VI.] THE MOTIVE POWERS OF THE UNIVERSE. 65 



parts of tlie original nebulous mass that lias condensed 

 into the solar system, has been thus transformed into heat 

 and radiated away, except xst^^^ part, which still remains.^ 

 Part of this is the potential energy due to the distances 

 of the planets from the sun, and would be transformed 

 first into energy of motion and then into heat if the planets 

 were to fall into the sun ; part is the energy of the planetary 

 motions. In this calculation the lieat of the sun and of 

 the heated centres of the planets is left out of the account, 

 as being almost infinitely small in comparison with the 

 energy just mentioned as due to the planetary distances 

 and the planetary motions.- 



In a few words, the history of the transformations of 

 energy in the condensation of a nebula is this : Fii-st it is 

 potential energy due to the gravitation of the parts of the 

 nebula towards each other ; this is transformed, first into 

 energy of motion, and then into heat ; and finally tlie heat 

 is transformed into radiance, and lost in space. 



We have seen that the condensation of a nebula must 

 necessarily result in the aggregation of masses with pro- 

 duction of heat. It follows from the law of the conserva- 

 tion of rotation,^ that, supposing the original nebula to have Eotation of 



a slight initial velocity of rotation, the masses will rotate ^ °^^^^°^^^ 

 o _ •' ' mass. 



on their axes and revolve round their common centre of 

 gravity, as we find those which constitute the solar system 

 to do. But it also follows from the conservation of rota- 

 tion, that if the nebula has no initial rotation, no mutual 

 actions of its parts can cause the nebula, or the sum-total 



1 In this calculation it is assiiined that the volume occupied by the 

 nebula was infinite. This of course cannot be true, but it probably gives 

 a nearly true result. 



The total mass, or weight, of the bodies composing the solar system 

 being approximately known, it is a matter of calculation to determine how 

 much energy, from being potential, would become actual, in the con- 

 densation of that mass of matter from a state of infinitely wide diffusion 

 into a globe having the density of the sun. 



^ The nebular theory was first advanced by Laplace ; the best account 

 of it that I know of is to be found in Herbert Spencer's Essays, where a 

 wonderful amount of cumulative proof has been assembled. Of course I do 

 not offer my slight sketch of the thenry as anything like complete proof, 



3 P. 17. 



F 



