruption, Laplace filled up the gaps in 

 the hypothesis with the aid of well-known laws of 

 gravitation and motion. He assumed that the primi- 

 tive mass of cosmic matter which was destined to 

 form our solar system was revolving on its axis 

 even at a time when it was still nebular in character, 

 and filled all space to a distance far beyond the 

 present limits of the system. As this vaporous mass 

 contracted through loss of heat, it revolved more 

 and more swiftly, and from time to time, through bal- 

 ance of forces at its periphery, rings of its substance 

 were whirled off and left revolving there, subsequently 

 to become condensed into planets, and in their turn 

 whirl off minor rings that became moons. The main 

 body of the original mass remains in the present as the 

 still contracting and rotating body which we call the 

 sun. 



Let us allow Laplace to explain all this in detail : 

 " In order to explain the prime movements of the 

 planetary system," he says, "there are the five follow- 

 ing phenomena: The movement of the planets in the 

 same direction and very nearly in the same plane ; the 

 movement of the satellites in the same direction as 

 that of the planets; the rotation of these different 

 bodies and the sun in the same direction as their revo- 

 lution, and in nearly the same plane ; the slight eccen- 



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