io EVOLUTION OF THE EARTH 



HYPOTHESIS OF PLANETS DERIVED FROM A PRIMAL NEBULA 



The original hypothesis of Kant and Laplace. In 1754 

 and 1755 Immanuel Kant, the philosopher, published the most 

 remarkable papers which had appeared up to that time upon 

 the evolution of the solar system. He conceived matter to 

 have been originally diffused and cold. From a position of 

 rest it began to converge under the influence of gravitation 

 and gave rise to the sun. In some manner he held that the 

 matter in converging acquired a movement of rotation. 

 Certain nuclei grew up independently from the center and 

 gave rise to the planets and satellites. In 1785 he developed 

 the idea that the contraction of the sun's mass would develop 

 its heat, a view elaborated by Helmholtz in 1854 and generally 

 held by astronomers at the present time. Thus Kant sought, 

 and with a large measure of success, to evolve the present state 

 of the universe from the simplest condition by means of 

 mechanical laws alone. 



In 1796 Laplace, one of the most eminent of French 

 astronomers, published a general work on astronomy, and in 

 a short note at the end of the appendix proposed a theory of 

 the origin of the solar system which shortly became widely 

 known as the nebular hypothesis. He was evidently unaware 

 of Kant's work published forty-one years previously. Laplace 

 is most noted for his mathematical work on celestial mechanics, 

 yet he did not develop his hypothesis along such lines and 

 apparently did not attach much importance to it. Neverthe- 

 less, it became the dominant idea in cosmic evolution through- 

 out the next century. 



Laplace postulated an original nebula as a very hot, gaseous 

 mass extending beyond the orbit of the farthest planet and 

 possessing a uniform rotation throughout, as if it were a solid 

 body. Its size was the result of a balance between expansion 

 from its heat and contraction from its gravitation. As it lost 



