FUNDAMENTAL PROBLEMS OF GEOLOGY. 219 



It is thus conceived that a spiral nebula, having two dominant 

 arms, opposite one another, each knotty from irregular pulsations, 

 and rotatory, the knots probably also rotatory, and attended by sub- 

 ordinate knots and whirls, together with a general scattering of the 

 larger part of the mass in irregular nebulous form, would arise 

 from the simple event of a disruptive approach. 



The ejected matter, at the outset, must have been in the free 

 molecular state, since by the terms of the hypothesis it arose from 

 a gaseous body ; but the vast dispersion and the enormous surface 

 exposed to radiation doubtless quickly reduced the more refractory 

 portions to the liquid and solid state, attended by some degree of 

 aggregation into small accretions ; hence the continuous spectrum 

 which this class of nebulae present. 



The problem of the luminescence of nebulae is confessedly a puz- 

 zling one. There is little ground for assigning general incandes- 

 cence to matter so obviousl}' scattered and tenuous and possessed of 

 such an enormous radiating surface. The assignment of the light 

 to the collision of meteorites, as done by I^ockyer, encounters both 

 dynamic and spectroscopic diflSculties. The recent discoveries of 

 the luminescent properties of radio-active matter and of its power 

 to awaken luminescence in other matter offers some hope of a solu- 

 tion. The fact that these properties are not necessarily dependent 

 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 excitating 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 dependent on heat, and hence objections to a planetesimal organi- 

 zation on the ground of low temperature do not seem to me to have 

 much force. 



As previously remarked, the verity of this particular mode of 

 origin of spiral nebulae is not essential to the acceptance of the 

 planetesimal hypothesis. It is merely necessary that two simple 

 a.ssumptions should hold good, viz, (1) that the nebular matter 

 of the spiral be in a finely divided solid or liqviid condition, as the 

 continuous spectrum implies, and (2) that the particles of this 

 scattered material revolve in elliptical orbits about the central mass. 



In attempting to follow the probable evolution of such a spiral 



