60 GEOLOGY. 



a solution. The fact that these properties are not necessarily depend- 

 ent on heat greatly relieves the stress of the problem. Whatever of 

 radio-active material there might be in the matter dispersed into 

 nebulous form would, by such dispersion, be set free for action, and 

 whatever other matter was subject to its excitation would also be set 

 free to receive the exciting influence. 



The solution of the problem may, however, lie along electrical 

 lines. At present it seems more probable that the luminescence arises 

 from some agency that acts at low temperatures than that it is depend- 

 ent on heat, and hence objections to a planetesimal organization on 

 the ground of low temperature cannot have much force. 



The assigned nebular origin not vital. — Whether the particular 

 mode of origin of the spiral nebula just sketched be true or not, is not 

 vital to the planetesimal hypothesis. It is offered as a definite genetic 

 picture and as a contribution to the natural demand for a conceivable 

 mode of genesis. It is only vital to the planetesimal hypothesis, as 

 here developed, that the scattered nebular material shall revolve 

 around the central mass in elliptical orbits, and this may perhaps 

 be sufficiently implied by the forms of the spiral nebulae without rais- 

 ing the question of their origin. It is true that their forms have been 

 interpreted as implying streams of in-running matter following spiral 

 courses, but a system of dynamics, on known principles and consist- 

 ent with known conditions, has never been worked out for such a 

 process, so far as we are aware, and it is not evident how it could be. 



As a basis for following onward the evolution, two simple assump- 

 tions only are necessary, (1) that the nebular matter of the spiral is in 

 a finely divided solid or liquid condition, as the continuous spectrum 

 implies, and (2) that the particles of this scattered material revolve in 

 elliptical orbits about the central mass. 



The evolution of the nebula into planets. 



The three essential factors. — The knotty spiral form of the nebula 

 gives ground for recognizing three factors as the chief participants in 

 the evolution into a planetary system, (1) the central mass (to become 

 the sun), (2) the knots (to become the nuclei of the planets), and (3) 

 the diffuse nebulous matter (to be added to the nuclei to form the 

 planets or to be added to the sun). Fitting the conception to the special 

 case of the solar nebula, the present distribution of mass and energy 



