THE NEBULAR HYPOTHESIS. 



By E. P. RIDLEY. 



^kj^J'HE hypothesis of the 

 original nebular con- 

 dition of the solar 

 system, with the 

 consequent explana- 

 tion of planetary 

 formations and 

 movements of rings 

 and satellites, was 

 first published by 

 Kant in 1755 in his 

 "General History 

 and Theory of the 

 Heavens." It was 

 subsequently inde- 

 pendently pro- 

 pounded by Laplace 

 in his Systbne du 

 Monde, published 

 about 1797 — in 

 which work it ap- 

 peared as a note, and was given, as the author stated, 

 " with the diffidence which one ought always to feel 

 in connection with a subject which is not the result 

 either of observation or of calculation." In the 

 present day the hypothesis is more usually ascribed 

 to Laplace than to Kant. 



The nebular hypothesis may be briefly stated as 

 follows. The solar system ages ago consisted of an 

 irregular mass of nebula intensely hot and in a state 

 of extreme gaseous tenuity, extending from its centre 

 outwards far beyond the limits of the outermost 

 planet, Neptune. It is practically impossible to 

 imagine such a mass at rest ; and, granted that it had 

 some motion (however indefinite), the movement must 

 finally end in a definite rotation in one direction 

 round the centre of gravity, and, no matter what its 

 primitive shape, it must at last inevitably assume the 

 form peculiar to rotating bodies, in which the 

 particles move freely upon each other, and become 

 an oblate spheroid, flattened at the poles and bulging 

 at the equator. The mass would, by pouring out heat 

 into space and by the action of gravity, contract, and, 

 No. 301. — January 1890. 



as it contracted, it must rotate faster and faster, the 

 velocity increasing as the space traversed diminished. 

 Contraction generates heat, which, pouring outwards 

 into space, would act as a centrifugal force tending 

 to drive part of the mass outwards, while gravity 

 would act as a centrifugal force drawing the mass 

 in towards the centre. As the solar nebula continued 

 to radiate heat and contract, it rotated with ever- 

 increasing velocity, its poles becoming more and 

 more flattened, and its equatorial zone protruding'more 

 and more, until at last the centrifugal tendency at 

 the equator became greater than the force of gravity. 

 Then the bulging equatorial zone, no longer able to 

 keep pace with the rest of the mass in its contraction, 

 was left behind as a detached ring, girdling, at a 

 small but steadily-increasing distance, the retreating 

 mass. This ring, unless subjected to absolutely 

 symmetrical forces in all directions — evidently an 

 infinitely improbable supposition — would, still con- 

 tracting, break, much as a dish breaks when dropped 

 on to the floor, into a host of fragments of very un- 

 equal dimensions. Instead of a continuous ring, we 

 should now have a host of satellites surrounding the 

 solar equator, revolving in the direction of the solar 

 rotation, and following each other in the same orbit. 

 Each large fragment would by its gravitative force 

 retard the smaller fragment in front of it, and 

 accelerate the smaller fragment behind it. This 

 process would continue until all the fragments were 

 finally united with a spheroidal body having a velocity 

 compounded of the several velocities of the fragments 

 and a rotation made up of their several rotations. 



The central mass continued to radiate heat and 

 to contract, until a second equatorial belt was left 

 behind to form in due time a second planet as before. 

 In like manner, the other planets were formed one 

 after the other, the planets themselves, by a similar 

 process, forming their satellites. 



Having now a general idea of the hypothesis, we 

 shall proceed to consider what relation the various 

 phenomena existing in and exhibited by the solar 

 system bear to it. 



Not only the sun, but every member of our 



B 



