1912] The Ottawa Naturalist. 57 



but faulty in some of its arguments from his imperfect know- 

 ledge of physical laws. 



The theory of the evolution of the solar systems commonly 

 called the nebular hypothesis, which held undisputed sway for 

 nearly a century, and which still, in spite of many contradictions 

 recently discovered, occupies the premier position in the minds 

 of scientific men generally, was enunciated by the great French 

 scientist, Laplace in 1796. 



Laplace called attention to the fact that all the motions of 

 rotation and revolution in the solar system then known were in 

 the same direction and almost in the same plane. He computed 

 that the probability of this being a mere accident was about one 

 part in 500,000,000, conclusively showing it to be due to some 

 initial state from which the system had developed. 



This theory as amended by himself, and with some later 

 additions is, that our system was originally a nebula probably 

 somewhat condensed towards the centre, which extended beyond 

 the orbit of the farthest planet, that it rotated as one body in 

 the direction in which the planets now move and that it grad- 

 ually condensed and got hotter under the mutual gravitation of 

 its parts, exactly as we have already postulated. Simultaneously 

 with the contraction, the rate of rotation necessarily increased 

 from a well known dynamical law. After some time the centri- 

 fugal force at the equator became equal to the central attraction, 

 and a ring of nebulous matter was left off, the remainder con- 

 tinuing to contract and leave off rings at the distances of the 

 planets. The rate of rotation and the temperature of course 

 increased with the contraction. 



The rings left off scarcely could have had a uniform structure 

 and, separating at some point, would coalesce forming the 

 planets, while the satellites would be formed from rings left off 

 from the contracting planets, Saturn's ring being an example 

 still remaining. By the time these rings had formed planets and 

 these latter had cooled down to a solid condition, the central 

 part, the sun, would have gone through some of the changes 

 outlined previously, and would have reached its present con- 

 dition of approximate equilibrium, the loss of heat due to radia- 

 tion being compensated by the gain due to contraction. 



Such is the nebular hypothesis which remained unquestioned 

 for more than half a century, and which has exercised an in- 

 calculable influence on the science and philosophy of the nine- 

 teenth century. Unfortunately the nebular hypothesis, beauti- 

 ful and complete as it is, can not, in the form it was left bv Laplace 

 be made to account for the facts as they are now known. It 

 has, since about 1860, been subject to continuous attacks and 

 if now accepted must be in a considerably modified form. A 



